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Role of Water in the Formation, Transformation and Fate of

Secondary Organic Aerosol

by

Jenny Pui Shan Wong

A thesis submitted in conformity with the requirements

for the degree of Doctor of Philosophy

Graduate Department of Chemistry

University of Toronto

© Copyright by Jenny Pui Shan Wong (2015)

ii

Role of Water in the Formation, Transformation and Fate of

Secondary Organic Aerosol

Jenny Pui Shan Wong

Doctor of Philosophy

Department of Chemistry

University of Toronto

2015

Abstract

Particle-phase water is the most abundant atmospheric aerosol constituent, yet the importance of

water for the formation, transformation and fate of secondary organic aerosol (SOA) remains not

fully characterized. In order to address this knowledge gap, this thesis explores the role of water

in SOA formation, chemical aging and fate as cloud condensation nuclei.

The formation of SOA from the photooxidation of isoprene in the presence of various sulfate

seed particles was investigated using a flow tube reactor. Under constant environmental

conditions, particle-phase water was found to have the largest effect on the amount of SOA

formed where this additional organic material was highly oxidized, likely arising from enhanced

uptake of organic acids due to their high water solubility. The amount of high molecular weight

compounds increases with acidity, suggesting the role of acidity in governing organic

composition.

The relative humidity (RH) dependence of SOA aging by photolysis was examined using

particles containing water-soluble α-pinene SOA material in an environmental chamber at three

RH conditions (5, 45 and 85 %). Photolysis led to substantial mass loss where the rate of mass

loss increased with increasing RH, suggesting that moisture-induced changes in SOA phase have

implications to particle reactivity.

iii

Aging of ambient SOA sampled at Whistler, British Columbia found that aging by both gas and

aqueous-phase OH increased the degree of oxygenation and CCN activity of the organic

material, confirming the hypothesis that there is a simple relationship between the hygroscopicity

of organic aerosol and its oxygen-to-carbon ratio.

Addition of various types of organic material onto sulfate particles resulted in the suppression of

water uptake during droplet growth. Experiments using sulfate particles with different acidity

suggest that high molecular weight compounds, formed via acid-catalyzed condensed phase

reactions, are the species affecting water uptake kinetics.

iv

Acknowledgements

First and foremost, this thesis would not be possible without the support and guidance from my

supervisor, Jon Abbatt. A short acknowledgement simply does not reflect the magnitude of

appreciation I have for the role Jon has played in my development as a scientist throughout the

years. Being believed in – it made a huge difference for me. Jon, thank you for everything.

Many thanks to my committee members, Jennifer Murphy and Jamie Donaldson, for their

mentorship and encouragements. I've had the opportunity to work as a RAP student at

Environment Canada under the supervision of John Liggio - thank you for making the

opportunity possible and for all your advice. I am lucky to have had the opportunity to work in

Jamie's and Rebecca Jockusch's research groups as an undergraduate. Thank you for fostering

my interest in research and leading me to believe that my ideas are worth pursuing.

I am so grateful to have shared this journey with past and present members of the Abbatt group.

There is great camaraderie among the members of the group and I value all the interactions I

have with each and every one of you, both in and outside the lab. Special thanks to the

phenomenal post-docs I have had the pleasure to work with: Jay Slowik, Shouming Zhou and

Alex Lee. From working with all of you, not only have I learned a lot about the science itself, but

how to be great scientists as well.

The environmental chemistry bunch has been an amazing - the people really makes this a very

special place.

Friends and family - thank you. Joshua, you have been a pillar of support. Your kindness and

patience are immeasurable. Lastly, I would like to thank my siblings and mother - for your

unwavering strength, courage and unity, despite all odds. I am so immensely proud and honoured

to be your sister and daughter.

v

Table of Contents

ACKNOWLEDGMENTS IV

TABLE OF CONTENTS V

LIST OF TABLES VIII

LIST OF FIGURES IX

PREFACE XIV

1 INTRODUCTION 1

1.1 Motivation 2

1.2 Formation and Aging of SOA 2

1.2.1 SOA Formation 3

1.2.2 SOA Aging Processes 5

1.3 Secondary Organic Aerosol Particles as Cloud Condensation Nuclei 10

1.3.1 CCN Activation 11

1.3.2 Droplet Growth Kinetics 14

1.4 Experimental Methods for SOA Studies 15

1.4.1 Approaches for SOA Formation and Aging 15

1.4.2 Chemical Analysis of SOA by Mass Spectrometry 17

1.4.3 Approaches for CCN Measurements 18

1.5 Summary of Research Objectives 19

1.6 References 20

2 IMPACTS OF SULFATE SEED ACIDITY AND WATER CONTENT ON

ISOPRENE SECONDARY ORGANIC AEROSOL FORMATION 30

2.1 Abstract 31

2.2 Introduction 31

2.3 Experimental Section 32

2.3.1 Seed Particle Generation and Phase Characterization 33

2.3.2 SOA Formation 34

2.3.3 Particle Measurements 35

2.4 Results and Discussion 37

2.4.1 Ammonium Sulfate Particles 37

2.4.2 Acidic Sulfate Particles 40

2.5 Atmospheric Implications 42

vi

2.6 Supporting Material 43

2.7 References 43

3 CHANGES IN SECONDARY ORGANIC AEROSOL COMPOSITION AND MASS

DUE TO PHOTOLYSIS: RELATIVE HUMIDITY DEPENDENCE 48

3.1 Abstract 49

3.2 Introduction 49

3.3 Experimental Section 51

3.3.1 SOA Generation and Collection 51

3.3.2 UV/Vis Absorption Spectroscopy 52

3.3.3 Photolysis of SOA 52

3.3.4 Particle Measurements 55


3.4.1 Absorption Cross Section of SOA 56

3.4.2 Photolytic Degradation of SOA 57

3.4.3 Kinetics of Photolysis 63

3.5 Conclusions and Atmospheric Implications 65


3.6.1 Absorption Cross Section Calculations 67

3.7 References 67

4 OXIDATION OF AMBIENT BIOGENIC SECONDARY ORGANIC AEROSOL BY

HYDROXYL RADICALS: EFFECTS ON CLOUD CONDENSATION NUCLEI

ACTIVITY 71

4.1 Abstract 72

4.2 Introduction 72

4.3 Experiment 73

4.3.1 Gas-phase OH Oxidation 74

4.3.2 Aqueous-phase OH Oxidation 75

4.3.3 Particle Composition and CCN Measurements 76


4.5 Summary 81


4.7 References 84

vii

5 SUPPRESSION IN DROPLET GROWTH KINETICS BY THE ADDITION OF

ORGANICS TO SULFATE PARTICLES 87

5.1 Abstract 88

5.2 Introduction 88

5.3 Experiment 90


5.4.1 α-Pinene SOA 94

5.4.2 Carbonyls 97

5.4.3 Engine Exhaust 99

5.5 Interpreting the Degree of Droplet Growth Suppression 100

5.6 Conclusions and Atmospheric Implications 101


5.8 References 103

6 CONCLUSIONS AND FUTURE RESEARCH 109

6.1 Summary and Future Direction of SOA Formation Studies 109

6.2 Summary and Future Direction of SOA Aging Studies 109

6.3 Summary and Future Direction of SOA-CCN Studies 111

6.4 References 112

viii

List of Tables

Table 3.1 Total Organic Mass Loss Rate Constants for Different RH

Conditions

64

Table 4.1 Control experiments: particle properties of ambient organic aerosol.

Absolute mass (µg m-3

) of the various components is noted in the

brackets.

81

Table 4.2 Particle properties, and CCN activity of ambient organic aerosol.

Absolute mass (µg m-3

) of the various components is noted in the

brackets for the gas-phase OH oxidation experiments. For aqueous

OH oxidation experiments, 24-hr particle filter sample was obtained

from 08:00 to 08:00 of the following day.

83

ix

List of Figures

Figure 1.1 General schematic of the SOA formation pathway (adapted from

Seinfeld & Pankow6).

3

Figure 1.2 Reaction scheme for gas-phase OH + isoprene21–23

and products

formed in the particle-phase following reactive uptake.16,17,19

4

Figure 1.3 Schematic diagram of the processes involved in the reactive uptake

of a gas molecule onto a particle (adapted from Abbatt et al.31

).

6

Figure 1.4 Non-oxidative condensed phase reactions that increases the

molecular weight of organic compounds in SOA (adapted from

Hallquist et al.3).

7

Figure 1.5 The evolution of aerosol mass (left axis) and O:C ratio (right axis)

with OH exposure (data from Kroll et al.34

).

9

Figure 1.6 An illustrated representation of the formation, transformation and

fate of SOA in the atmosphere. Research questions addressed in

specific chapters are shown.

20

Figure 2.1 a) Schematic of the experimental setup. For each corresponding

region of the setup, the physical state of the particles is illustrated in

b) for sulfate seed particles, effloresced AS is represented by the red

hexagon and deliquesced AS is represented as the small red circles.

Abbreviations are described in the main text.

33

Figure 2.2 Mass spectra (normalized by total sulfate mass) for the organics on

wet AS (blue) and dry AS (red) seed particles. The molecular

formulae of the dominant organic fragment for significant ions are

identified in the main text.

37

Figure 2.3 Mass spectral difference (Wet AS – Dry AS) of the aerosol organics

normalized by total organic mass. Positive values (blue) indicate the

mass fragments that are enhanced for organics on wet AS seed, and

the negative values (red) indicate enhancement for dry AS seed. The

molecular formulae of the dominant organic fragments for

significant ions are labeled.

38

Figure 2.4 Mass spectral difference of the organics (normalized by total

organic mass) for a) wet AS and b) dry AS to illustrate the

reversibility of uptake due to water evaporation. Positive values

(blue) indicate the mass fragments that are enhanced for organics on

dried sulfate-SOA particles, and the negative values (red) indicate

enhancement for not-dried sulfate-SOA particles. The molecular

formulae of the dominant organic fragment for significant ions are

40

x

identified in the main text.

Figure 2.5 Mass spectral difference (wet ABS – wet AS) of the organics

(normalized by total organic mass). Positive values (blue) indicate

the mass fragments that are enhanced for organics on wet ABS seed,

and the negative values (red) indicate enhancement for wet AS seed.

The insert is a zoomed-in view of the mass spectrum at higher m/z.

41

Figure 2.6 Mass spectral difference of the organics (normalized by total organic

mass) for wet ABS to illustrate the reversibility of uptake due to

water evaporation. Positive values (blue) indicate the mass

fragments that are enhanced for organics on dried sulfate-SOA

particles, and the negative values (red) indicate enhancement for

not-dried sulfate-SOA particles. The molecular formulae of the

dominant organic fragment for significant ions are identified in the

main text.

42

Figure 2.7 Mass spectra (normalized by total organics) of the organics on wet

ABS (blue) and wet AS (red) seed particles. The insert illustrates

the zoomed-in view at higher m/z. The molecular formulae of the

dominant organic fragment for significant ions are identified in the

main text.

43

Figure 3.1 Typical volume distribution of SOA formed during filter collection,

measured by SMPS.

52

Figure 3.2 Sized-resolved mass distribution of organics (green) and sulfate

(red) measured by the AMS in PTOF mode for mixed SOA-AS

particles before UV light exposure

53

Figure 3.3 Measured photon flux in the chamber from UVB and UVA lamps

compared to the actinic flux for a clear-sky summer day. The actinic

flux was obtained from “Quick TUV Calculator”, available at

http://cprm.acd.ucar.edu/Models/TUV/Interactive_TUV/ using the

following parameters: SZA = 0, June 30, 2000, 300 Dobson

overhead ozone, surface albedo of 0.1, and 0 km altitude.

54

Figure 3.4 Typical time trace of org:sulfate ratio of the mixed particles due to

evaporation in the dark (control) and exposure to UV irradiation

under high RH conditions.

56

Figure 3.5 Absorption cross section of bulk SOA solution (this work) and

aqueous cis-pinonic acid.20

57

Figure 3.6 Time series profiles of the org:sulfate ratios measured by the AMS

for the photolysis of SOA by UVB radiation under high RH (solid

line), mid RH (dashed line), and low RH (dotted line) conditions.

The shaded areas for each RH condition represent the variability

(±1σ) between multiple experiments.

58

xi

Figure 3.7 Time series profiles of the sulfate normalized (a) m/z 29, (b) m/z 43,

and (c) m/z 44 for the photolysis of mixed SOA–sulfate particles by

UVB radiation at high RH (solid line), mid RH (dashed line), and

low RH (dotted line) conditions. The shaded areas for each RH

condition represent the variability (±1σ) between multiple

experiments.

59

Figure 3.8 Time series profiles of the sulfate normalized (a) total organic mass

(org:sulfate ratio, black), (b) m/z 29, (c) m/z 43, and (d) m/z 44 for

the photolysis of mixed SOA–sulfate particles by UVB (solid lines)

and UVA (dashed lines) radiation under high RH conditions. The

shaded areas for each light source represent the variability (±1σ)

between multiple experiments.

60

Figure 3.9 Aerosol composition described by the fraction of organic mass at

m/z 43 and 44 (f43 vs f44) before (open circles), during (closed

circles), and after (open circles) UV light exposure for (a) high RH,

(b) mid RH, (c) and low RH conditions. The inset shows the

zoomed-out view of the f43 vs f44 space (with dimensions of Figure

3.9a–c illustrated by the red box), where ambient SOA is distributed

within the triangular area.

62

Figure 3.10 Kinetic plots for the loss of total organic mass due to photolysis of

SOA at high RH (solid line), mid RH (dashed line), and low RH

(dotted line). The shaded region for each RH condition represents

the variability (±1σ) between experiments. The mass loss rate

constant was obtained from the linear best fit of the first 10 minutes

of photolysis.

63

Figure 4.1 Toronto Photo-Oxidation Tube (TPOT) setup for the study of the

gas-phase OH oxidation of ambient organic aerosol and their CCN

activity.

74

Figure 4.2 Activated particle fraction (CCN/CN) plotted as a function of the

water supersaturation (SS) for unoxidized and oxidized ambient

particles by gas-phase OH oxidation (Dm = 75 nm; July 22, 2010,

00:02–02:50) (red circle) and aqueous-phase OH oxidation (Dm =

100 nm; July 8, 2010) (blue square).

78

Figure 4.3 Relationship between O:C and κorg,CCN from the current study and

from several laboratory studies of model POA and biogenic

SOA,14,22,31,32

and a field CCN closure study21

for comparison. The

Lambe et al.32

solid line represents a fit for SOA from a number of

precursors. For each pair of points, the samples of the lowest and

highest O:C values are shown from each study. Dashed lines are

included only to indicate the relationship between these two points.

The following were the OH exposures used for the laboratory

studies of: BES (3.0 × 1012

molecules cm−3

s),31

O3-α-pinene SOA

80

xii

(1.3 × 1012

molecules cm−3

s),22

OH-α-pinene SOA (1.2 × 1012

molecules cm−3

s)14

and OH-α-pinene SOA (1.7 × 1012

molecules

cm−3

s).32

The shaded region illustrates the uncertainty of the

κorg,CCN and O:C relationships derived from the CCN closure study21

and laboratory generated organic aerosol.32

The uncertainties in the

calculated values of O:C and κorg,CCN are mainly influenced by the

systematic errors associated with each measurement, as discussed in

the main text. Also, daily averages are shown for the gas-phase OH

oxidation experiments, and the average of 4 one-day aqueous-phase

OH oxidation experiments (results from individual experiments

included in the supporting material).

Figure 4.4 AMS mass spectrum of organic and inorganic component of

particles subjected to “hv light” condition.

82

Figure 4.5 AMS mass spectrum of organic and inorganic component of

particles subjected to “OH Oxidation” condition.

82

Figure 5.1 Geometric mean diameter of droplets plotted as a function of seed

particle number concentration (number cm-3) for α-pinene SOA

condensed onto (a) sulfuric acid and (b) ammonium sulfate particle.

Each data point and the corresponding error bars for seed particle

number concentration and droplet diameters reflect the 5 min

averages of the measurements and ±1σ of measurement variability.

The results for controls (bare inorganic particles) are indicated by

the dashed lines (grey area, ±1σ).

94

Figure 5.2 Fraction of the geometric mean diameter of droplets with (Dd) and

without (Dd, control) α-pinene SOA on sulfuric acid (red circle) and

ammonium sulfate (blue square) seed particles, plotted as a function

of the org:sulfate mass ratio. The dashed line indicates no change in

droplet diameter, drawn to guide the eye.

95

Figure 5.3 Time series of various particle components measured by the AMS

for α-pinene SOA condensed onto (left) sulfuric acid and (right)

ammonium sulfate seed particles. The decrease in the mass loadings

for the various components is due to particle loss to the chamber

walls.

96


without (Dd, control) 2-pentanone and 1-octanal on sulfuric acid at RH

11 % (red circle) and RH 40 % (open circle), as well as ammonium

sulfate (blue square) seed particles, plotted as a function of the

org:sulfate ratio.

98

Figure 5.5 Time series of various particle components measured by the AMS

for carbonyls condensed onto sulfuric acid seed particles at (left)

RH 11 % and (right) 40 %.

99

xiii


without (Dd, control) engine exhaust on acidic sulfate with NH4+:SO4

2−

ratio of 1:4 (red circle), and NH4+:SO4

2− ratio of 2:5 (green circle),

plotted as a function of the org:sulfate ratio.

100

Figure 5.7 Mass spectra of the organic component measured by the AMS for

engine exhaust condensed on acidic sulfate particles with

NH4+:SO4

2- ratio of 1:4 (red) and 2:5 (green).

100

Figure 5.8 Mass spectra of the organic component measured by the AMS for

carbonyls condensed onto sulfuric acid seed particles at RH 11 %

(red) and 40 % (pink).

103

xiv

Preface This thesis was arranged in a series of chapters that are based upon several manuscripts in

preparation for submission to (Chapter 2) or have been published in peer-reviewed scientific

journals (Chapters 3 - 5). The following outlines the contributions made to each chapter.

Chapter 1: Introduction

Contributions: The chapter was written by Jenny P. S. Wong with critical comments from

Jonathan P. D. Abbatt.

Chapter 2: Impacts of Sulfate Seed Acidity and Water Content on Isoprene Secondary

Organic Aerosol Formation

Contributions: All experiments were designed and performed by Jenny P. S. Wong. Data

analysis was performed by Jenny P. S. Wong with data interpretation assistance from Alex K. Y.

Lee. The manuscript was written by Jenny P. S. Wong with critical comments from Alex K. Y.

Lee and Jonathan P. D. Abbatt.

Chapter 3: Changes in Secondary Organic Aerosol Composition and Mass due to

Photolysis: Relative Humidity Dependence

Contributions: All experiments were designed and performed by Jenny P. S. Wong and

Shouming Zhou. Data analysis and interpretation were by performed by Jenny P. S. Wong. The

manuscript was written by Jenny P. S. Wong with critical comments from Shouming Zhou and


Chapter 4: Oxidation of Ambient Biogenic Secondary Organic Aerosol by Hydroxyl

Radicals: Effects on Cloud Condensation Nuclei Activity

Contributions: All gas-phase OH oxidation experiments were designed, performed and analyzed

by Jenny P. S. Wong and Jay G. Slowik. All aqueous-phase OH oxidation experiments were

designed, performed and analyzed by Alex K. Y. Lee. Experiments were conducted at Raven’s

Nest site at Whistler, British Columbia with the assistance of W. Richard Leaitch and Ann-Marie

Macdonald from Environment Canada. The CCN instrument was provided by Droplet

xv

Measurement Technologies with the support from Dan J. Cziczo. The manuscript was written by

Jenny P. S. Wong with critical comments from Alex K. Y. Lee, Jay G. Slowik, Dan J. Cziczo,

W. Richard Leaitch, Ann-Marie Macdonald and Jonathan P. D. Abbatt.

Chapter 5: Suppression in Droplet Growth Kinetics by the Addition of Organics to Sulfate

Particles

Contributions: All experiments were designed, performed and analyzed by Jenny P. S. Wong

except for experiments conducted at Environment Canada’s engine lab, where substantial

assistance was provided by John Liggio and Shao-Meng Li. Modelling of experimental results

was performed by Athanasios Nenes. The manuscript was written by Jenny P. S. Wong with

critical comments from John Liggio, Shao-Meng Li, Athanasios Nenes and Jonathan P. D.

Abbatt.

Chapter 6: Conclusions and Future Research

Contributions: The chapter was written by Jenny P. S. Wong with substantial comments from


1

Chapter 1

1 Introduction

Introduction

2

1.1 Motivation

An aerosol is defined as a suspension of solid or liquid in a gas. The sizes and chemical

compositions of atmospheric aerosols are highly variable, both temporally and spatially. Particle

size ranges from 10-9

m to 10-5

m and can be comprised of inorganic (e.g., sulfate, nitrates),

carbonaceous matter (organic matter and elemental carbon), and water.1 Not only are organic

components ubiquitous in the atmosphere where they can constitute > 90 % of particle by mass,

they consist of a highly complex and variable mixture, most of which remains uncharacterized.2,3

The term organic aerosol refers to the organic fraction of the total aerosol mass. Furthermore,

most of the components in organic aerosol are secondary in nature (i.e., formed in the

atmosphere) and are referred to as secondary organic aerosol (SOA).

Atmospheric aerosols play important roles in climate and air quality. On one hand, particles

partially counteract the warming induced by greenhouse gases directly by interaction with solar

radiation or indirectly by acting as cloud nuclei. In fact, these climatic effects represent one of

the largest uncertainties in the assessments of global radiative forcing.4 On the other hand,

particles contribute to air pollution, with detrimental effects on human health.5 Ultimately, the

influence aerosols have on these effects is controlled by their physical and chemical properties. It

is thus critical to understand the formation, transformation, and fate of SOA in the atmosphere.

1.2 Formation and Aging of SOA

SOA is formed in the atmosphere following the oxidation of volatile organic compounds (VOCs)

when oxidation products of sufficiently low vapour pressure partition to the condensed phase. As

the air mass gets transported away from the source region, following its formation, SOA is

subjected to further physical and chemical transformations (“aging”), such as evaporation and

heterogeneous oxidation. In fact, such aging processes can result in additional SOA formation or

loss. In this framework, the formation and aging processes of SOA are intrinsically coupled

processes. However, in the context of this thesis, formation refers to the processes that lead to the

initial generation of SOA while aging refers to processes that alter the properties of existing

SOA.

3

1.2.1 SOA Formation

Both biogenic (vegetation) and anthropogenic (human activities) sources of VOCs can form

SOA. However, biogenic VOCs are the dominant global source of SOA due to their larger

emissions.3 The formation of SOA is initiated from the oxidation of VOCs (see Figure 1.1),

where the vapour pressure of these oxidation products largely determines their potential to

partition to the condensed phase.

Figure 1.1. General schematic of the SOA formation pathway (adapted from Seinfeld &

Pankow6).

The gas-phase oxidation of SOA precursors by OH, O3, and NO3 favours the formation of

products that are less volatile as these reactions typically result in the addition of more

oxygenated/polar functional groups.7 Generally, multiple oxidation steps are required to form

products of sufficiently low volatility to condense onto existing particles. In pristine

environments, these low volatility products may undergo homogeneous nucleation to form new

particles.7,8

1.2.1.1 Emerging Research Questions

Using the “traditional” SOA formation framework shown in Figure 1.1, atmospheric models with

parameterizations based on gas and gas-particle reactions studied in the laboratory, cannot

predict ambient SOA mass loadings.9,10

This discrepancy may result from numerous reasons,

4

including: i) yields of SOA measured in the laboratory may under-represent those in the

atmosphere; ii) the existence of unidentified precursors; and iii) formation mechanisms from

known precursors not being well understood.

About a decade ago, it was hypothesized that water-soluble oxidation products, despite their high

volatility, can dissolve into particle-phase water where subsequent condensed phase reactions

can lead to the formation of SOA.11

In fact, the partitioning of glyoxal to particle-phase water

was estimated to contribute 10 – 30 % of SOA mass measured in Mexico City.10

The degree to

which water partitions to organic aerosol will be discussed later in Section 1.3. The following

discussion will focus on SOA formation from isoprene as the work in this thesis focuses on

isoprene SOA formation.

Figure 1.2. Reaction scheme for gas-phase OH + isoprene21–23

and products formed in the

particle-phase following reactive uptake.16,17,19

The oxidation of isoprene is estimated to be the largest source of SOA globally as it is the most

abundant non-methane hydrocarbon emitted to the atmosphere.12

In particular, isoprene

epoxydiols (IEPOX), which are second generation oxidation products of isoprene photooxidation

under low NOx conditions, have been identified as key contributors to SOA formation.13–15

As

illustrated in the upper reaction branch of Figure 1.2, the reactive uptake of IEPOX and

subsequent reactions in the condensed phase lead to the formation of low volatility products such

5

as alcohols, dimers, and organosulfates.16–18

Laboratory studies have shown that the reactive

uptake of IEPOX onto acidic seed particles is enhanced due to acid-catalyzed nucleophilic

addition to the epoxide ring.19,20

Contrary to laboratory evidence demonstrating the acid enhancement of the reactive uptake of

IEPOX, multiple ambient studies have observed weak correlations between particle acidity and

IEPOX-derived SOA,24–27

suggesting that outstanding factors are likely to modulate isoprene

SOA formation in the atmosphere. Given that acidic seed particles are hygroscopic, even at the

low relative humidity (RH) conditions used by the IEPOX studies mentioned previously, the

conclusions from these previous studies do not preclude the role of particle-phase water on the

reactive uptake of IEPOX.

In addition to IEPOX, the oxidation of isoprene generates other products, mostly carbonyl

compounds, as shown in the bottom branch of the reaction scheme in Figure 1.2. These products

can undergo hydration and form low volatility products via subsequent reactions.23

The relative

importance of particle acidity and water on these other oxidation products remains unknown and

will be explored in Chapter 2.

1.2.2 SOA Aging Processes

Away from its sources, SOA is continually subjected to chemical aging that transforms the

particles’ physico-chemical properties. Aging processes include heterogeneous oxidation, gas-

phase oxidation of semi-volatile components, and reactions in the condensed phase, such as

photolysis, oxidation, and oligomerization.28,29

Generally, aging processes increase the degree of

oxygenation of the SOA. The following sections include brief summaries of the various types of

SOA aging that can occur. For oxidation reactions, only aging processes by OH radicals will be

considered, as most SOA compounds react with OH much more quickly than they do with

ozone. While it is useful to understand these processes individually, these transformations are

dynamic. Therefore, a conceptual framework will also be discussed to understand the overall

implications of aging processes on the molecular properties and consequently the physico-

chemical properties of SOA.

6

1.2.2.1 Heterogeneous Oxidation

Oxidation of SOA can occur heterogeneously by gas-phase OH radicals. This process can be

kinetically parameterized by the reactive uptake coefficient (γ), which describes the probability

of a reaction following gas-surface collision. The reactive uptake of a molecule is governed by

several processes (illustrated in Figure 1.3): i) diffusion to and on particle surfaces; ii) reaction

on the surface; iii) diffusion and solubility of solvated radical species in the particle bulk; iv)

bulk phase reactions.30,31

The uptake coefficient is determined in the laboratory by monitoring

the loss of either gas-phase or particle-phase reactants. The uptake coefficient of gas-phase OH

on model organic aerosol/surfaces typically range from 0.1 to 1. These values of γ indicate that

reactive uptake of OH onto SOA is an efficient process.32,33

Heterogeneous oxidation is

important for the oxidation of very low volatility compounds that only exist in the particle-phase.

Laboratory studies of the heterogeneous oxidation of model organic compounds indicate that the

reaction can result in either an increase in oxygen content or the loss of carbon due to

fragmentation of the carbon-carbon backbone.29,34

This will be further explored in Section

1.2.2.4.

Figure 1.3. Schematic diagram of the processes involved in the reactive uptake of a gas

molecule onto a particle (adapted from Abbatt et al.31

).

1.2.2.2 Gas-phase Oxidation of Semi-Volatile Compounds

The semi-volatile nature of organic compounds in SOA means that they are present in significant

concentrations in both the gas and particle-phase, where the semi-volatile compounds can be

oxidized by OH in the gas-phase.35

For semi-volatile compounds, oxidation in the gas-phase will

7

be more rapid compared to diffusion-limited heterogeneous oxidation. Aging of laboratory SOA

has indicated that gas-phase oxidation results in a higher degree of oxygenation and the

generation of lower volatility products that results in additional SOA mass.36

1.2.2.3 Condensed Phase Reactions

Non-oxidative reactions in the condensed phase, such as condensation reactions, often increase

the molecular weight and thus decrease the volatility of organic compounds. For example,

formation of dimers from organic compounds with 6 – 8 carbon atoms can lead to a reduction in

vapour pressure by two orders of magnitude.7 Most of these condensed phase reactions are acid-

catalyzed and are summarized in Figure 1.4. Numerous laboratory and ambient measurements

have observed products of these condensation reactions, further demonstrating their

importance.37–39

Figure 1.4. Non-oxidative condensed phase reactions that increases the molecular weight of

organic compounds in SOA (adapted from Hallquist et al.3).

Oxidation in the condensed phase, particularly in the aqueous-phase has been an emerging area

of research in the last decade.40

Aqueous-phase oxidation of water-soluble compounds leads to

the formation of low volatility products that are highly oxidized. For example, Turpin and

coworkers have shown that aqueous oxidation of dicarbonyls leads to the formation of highly

8

oxidized compounds such as carboxylic acids and hydroxyhydroperoxides.41,42

Taking the lab to

the field, on-site aqueous OH oxidation of ambient aerosol filter extracts (i.e., water-soluble

components) resulted in the formation of highly oxidized products.43

Finally, direct photochemical aging in the presence of UV radiation (i.e., photolysis) has been a

less explored area of aging processes in the condensed phase. The importance of photolysis as a

process that can affect the reactivity of organic aerosol was suggested by early studies of SOA

yield, where exposure to direct UV radiation was observed to decrease α-pinene, d-limonene and

isoprene SOA yield by as much as 20 – 40 %.44–46

In particular, studies by Seinfeld and co-

workers observed that the fraction of peroxides in isoprene SOA decreased with UV irradiation

time, demonstrating that certain species in organic aerosol are photolabile and their

photodegradation can be a significant loss process.16,46

Studies by Nizkorodov and co-workers

also confirmed that the photolysis of carbonyls contributes significantly to the photodegradation

of d-limonene SOA.47,48

With the increasing awareness of the importance of cloud processing on SOA composition,

recent studies have investigated the photolysis of SOA solutions dissolved in water. Similar to

earlier studies conducted under dry conditions, the photolysis of aqueous SOA solutions leads to

significant photodegradation of carbonyl compounds and the formation of small molecules in the

gas-phase (i.e., CO, CH4, acetone).49,50

1.2.2.4 Conceptual Framework

It is important to note that the aging processes discussed previously do not occur in isolation, but

that they are dynamic and intrinsically coupled to each other. For example, heterogeneous

oxidation can lead to the formation of smaller, more volatile compounds, which following

partitioning to the gas-phase, can be further oxidized by gas-phase OH, reducing their volatility.

These coupled processes in the gas-phase, at the interface, and in the condensed phase are

referred to as multiphase processing and it has been shown that the evolution of SOA occurs via

a set of fundamental processes: functionalization, fragmentation, and oligomerization.8,34

Functionalization includes all processes that result in the addition of an oxygenated functional

group to the SOA whereas fragmentation results in the cleavage of carbon-carbon bonds. These

two steps have different implications for SOA physico-chemical properties, for example, both

processes will increase the oxygen-to-carbon atom ratio (O:C) but functionalization leads to a

9

reduction while fragmentation leads to an increase in average aerosol volatility.51

Oligomerization, will result in a decrease in O:C ratio, but as mentioned previously, will lead to

a reduction in volatility.52

Laboratory studies of the heterogeneous OH oxidation of squalane (C30H62) elucidated the

relative contributions of functionalization and fragmentation processes during different stages of

the reaction.34

The results of this study are shown in Figure 1.5 where both organic aerosol mass

and O:C ratio evolve with OH exposure. At low OH exposures, increases in both O:C and

aerosol mass suggest that the functionalization pathway dominates. However, at higher OH

exposure (~ 1.5 × 1012

molec cm-3

s), a decrease in aerosol mass and less net change in O:C were

observed, suggesting that fragmentation-induced volatilization was the dominant process. These

results also suggest that for less oxidized organic compounds, functionalization is the dominant

pathway whereas fragmentation is the dominant pathway for more oxidized compounds (i.e.,

more aged SOA is more susceptible to volatilization).

Figure 1.5. The evolution of aerosol mass (left axis) and O:C ratio (right axis) with OH exposure

(data from Kroll et al.34

).


As noted previously, research into the importance of photolysis as an aging process for SOA is

still in its infancy. The photolysis studies discussed previously have demonstrated that photolysis

can lead to the fragmentation of organics under both dry/low relative humidity (RH) and

saturated conditions. Recent work in the Abbatt group illustrated that the rate of heterogeneous

10

oxidation of benzo[a]pyrene embedded within an SOA matrix is suppressed, where the extent of

this suppression was shown to depend on RH.53

These results suggest that RH affects particle

reactivity as the viscosity of the SOA material has been shown to decrease with increasing water

content.54,55

In this context, diffusion of reactants is more rapid in low viscosity material,

resulting in the observed higher reactivity. Recent work by Lignell et al., have also shown that

matrix effects, such as changes in viscosity, can have implications to the photolysis of an

embedded substrate.56

In fact, there is growing experimental evidence that the viscosity of SOA

material varies with environmental conditions such as RH.54

While the study by Zhou et al. has

shown that the reactivity of a substrate buried in SOA matrix is dependent on RH, it remains

unclear whether the reactivity of the SOA itself is dependent on the RH conditions. This question

will be addressed in Chapter 3.

From the discussion in the previous sections, it can be seen that aging processes generally lead to

more functionalized and/or smaller compounds that are more water-soluble (i.e., more

hygroscopic). The hygroscopicity of organics will be discussed in detail in Section 1.3. The

degree to which aging processes increase the hygroscopicity of SOA remains unclear. This

question will be addressed in Chapter 4.

1.3 Secondary Organic Aerosol Particles as Cloud Condensation

Nuclei

The average atmospheric lifetime of SOA is approximately one week, where the particles can be

removed from the atmosphere either via dry deposition onto Earth’s surfaces or wet deposition

that is initiated both below and in clouds.57

SOA can initiate cloud formation by acting as cloud

condensation nuclei (CCN) and can eventually be lost via precipitation. SOA below the cloud

can also be scavenged by the precipitation. The following sections include an overview of the

theory that describes the ability of organics to act as CCN at supersaturated water vapour

conditions (i.e., RH < 100 %) and the kinetics of droplet growth. It is important to note that

water uptake on particles can also occur under subsaturated conditions (i.e., RH < 100 %). While

the hygroscopic response of pure inorganic particles has been well studied, recent studies have

demonstrated that the water content of organic-containing particles also increases with increasing

RH conditions.58,59

11

1.3.1 CCN Activation

At supersaturated water vapour (S) conditions (i.e., RH > 100 %), aerosol particles can activate

as CCN and grow into cloud droplets. This process occurs via the condensation of water vapour

and is described as an equilibrium process using the Kohler equation:60

Equation 1.1

where s is the ratio of the partial pressure of water above the droplet to the equilibrium vapour

pressure of water, S is the supersaturation, aw is the water activity of the solution, σ is the surface

tension of the droplet, Mw is the molecular weight of water, ρw is the density of water, R is the

universal gas constant, T is temperature, and D is the diameter of the droplet. Typical

supersaturations in clouds in the atmosphere are 0.2 – 2.0 %.61

The water activity can also be

expressed using Raoult’s Law:

Equation 1.2

where nsolutes is the moles of soluble species in the initial particle, and D is the diameter of the

droplet. Combining equation 1.1 and 1.2, it can be seen that two competing factors affect the

condensation of water, which occurs when ambient water vapour pressure exceeds the

equilibrium water vapour pressure over the aerosol particle: the Raoult effect (lowering of

vapour pressure of water by a solute) and the Kelvin effect (increased vapour pressure over a

curved surface). A particle requires a minimum or critical supersaturation (Sc) in order to convert

into a CCN. When exposed to the Sc, the particle must grow to a critical diameter (Dpc) to

overcome the Kelvin effect, following activation as a CCN, the droplet will reach a mode of

spontaneous growth (up to approximately 10 µm). As such, the Sc and Dpc are controlled by the

particle size and its chemical affinity for water (commonly referred to as “hygroscopicity”).

To better understand the CCN activity of organic aerosol, the earliest CCN-studies examined the

hygroscopicity of model organic compounds.62–64

These studies have demonstrated that highly

water-soluble compounds do indeed act as good CCN by increasing the concentration of solutes

(e.g., Raoult’s effect) and that the CCN activity of these compounds can be predicted using the

Kohler equation if their molecular properties are known. However, given that ambient organic

12

aerosols contains a myriad of compounds, most of which are still unidentified and unquantified,

their CCN activity cannot be predicted based on first principles. As a result, a modified Kohler

theory was developed by Petters and Kreidenweis65

where water activity is represented as:

Equation 1.3

where D is the droplet wet diameter, Di is the initial particle diameter, and κ is the hygroscopicity

parameter of the particle. This extension of the Kohler theory uses one parameter (κ) to describe

average molecular properties (e.g., molecular weight and solubility) that control the particle’s

water uptake characteristics. Using single component organic aerosols composed of model

compounds, κ values were empirically determined by numerous laboratory studies. For example,

κ of a hydrophobic compound, bis(2-ethylhexyl) sebacate, is 0,66

and for a hydrophilic

compound, malonic acid, the κ value is 0.23.64

In comparison, κ values of typical inorganic

particles in the atmosphere are 0.61 for ammonium sulfate and 0.90 for sulfuric acid.67

Given that ambient particles are typically composed of both inorganic and organic components,

the overall κ of the particle (κtotal) can be represented as a volume-fraction weighted average κ of

individual components:65

Equation 1.4

where εinorg, εorg are the volume fractions of the inorganic and organic components, and κinorg, κorg

are the average κ values of the individual components for inorganic and organic compounds.

This approach is useful to determine the average κorg of ambient organic aerosols since the

organic component contains many compounds that have yet to be completely speciated.

Additionally, from equation 1.4, it can be seem that the hygroscopicity of organics can contribute

significantly to the total aerosol hygroscopicity if they constitute a large fraction of the total

particle material and/or if the organics are highly hygroscopic (i.e., the product of the organic

terms is comparable to the product of the inorganic terms).

While the above discussion considers the contribution of organics to particle hygroscopicity

through solubility, organic compounds can also affect CCN activity by altering surface tension,

and thus the water activity. Surface active organics, which have both hydrophilic and

13

hydrophobic moieties, will partition to the gas-particle interface, resulting in a reduction in the

aerosol surface tension (i.e., reduction in the Kelvin term) and a more CCN-active particle.

Laboratory studies show that bulk solutions of organics do indeed have lower surface tension

compared to water.68–70

These findings have also been corroborated by laboratory studies of

organic aerosol composed of pure slightly soluble compounds such as large organic acids.71

Reactions of organics in the condensed phase have been shown to enhance the CCN activity of

inorganic particles.72

However, the same organic acids that were shown to increase CCN activity,

when mixed with hygroscopic inorganic particles, actually decreased the CCN activity.73,74

It is

thought that mixtures of surface active organics and inorganic compounds result in a decrease in

CCN activity due to a decrease in soluble ions when the surfactant partitions to the surface. At

this point, uncertainties remain for the overall effect surface active organics have on CCN

activity.


Although it is a simplification, unless one assumes that one single κ value is accurate for all

aerosol organic matter, the use of κ-Kohler theory still requires the quantification and some

chemical speciation of organic aerosol, which despite recent analytical advances, remains

challenging. As discussed earlier, organic aerosol undergoes chemical transformations during its

atmospheric lifetime, which can affect its CCN activity, thereby further complicating the

prediction of the CCN activity of ambient particles.8 Motivated by the need to simplify the

experimental approach to studying organic hygroscopicity, laboratory and ambient studies have

proposed that organic aerosol hygroscopicity can be characterized based on the organic

component’s degree of oxygenation, as quantified by the average O:C ratio.8,75,76

In this context,

oxidation of organic aerosols in the atmosphere, which typically gives rise to more

functionalized/smaller solute molecules (as discussed in Section 1.2.2), is likely to render the

aerosol more hygroscopic. This approach not only simplifies predictions of CCN activity of

ambient particles, but it also accounts for chemical reactivity.

Previous work in the Abbatt group has postulated that there is a simple linear relationship

between the hygroscopicity of organic aerosol and its O:C ratio.75

However, the extent to which

aging processes actually result in an increase in the O:C ratio and the hygroscopicity of ambient

organic aerosol remain unclear. The goal of Chapter 4 is to test this hypothesis.

14

1.3.2 Droplet Growth Kinetics

While the thermodynamic properties of organic aerosol and their relationship to droplet

activation are becoming well understood, much less is known regarding how organics affect the

rate of water vapor transport during CCN activation (i.e., kinetic effects). Changes to the droplet

growth rates will change the amount of water in the droplet, thus affecting cloud droplet size

distributions. In fact, the assumption that the activation of aerosols to CCN can be modelled

purely as an equilibrium process has been shown to be invalid in some conditions.77–79

While

highly uncertain, it appears that organics can result in kinetic limitations due to slow solute

dissolution associated with highly viscous organic particles54,80,81

or that organics can form a film

at the surface, affecting the mass-transfer rate of water vapor.82,83


Slow droplet growth rates have been observed in the laboratory and ambient measurements, yet

the extent to which growth rate suppression occurs in the atmosphere remains unclear as only a

limited number of studies have investigated this process, with contrasting results. In this context,

suppression in droplet growth kinetics is assumed to be responsible if the addition of an organic

compound to an inorganic particle gives rise to a smaller droplet size compared to that of the

bare inorganic particles, which have known rapid growth kinetics. Laboratory studies have

demonstrated that the following organic particles exhibited slow droplet growth kinetics:

carboxylic acid particles,84

highly viscous sucrose/sodium chloride particles,85

organic films

consisting of hydrocarbons with carbon chain length larger than 12,86

and secondary organic

aerosol from β-caryophylene ozonolysis.80

In addition, ambient observations of kinetic

limitations have been observed from various sources, including bovine emissions,87

and

continental particles containing anthropogenic organic components.88

However, using

computational fluid dynamics models of the conditions inside a CCN counter, Raatikainen et al.

analyzed 10 ambient data sets and represented growth kinetics via an effective water uptake mass

accommodation coefficient (α), a parameter describing the probability that a water molecule

striking the particle surface will be taken up by the growing droplet. This study constrained α

between the values of 0.1 and 1, indicating that rapid growth kinetics are globally prevalent.89

These contrasting results from laboratory and field data pose a challenge to determining whether

the kinetic limitation of droplet growth is an important process, and to what degree it actually

occurs in the atmosphere. While previous work has demonstrated that certain compounds and

15

particle sources can give rise to suppression in growth kinetics, one of the goals of Chapter 5 is

to investigate the droplet growth kinetics of various organic compounds with different

functionalities.

1.4 Experimental Methods for SOA Studies

In order to investigate SOA in the laboratory, a suite of experimental techniques is required.

Generally, a reactor is required to generate or age the SOA and subsequent measurement

techniques are required to investigate the physico-chemical properties of the SOA. To investigate

ambient SOA, other technical requirements must be met, generally those regarding the

portability of instrumentation and good time resolution. While there are many existing

approaches and techniques, the following sections focus the discussion on the techniques and

instrumentation used for this thesis.

1.4.1 Approaches for SOA Formation and Aging

Multiple approaches have been used to study the formation and aging of SOA in the laboratory.

Historically, environmental chambers were used to simulate atmospheric conditions and these

seminal experiments have led to much of our current understanding of SOA formation. The use

of flow tube reactors has emerged more recently. Note that other techniques have also been

employed to study SOA formation and aging, for example, reactions on SOA collected on filters

or dissolved in bulk aqueous solutions.48

The following sections provide a brief overview of

environmental chamber and flow tube techniques as they were used for the work in this thesis.

1.4.1.1 Environmental Chambers

Environmental chambers are generally bags made out of Telfon film with large volumes (0.01 –

250 m3) in which the reactions take place.

90 The large volume of these chambers result in a large

volume-to-surface area ratio to minimize wall effects, which is important for studying SOA

formation as it minimizes the interaction of condensable organic vapours with walls. However,

recent work by Zhang et al. demonstrated that SOA formation can still be suppressed due to the

loss of SOA-forming vapours onto the walls.91

To investigate photochemical reactions, broad-

band artificial light sources are placed outside the chambers. While these light sources are used

to simulate actinic radiation, the relative intensities and wavelengths may not be comparable.

There are also outdoor environmental chambers that utilize actual solar radiation as their light

16

source.92,93

Environmental chambers are typically operated in batch or continuous modes, which

can theoretically provide aerosol residence times from hours to days. However, aerosol residence

times are generally limited by the wall loss of particles, resulting in stimulations of ambient

reaction times up to a few (2 - 3) days at most. Environmental chambers were used in the work

described in Chapters 3 and 5.

1.4.1.2 Flow Tubes

The use of flow tube reactors to investigate aerosol chemistry emerged in the 1980’s as the

importance of heterogeneous reactions on the surfaces of polar stratospheric clouds was

recognized.94

The use of flow tubes to study the chemistry of organic aerosol only began in the

last decade. The volumes of these reactors (generally < 0.001 – 0.02 m3) are much smaller

compared to environmental chambers, which result in short residence times (minutes) and

potentially significant wall effects. Indeed, an evaluation of two flow tube reactors with different

volume-to-surface area ratios demonstrated that differences in wall-interactions resulted in

different measured SOA yields.94

However, compared to environmental chambers, the total wall

surface areas are much smaller, and therefore it is possible to pacify the walls through

conditioning. Sheath flows have also been used in order to minimize wall interactions.95

Given

the small volumes, high concentrations of gas and particle species are typically used to drive up

reaction rates, resulting in simulation of atmospheric processing times of up to two weeks. While

this property renders flow tubes useful for investigating SOA aging processes that occur

following its formation, the chemical reactions occur on an accelerated time scale compared to

the rate at which they occur in the atmosphere. Flow tube reactors have been constructed out of

Pyrex and stainless steel, with the former subject to electrostatic wall loss of charged particles.

Similar to environmental chambers, photochemical reactions have been investigated using lamps

placed inside or outside the flow tubes, however, the light sources are not necessarily comparable

to actinic radiation (e.g., UV light from mercury lamps). From a practical standpoint, flow tubes

enable rapid switching between experimental conditions (e.g., seed particle type, RH, light

source) compared to environmental chambers due to differences in volumes. This is particularly

advantageous for the study described in Chapter 2. Also, being compact in size compared to

environmental chambers, flow tubes can be transported and used in the field to investigate aging

processes on ambient aerosols. These properties of flow tube reactors enabled the study

described in Chapter 4.

17

1.4.2 Chemical Analysis of SOA by Mass Spectrometry

The analytical techniques used for the chemical characterization of SOA span a wide range, from

optical spectroscopy to mass spectrometry. Nevertheless, complete characterization of SOA

chemical composition is still impossible.3 This section will focus on mass spectrometry

techniques used to investigate SOA processes in the Abbatt group. Note that other mass

spectrometry ionization techniques have been used to characterize SOA, such as electrospray

ionization (ESI), tunable vacuum ultraviolet (VUV) ionization, etc.96

The commercially developed Aerosol Mass Spectrometer (AMS) by Aerodyne, Inc. has become

central to many SOA studies over the past decade.97

The AMS generates gas-phase species by

thermal desorption and ionization by electron impact ionization. This quantitative technique

provides size-resolved mass spectra of inorganic and organic compounds with fast time

resolution (~ one minute). The instrument consists of three main regions: i) particle inlet and

sizing, ii) vaporization and ionization, and iii) time-of-flight mass spectrometer. After passing

through the inlet, the particle beam is focused by a series of aerodynamic lenses with 100 %

transmission for particles with diameters between 70 – 500 nm. The resulting focused particle

beam passes through a chopper which sends bursts of particles through the sizing chamber, from

which the particle flight time can be used to determine the particle aerodynamic vacuum

diameter. The non-refractory components of the particles (e.g., sulfate, nitrate, organics) are

vapourized upon impaction on a heated surface (~600°C) and are ionized by 70 eV electrons.

While earlier versions of the AMS used a quadrupole mass spectrometer, new versions employ

time-of-flight (ToF) mass spectrometers, of which a compact version (c-ToF) as well as a high

resolution version (HR-ToF) are available. While quantitative, the use of EI results in extensive

fragmentation of molecular compounds, where the identity of the parent compound is lost.

However, classes of compounds (e.g., hydrocarbons vs. oxidized carbons) can be identified from

the mass spectra, and coupled with factor analysis, the mass spectra can be deconvoluted into

components with different chemical or source properties. The use of the AMS is central to this

thesis.

More recently, the use of chemical ionization mass spectrometry (CIMS) to investigate SOA has

been increasing. The chemical information provided by the CIMS is generally complementary to

that provided by the AMS.98

Similar to the AMS, aerosol-CIMS employs thermal desorption to

18

volatilize the organic constituents of the particle, but only temperatures up to around 200°C have

been used (i.e., inorganics will not be vapourized).99

Along with the relatively low temperatures

used for vapourization, the soft ionization technique used by the CIMS means the analyte ions

will not fragment significantly. Various reagent ions have been used (i.e., (H2O)nH+, I

-,

CH3COO-) and they provide selective detection of various classes of organic compounds.

100

Recently, an inlet has been developed (FIGAERO, Filter Inlet for Gas and Aerosols) that enables

semi-continuous detection of both particle-phase and gas-phase compounds by the CIMS.101

Here, particles are collected on a particle filter while gas-phase measurements are conducted.

When particle-collection is completed, the filter is heated gradually as the organics volatilize as a

function of their vapour pressure, allowing for particle molecular composition measurements.

While not employed for any of the studies in this thesis, this upcoming technique is a powerful

tool for future SOA studies.

1.4.3 Approaches for CCN Measurements

In order to investigate the ability of particles to act as CCN, all CCN counters (CCNc) must be

able to generate a known supersaturation of water vapour. While the approach to generating a

supersaturation is not identical for all CCNc, because the CCN work in this thesis is based on the

CCNc commercially developed by Droplet Measurement Technology, the following will focus

on the operational principles of that instrument.102

Particles, which are continuously sampled by

the CCNc, enter a CCN column, a region where supersaturation is generated and the particles

activate into droplets. In the column, which is coated by liquid water, a positive temperature

gradient is generated in the streamwise direction and a supersaturation results since the rate of

mass transfer is greater than heat transfer (i.e., because water molecules diffuse faster than do

nitrogen and oxygen molecules). A range of supersaturations can be achieved (0.07 – 2.0 %) by

changing the temperature gradient down the column. The supersaturated water vapour condenses

onto particles and the newly formed droplets. The droplets are counted and sized using an optical

particle counter, which can measure droplets from 0.75 – 10 µm.

The following includes a brief overview of the two different approaches commonly used to

investigate the CCN activity of organic aerosol. The main difference between the two approaches

is that one uses polydisperse aerosols (hence measuring the average hygroscopicity of the aerosol

population) while the other measures the hygroscopicity of monodisperse (size-selected)

19

particles. For the first approach, polydisperse particles are sampled by a CCNc at a fixed

supersaturation. Paired with particle size distribution measurements, the minimum particle

diameter that corresponds to the fixed supersaturation of the CCNc is considered to be the

critical diameter (Dpc). This approach, called CCN-closure, compares the measured CCN

concentration to that predicted using Kohler theory. Closure is achieved if the measurements

agree with predictions. However, the presence of multiply charged particles has been

demonstrated by Petters et al. to skew the results from this approach.103

Additionally, another

challenge of this type of CCN study is that the particle mixing state can play an important role in

activation. For example, the composition of the particles with the critical diameter could be

different from those measured or assumed.104

For the second approach, size-selected particles are

sampled both by the CCNc operated at a range of supersaturations, and by a particle counter.

Here, the supersaturation at which 50 % of the counted particles activate as CCN is considered

the critical supersaturation (Sc). Ambient studies have shown that monodisperse CCN

measurements can be used to discern the mixing state of hygroscopic aerosols.105

For the CCN

work in this thesis, the second approach was used.

1.5 Summary of Research Objectives

Particle-phase water is an abundant atmospheric constituent that is estimated to exceed the total

dry particle mass by 2 – 3 times globally.106

Despite its abundance, the importance of water on

the formation, aging and fate of SOA remains poorly characterized and the goal of this thesis is

to address the gaps in knowledge shown in Figure 1.6.

20

Figure 1.6. An illustrated representation of the formation, transformation and fate of SOA in the

atmosphere. Research questions addressed in specific chapters are shown.

In Chapter 2, the first question is examined by quantifying the amount of isoprene SOA formed

on different sulfate seed particles with different acidity and water content. The importance of

photolysis as an aging process of SOA and the dependence of the kinetics of this process on

relative humidity are examined in Chapter 3. The modification of CCN activity of ambient SOA

by OH oxidative aging was observed and a simple relationship of organic CCN activity and

degree of oxygenation was demonstrated in Chapter 4. The last question is addressed in Chapter

5, where the droplet growth kinetics of neutral and acidic sulfate particles with three types of

organics (SOA, carbonyls, and diesel engine exhaust) were determined. The results obtained in

these studies provide insight into the role of water on the formation, transformation and fate of

SOA in the atmosphere. A summary of the main results along with a discussion of future

research directions conclude this thesis in Chapter 6.

1.6 References

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Climate Change; John Wiley & Sons, Inc.: New York, 1998.

21

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30

Chapter 2

2 Impacts of Sulfate Seed Acidity and Water Content on

Isoprene Secondary Organic Aerosol Formation

Impacts of Sulfate Seed Acidity and Water Content on

Isoprene Secondary Organic Aerosol Formation

J. P. S. Wong, A. K. Y. Lee, J. P. D. Abbatt

In preparation for submission to Environmental Science and Technology

31

2.1 Abstract

The effects of particle-phase water and acidity of pre-existing sulfate seed particles on the

formation of isoprene secondary organic aerosol (SOA) was investigated. SOA was generated

from the photooxidation of isoprene in a room temperature flow tube reactor at 70 % relative

humidity (RH) in the presence of 3 different sulfate seeds (effloresced and deliquesced

ammonium sulfate, ammonium bisulfate). Particle-phase water had the largest effect on the

amount of SOA formed, with 60 % more SOA formation occurring with deliquesced ammonium

sulfate seeds as compared to effloresced ones. The additional organic material was highly

oxidized. While the amount of SOA formed did not exhibit a dependence on the range of seed

particle acidity examined, the amount of high molecular weight material increases with acidity,

indicating the role of acidity in governing organic composition. While the uptake of organics was

partially reversible upon drying, the results nevertheless indicate that the presence of particle-

phase water will enhance the amount of organic aerosol material formed in the atmosphere and

that the RH cycling of sulfate particles mediates the extent of isoprene SOA formation.

2.2 Introduction

Isoprene is the most abundant non-methane hydrocarbon emitted to the atmosphere, and its

oxidation is estimated to be the largest source of secondary organic aerosol (SOA).1,2

Despite the

importance of SOA on climate and human health, there is a lack of understanding of its

atmospheric formation and evolution pathways.3 Traditionally, SOA formation is thought to

occur via the partitioning of semi-volatile organics from gas-phase photochemistry. Current

atmospheric models, based on this volatility-driven partitioning theory, underestimate observed

SOA mass, suggesting unidentified formation mechanisms.4

Previous studies have shown that water-soluble isoprene photooxidation products, even if highly

volatile, can readily partition to particle-phase water where subsequent condensed phase

reactions can lead to the formation of SOA.5 In particular, the formation of epoxydiols (IEPOX)

from isoprene photooxidation under low NOx conditions and their reactive uptake to the particle-

phase have been proposed as key processes for isoprene SOA formation.6,7

Studies have shown

that the reactive uptake of IEPOX is enhanced under acidic seed particle conditions, leading to

substantial SOA formation via acid-catalyzed nucleophilic addition to the epoxide ring.8–10

However, ambient studies have observed weak correlations between particle acidity and IEPOX-

32

derived SOA,11–14

suggesting that outstanding factors are likely to modulate isoprene SOA

formation. Given that acidic seed particles are hygroscopic, even at the low RH conditions used

by most isoprene SOA studies, the relative importance of seed particle acidity and particle-phase

water on the formation of isoprene SOA remains uncertain.

Recent laboratory studies have focused on elucidating the effects of particle acidity and particle-

phase water on the reactive uptake of IEPOX.15,16

Collectively, these studies have shown that

particle acidity is the dominant factor affecting the reactive uptake rate of IEPOX for particles

with pH values < 3 while for less acidic particles (pH > 3), the availability of efficient

nucleophiles (e.g., NH3) is key for the reactive uptake process to occur. Recently, Liu et al.,

quantified that the uptake of IEPOX onto acidic sulfate seed particles under low RH conditions

accounts for 50 % of the SOA mass formed.17

Although the reactive uptake process of IEPOX

and its contribution to the SOA burden are becoming better understood, it is important to note

that other isoprene oxidation products are water-soluble (e.g., carbonyls), and that the effects of

acidity and particle-phase water on such oxidation products remain unknown.

In this chapter, we present laboratory measurements of SOA formation from the photooxidation

of isoprene under high RH (70 %) conditions onto various seed particles in a flow tube reactor:

effloresced and deliquesced ammonium sulfate, as well as acidified sulfate particles. In

particular, by operating at constant relative humidity, experimental issues related to varying gas-

phase chemistry and/or wall loss rates are minimized. And by taking advantage of the hysteresis

in the phase of ammonium sulfate aerosol particles as a function of increasing and decreasing

relative humidity, we can operate with seeds that have the same chemical composition but

different phases (i.e., solid vs. aqueous). Overall, this study aims to i) isolate the roles of

particle-phase water and acidity on the nature of isoprene SOA formation and ii) investigate the

reversibility of the uptake processes leading to SOA formation that occurs upon changes in

relative humidity.

2.3 Experimental Section

A schematic of the experimental setup used to investigate seed particle effects on isoprene SOA

formation is shown in Figure 2.1. Key components of the setup are described in detail below. We

emphasize that the overall experimental requirements to achieve our science goals are i) to have

33

control over the phase of the ammonium sulfate seeds, i.e., whether the particles are deliquesced

or effloresced, and ii) for the oxidation chemistry that forms the SOA to be conducted at the

same relative humidity independent of seed phase or composition.

Figure 2.1. a) Schematic of the experimental setup. For each corresponding region of the setup,

the physical state of the particles is illustrated in b) for sulfate seed particles, effloresced AS is

represented by the red hexagon and deliquesced AS is represented as the small red circles.

Abbreviations are described in the main text.

2.3.1 Seed Particle Generation and Phase Characterization

Sulfate seed particles were generated by atomizing aqueous sulfate solutions with a constant

output atomizer (TSI 3076) with N2 as carrier gas. For ammonium sulfate seed particles, a dilute

solution (0.12 mM) of ammonium sulfate (AS, Sigma-Aldrich) was used. For acidified sulfate

seed particles, the above AS atomizer solution was acidified using sulfuric acid (Fisher

Scientific) resulting in a 1:1 ammonium-to-sulfate molar ratio. After atomization, 300 sccm of

the atomizer output was dried using a silica gel diffusion dryer, which reduced the RH to < 30 %,

where the AS effloresced. The polydisperse particles were subsequently charge neutralized and

size selected at a mobility diameter of 100 nm (for AS) or 120 nm (for acidified AS) by a

differential mobility analyzer (DMA, TSI-3081). 120 nm was chosen for the size of the acidified

AS particles to match the size of the deliquesced AS particles (see below).

34

Following size-selection, the monodisperse particle flow, controlled by a 3-way valve, was

alternatively mixed with a humidified N2/O2 flow (1550 sccm) in the conditioning tube (RH 82

%) or in a bypass tube (RH < 30 %). AS particles mixed with the humidified flow at this point

deliquesce in the conditioning tube (residence time approximately 1 min), otherwise they remain

effloresced in the bypass tube.

The resulting deliquesced AS particle flow was combined with a dry flow containing ozone (150

sccm, generated by passing O2 through a mercury lamp) in a mixing volume. For the effloresced

AS particle flow, the same dry ozone flow was first mixed with the 1550 sccm N2/O2 humidified

flow (see above) which was then added to the particle flow in the mixing volume. It is important

to note that i) the RH of the latter particle flow never exceeds 80 % and so the AS particles

remain fully effloresced, and that ii) within the mixing flow, the relative humidities are the same

for both configurations, but the phase of the AS particles is not.

Control experiments were conducted to ensure that the AS particles deliquesced in the

conditioning tube. The particle size distributions were measured from the outflow of the mixing

volume using a scanning mobility particle sizer (SMPS, TSI-3075) whose sheath flow was

conditioned to the same RH conditions as the mixing volume for several hours. For effloresced

AS particles size-selected at a mobility diameter of 100 nm, exposure to RH 82 % in the

conditioning tube resulted in an increase of particle diameter to 120 nm, suggesting the AS

particles have deliquesced. For particles that flowed through the bypass tube, no change in

particle mean diameter was observed, indicating the particles remain effloresced. The observed

increase in particle diameter due to deliquescence is consistent with that expected from the

hygroscopic behaviour of AS.18

For the remainder of this paper, deliquesced AS will be referred

to as “wet AS”, effloresced AS as “dry AS,” and acidic sulfate as “wet ABS” for simplicity.

These sulfate seed particles were chosen as they are ubiquitous in the atmosphere and are

commonly used seed particles in previous isoprene SOA/IEPOX studies.

2.3.2 SOA Formation

SOA was formed from the oxidation of isoprene with OH radicals in the Toronto Photo-

Oxidation Tube (TPOT), which was previously described in detail elsewhere.19,20

Briefly, TPOT

is a flow tube reactor that produces high OH exposures via the photolysis of ozone (λ = 254 nm)

35

in the presence of water vapor. Since the use of this UV lamp increased the temperature inside

the reactor by 2 – 3 degrees above room temperature, the temperature inside the TPOT was

continuously measured using a thermocouple. The RH and temperature of the reactor’s outflow

(at room temperature) were continuously measured as well (Vaisala). These measurements were

used to calculate the dew point, and subsequently, the RH inside the TPOT. All the reported RH

values in this paper correspond to the corrected RH inside the TPOT reactor. Specifically, the

TPOT was operated using 8.5 × 1012

molecules cm-3

(350 ppb) of ozone, at RH 70 %, and for a

residence time of 100 seconds. These operating conditions resulted in an OH exposure of 3.4 ×

1011

molecules cm-3

s, which is equivalent to 2 - 3 days of OH exposure with ambient

concentrations of 1.5 × 106 molecule cm

-3. The OH concentration was determined by monitoring

the decay of SO2 (2.4 × 1012

molecules cm-3

) by the reaction of OH using a SO2 monitor (Thermo

40c), as described previously.19,20

The TPOT was cleaned before and after each experiment by

exposure to OH for several hours, until the SMPS and AMS measurements (described below)

reach background levels.

To initiate SOA formation, a small flow (20 sccm) of an isoprene mixture (17.5 ppm in nitrogen,

Scott Specialty Gas) was added to the particle flow in the mixing volume upstream of the TPOT,

resulting in a final isoprene concentration of 4.3 × 1012

molecules cm-3

(175 ppb). The system

was conditioned for 30 minutes, until stable SMPS and AMS measurements were observed.

From the SMPS measurements, no homogeneous nucleation (i.e., formation of small particles 10

nm and larger) was observed following the reaction of isoprene and OH. Control experiments

were conducted in which isoprene was oxidized solely by ozone in the TPOT and no organic

aerosol material formation was observed, indicating that the OH oxidation of isoprene generated

the SOA during typical experiments.

2.3.3 Particle Measurements

Following organic condensation onto seed particles, the resulting SOA-sulfate particles were

alternatively dried by passing through a silica gel diffusion dryer or not dried by passing through

a bypass tube. These conditions were investigated for two reasons. First, since it is hypothesized

that the presence of particle-phase water would result in the uptake of soluble organic gases,

water removal by the dryer explores the reversibility of the uptake of soluble organic gases

leading to SOA formation. Second, these conditions allowed for the characterization of the types

36

of organic materials that are associated with the particle-phase water. We note that all sulfate

particles (wet and dry AS, as well as wet ABS) experienced a 10 % particle loss in the diffusion

dryer and that this effect has been accounted for in the results presented below.

After passing through the dryer or dryer bypass, the particles were sampled online by a time-of-

flight aerosol mass spectrometer (ToF-AMS, Aerodyne, Inc.) for particle mass and composition

measurements. Most experiments were conducted with a lower mass resolution AMS (cToF-

AMS) whereas a high mass resolution AMS (HR-ToF-AMS), which can resolve different ions at

the same nominal m/z, was used on occasion to confirm the identity of the dominant organic ion

fragments. The operating and analysis procedure of the AMS has been reviewed previously in

detail.21

Briefly, the AMS provides size-resolved mass and chemical composition of the non-

refractory components of particles, including organics, sulfate, nitrate, and ammonium. The

ionization efficiency of the AMS (IENO3 and RIENH4) was calibrated routinely using size-selected

ammonium nitrate particles. Signals from major gases (N2, O2, H2O, Ar, and CO2) were

accounted for by obtaining multiple filter measurements (i.e., zero particle signal) throughout

each experiment.

To quantify the amount of SOA material condensed onto the sulfate seed particles, the organic

mass loading measurements from the AMS were used. For all experiments, the contribution from

background organics have been subtracted from those arising from SOA formation. Given that

the collection efficiency (CE, related to the particle bounce characteristics on the AMS

vaporizer) of wet AS is higher than that of dry AS,22

the sulfate mass was used to normalize the

organic signal for different aerosol types under the assumption of no sulfate reactivity. However,

for the comparison of SOA formation onto wet AS and wet ABS seed particles, given that they

have similar CE in the AMS, SOA formation will be reported as an absolute yield (using CE of

1.0), in order to facilitate comparison to previous isoprene SOA formation studies.5

In additional to AMS measurements, particle size distributions and total surface area (Sa)

concentrations were measured by a SMPS (TSI-3075). Efforts were made to conduct the

experiments under the same operating conditions as much as possible, and so typical Sa ranged

only from 1.0 – 1.4 × 109 nm

2 cm

-3, with the small variation arising from experimental variability

in particle generation efficiency and variations in Sa due to the presence of water for wet seed

37

particles. One goal for working at constant aerosol surface areas is that recent work has shown

that SOA formation is sensitive to seed particle Sa as the condensation of oxidation products is a

competitive process of all available surface area (i.e., seed particles, reactor’s walls).23

To

evaluate the effect of Sa on SOA formation in the TPOT, control experiments with varying Sa

were conducted, and the measured isoprene SOA yields were compared. Similar SOA yields

(within the experimental uncertainties) were observed when Sa was increased by 200 %,

indicating that the range of Sa used in this study does not affect SOA formation in the TPOT.

2.4 Results and Discussion

2.4.1 Ammonium Sulfate Particles

The formation of SOA onto dry and wet AS was compared in order to gain insight into the role

of particle-phase water. A major result is that a greater amount of isoprene SOA was observed

for wet AS compared to dry AS by a factor of 1.6 ± 0.2. This value is the average of 4

measurements and the uncertainty reflects one standard deviation in those results. We note that

the SOA mass yield for dry AS [(5.3 ± 3.6) × 10-3

] is consistent with the SOA yield observe by

Kroll et al. from isoprene photooxidation under NOx free conditions (0.001 – 0.004).24

Analysis

of the corresponding AMS organic mass spectra (normalized by sulfate to reflect absolute

signals) indicates that carbonyl compounds contribute significantly to the SOA formed, given the

high intensity of m/z 29 (Figure 2.2). High resolution fitting of the mass spectra confirm the peak

at m/z 29 as arising from the CHO+ fragment, which has previously been used as a tracer for

organic compounds with carbonyl functional groups.25

Figure 2.2. Mass spectra (normalized by total sulfate mass) for the organics on wet AS (blue)

38

and dry AS (red) seed particles. The molecular formulae of the dominant organic fragment for

significant ions are identified in the main text.

In addition to the absolute changes in SOA mass, to better understand the impact of particle-

phase water on SOA composition, the spectral differences between the organic-normalized

spectra of SOA on dry and wet AS are shown on Figure 2.3. Note that this spectrum represents

the fractional differences in organic composition. The organics associated with particle-phase

water (i.e., wet AS) were more oxidized, where the strong positive differences in m/z 28 (CO+)

and m/z 44 (CO2+) indicate that organic acids are likely associated with the presence of particle-

phase water.26

While the specific organic acids cannot be identified using an AMS, we speculate

that the uptake of isoprene oxidation products such as methylglyceric acid are likely to be

enhanced due to the presence of particle-phase water, due to their high water solubility.5 For dry

AS, the strong negative difference for m/z 29 (CHO+) indicates enhancement in carbonyl

compounds.

Figure 2.3. Mass spectral difference (Wet AS – Dry AS) of the aerosol organics normalized by

total organic mass. Positive values (blue) indicate the mass fragments that are enhanced for

organics on wet AS seed, and the negative values (red) indicate enhancement for dry AS seed.

The molecular formulae of the dominant organic fragments for significant ions are labeled.

It is important to note that the enhancement in organic acids due to particle-phase water could

also have arisen from aqueous photochemical reactions of carbonyls subsequent to uptake.27,28

For this study, the use of the UV lamp in the TPOT may have initiated these photochemical

reactions. We note that the organic fragment m/z 82 (C5H6O+) has been suggested as a tracer for

IEPOX-derived organics if it contributed to more than 4 ‰ of the total organic mass (f82) as the

contributions from other types of SOA are normally less than that amount.29

For the current

39

study, the f82 for SOA material on both dry and wet AS were less than 4‰, indicating that

IEPOX did not contribute to the SOA material in the current study. Additionally, given that the

lifetime of gas-phase IEPOX with respect to OH oxidation (kOH ~ 1 × 10-11

cm3 molec

-1 s

-1)

in the

TPOT is estimated to be less than a second,30,31

its reactive uptake is unlikely to be the dominant

fate in the TPOT.

The reversibility of the uptake processes leading to additional SOA ultimately determines their

importance for SOA formation in the atmosphere. This was determined by passing the SOA-

sulfate particles through a dryer, qualitatively simulating the evaporation of water in the

atmosphere due to RH cycling. For wet AS, the drying process led to a loss of 20 ± 15 % of SOA

organics, whereas a loss of 35 ± 15 % was observed for dry AS, indicating that the uptake of

SOA materials is partially reversible for both seed particles. The composition of the organics that

was removed can be observed from the spectral differences in the organic spectra (Figures 2.4a-

b). The drying process removed similar types of organics for both dry and wet AS, mainly

comprised of organic acids. It is important to note that the drying process can also accelerate the

rate of condensed phase reactions due to increasing concentrations of solutes during water

evaporation.32,33

Given that a net loss of organic mass was observed upon drying, we believe the

loss of organic acids was largely due to the reversibility of their uptake.

Considering the experiments described above, we conclude that the uptake of SOA material was

enhanced due to the presence of particle-phase water, and that this enhanced uptake of organic

acids is somewhat reversible. Accounting for the partial reversibility of SOA uptake on both wet

and dry AS, the net increase in the amount of SOA that can be taken up by wet AS was 1.7 times

the dry AS value.

40

Figure 2.4. Mass spectral difference of the organics (normalized by total organic mass) for a)

wet AS and b) dry AS to illustrate the reversibility of uptake due to water evaporation. Positive

values (blue) indicate the mass fragments that are enhanced for organics on dried sulfate-SOA

particles, and the negative values (red) indicate enhancement for not-dried sulfate-SOA particles.

The molecular formulae of the dominant organic fragment for significant ions are identified in

the main text.

2.4.2 Acidic Sulfate Particles

While the previous set of experiments demonstrates that particle-phase water affects the

formation of isoprene SOA, the role of acidity also needs to be investigated in order to

understand the relative importance of the two factors on aerosol yield. At 70 % RH, wet ABS

has similar particle water content compared to wet AS, while being more acidic.34

The pH of the

particles used in the current study was estimated using AIM-II from AMS measured ammonium

and sulfate mass, where the pH of wet ABS is estimated to be -0.31 (ammonium-to-sulfate molar

ratio: 0.94) and 0.59 for wet AS (ammonium-to-sulfate molar ratio: 1.72). Note that the

ammonium-to-sulfate molar ratio for AS deviates from the expected value of 2, indicating loss of

ammonium via the partitioning of ammonia to the gas-phase, likely due to flow dilution.

b)

a)

41

The observed isoprene SOA yield for wet AS is (9.3 ± 6.3) × 10-3

and (4.3 ± 0.3) × 10-3

for wet

ABS. The values are the average of 4 measurements for AS and 3 measurements for ABS - the

uncertainties reflect one standard deviation in those results. Thus, similar isoprene SOA yields

were observed for both sulfate seeds, indicating changes in particle acidity did not affect SOA

formation within experimental uncertainty. We note that the IEPOX tracer at m/z 82 was not

observed for wet ABS (i.e., < 4 ‰ of the total organic mass), further suggesting that the uptake

of IEPOX did not contribute to the SOA formed in the current study. The spectral differences of

the SOA on the different seed particles shown in Figure 2.5 indicate that carbonyls contribute a

higher fraction of the total organics in wet AS while a higher fraction of organic mass at m/z >

100 was observed for wet ABS (Figure 2.5 insert). Given the hard ionization technique used in

the AMS, organic fragments > m/z 100 arise from the fragmentation of even larger molecular

weight compounds. We speculate that the high molecular weight species were formed via acid-

catalyzed condensed phase reactions. These observations are consistent with previous studies

where high molecular weight species (i.e., esters and organosulfates) have been previously

identified in isoprene SOA on acidic seed, where their contributions increase with acidity.8,35

Figure 2.5. Mass spectral difference (wet ABS– wet AS) of the organics (normalized by total

organic mass). Positive values (blue) indicate the mass fragments that are enhanced for organics

on wet ABS seed, and the negative values (red) indicate enhancement for wet AS seed. The

insert is a zoomed-in view of the mass spectrum at higher m/z.

The reversibility of the uptake process leading to isoprene SOA formation on wet ABS was also

investigated, where the drying process resulted in a loss of 30 ± 10 % organics. Thus, the drying

process resulted in comparable mass loss to that of wet and dry AS, along with similar changes

42

in chemical composition (see Figure 2.6). These observations suggest that the differences in

particle-phase water and acidity do not affect the reversibility uptake behavior of isoprene SOA.

Figure 2.6. Mass spectral difference of the organics (normalized by total organic mass) for wet

ABS to illustrate the reversibility of uptake due to water evaporation. Positive values (blue)

indicate the mass fragments that are enhanced for organics on dried sulfate-SOA particles, and

the negative values (red) indicate enhancement for not-dried sulfate-SOA particles. The

molecular formulae of the dominant organic fragment for significant ions are identified in the

main text.

2.5 Atmospheric Implications

In this work, the importance of particle-phase water and acidity on isoprene SOA formation is

demonstrated and quantified for the first time at high relative humidity. Particle-phase water was

the dominant factor controlling the amount of SOA formed as it gave rise to 60 % more mass.

The resulting SOA material is more enriched in organic acids, likely due to their enhanced

uptake due to presence of particle-phase water. The uptake of these organic acids was also

observed to be partially reversible upon the removal of particle-phase water. Particle acidity

mainly contributed to differences in chemical composition of the resulting SOA likely due to

acid catalyzed condensed phase reactions where high molecular weight products were formed.

Considering all experiments in sum, the results indicate that the RH cycling of sulfate seed

particles in the atmosphere affects the SOA burden, and further illustrates how anthropogenic

components (i.e., sulfate) mediate the formation of SOA from biogenic precursors.36,37

The results of this study are largely consistent with ambient measurements where weak

correlations of isoprene-SOA and particle acidity have been observed.11–14

We note that IEPOX

did not contribute to the SOA formed in this study due to its short lifetime in the reactor flow

43

tube. Because of this, the distribution of gas-phase isoprene oxidation products probably

resembles that further away from isoprene sources, where IEPOX has been oxidized.

Also, we note that the presence of pre-existing organics on sulfate particles may affect the role of

particle-phase water and acidity on isoprene SOA formation. Gaston et al. observed that the

uptake of IEPOX on mixed sulfate/polyethylene glycol particles was suppressed even for acidic

sulfate seeds at high RH conditions.16

Given the complexity of seed particle effects on just one

SOA precursor, further investigations using a wide range of seed particle composition, SOA

precursors, and conditions (i.e., RH, effects of UV light) are warranted in order to understand

how seed particles mediate SOA formation in the atmosphere.

Acknowledgements

The authors thank Natural Sciences and Engineering Research Council for funding.

2.6 Supporting Material

Figure 2.7. Mass spectra (normalized by total organics) of the organics on wet ABS (blue) and

wet AS (red) seed particles. The insert illustrates the zoomed-in view at higher m/z. The

molecular formulae of the dominant organic fragment for significant ions are identified in the

main text.

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1877.

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Abbatt, J. P. D. Characterization of Aerosol and Cloud Water at a Mountain Site during

WACS 2010: Secondary Organic Aerosol Formation through Oxidative Cloud Processing.

Atmos. Chem. Phys. 2012, 12, 7103–7116.

(26) Ng, N. L.; Canagaratna, M. R.; Zhang, Q.; Jimenez, J. L.; Tian, J.; Ulbrich, I. M.; Kroll, J.

H.; Docherty, K. S.; Chhabra, P. S.; Bahreini, R.; et al. Organic Aerosol Components

Observed in Northern Hemispheric Datasets from Aerosol Mass Spectrometry. Atmos.

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of Glyoxal and Methylglyoxal by Aerosol Chemical Ionization Mass Spectrometry:

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Artaxo, P.; Brito, J.; Lee, J. D.; Coe, H. Airborne Observations of IEPOX-Derived

Isoprene SOA in the Amazon during SAMBBA. Atmos. Chem. Phys. 2014, 14, 11393–

11407.

(30) Jacobs, M. I.; Darer, A. I.; Elrod, M. J. Rate Constants and Products of the OH Reaction

with Isoprene-Derived Epoxides. Environ. Sci. Technol. 2013, 47, 12868–12876.

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H.; Stoltz, B. M.; Wennberg, P. O. Gas-phase Production and Loss of Isoprene

Epoxydiols. J. Phys. Chem. A 2014, 118, 1237–1246.

(32) Ervens, B.; Turpin, B. J.; Weber, R. J. Secondary Organic Aerosol Formation in Cloud

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Absorbing Organo-Nitrogen Species from Evaporation of Droplets Containing Glyoxal

and Ammonium Sulfate. Environ. Sci. Technol. 2013, 47, 12819–12826.

(34) Schlenker, J. C.; Martin, S. T. Crystallization Pathways of Sulfate-Nitrate-Ammonium

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47

(36) Carlton, A. G.; Turpin, B. J. Particle Partitioning Potential of Organic Compounds Is

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of Isoprene Markers and Reveal Key Role of Acidity in Aerosol Formation. Environ. Sci.

Technol. 2013, 47, 11056–11064.

48

Chapter 3

3 Changes in Secondary Organic Aerosol Composition and

Mass due to Photolysis: Relative Humidity Dependence

Changes in Secondary Organic Aerosol Composition

and Mass due to Photolysis: Relative Humidity

Dependence

J. P. S. Wong, S. Zhou, J. P. D. Abbatt, The Journal of Physical Chemistry A, Article ASAP,

doi:10.1021/jp506898c

Reprinted (adapted) with permission from The Journal of Physical Chemistry A

Copyright 2014 American Chemical Society

49

3.1 Abstract

This study is focused on the relative humidity (RH) dependence of water-soluble secondary

organic aerosol (SOA) aging by photolysis. Particles containing α-pinene SOA and ammonium

sulfate, generated by atomization, were exposed to UV radiation in an environmental chamber at

three RH conditions (5, 45 and 85 %), and changes in chemical composition and mass were

monitored using an aerosol mass spectrometer (AMS). Under all RH conditions, photolysis leads

to substantial loss of SOA mass, where the rate of mass loss decreased with decreasing RH. For

all RH conditions, the less oxidized components of SOA (e.g., carbonyls) exhibited the fastest

photodegradation rates, which resulted in a more oxidized SOA after photolytic aging. The

photolytic reactivity of SOA material exhibited a dependence on RH likely due to moisture-

induced changes in SOA morphology or phase. The results suggest that the atmospheric lifetime

of SOA with respect to photolysis is dependent on its RH cycle, and that photolysis may be an

important sink for some SOA components occurring on an initial time scale of a few hours under

ambient conditions.

3.2 Introduction

Organic compounds are ubiquitous in ambient aerosol, yet their formation and transformation

mechanisms in the atmosphere remain not fully characterized.1 These organic compounds are

largely secondary in nature (i.e., secondary organic aerosol, SOA), produced from the

condensation of oxidation products of volatile organic compounds. Away from sources, SOA is

continually subjected to further chemical processing, or aging, that transforms the particulate

physico-chemical properties, which are important for climate, air quality, and human health.

Current atmospheric models, with parametrizations based largely on gas- and gas-particle

reactions studied in the laboratory, have difficulty predicting the mass and the degree of

oxidation of ambient SOA, suggesting unidentified formation and aging mechanisms.2,3

Aging processes include heterogeneous oxidation, gas-phase oxidation of semi-volatile

components, and reactions in the condensed phase, such as photolysis, polymerization, and

aqueous oxidation.3 Historically, the aging of SOA material has been investigated using SOA

particles in environmental chambers or flow tube reactors, or particles collected on filters.

Typically, these studies have been conducted under dry or low RH conditions. On the other hand,

studies have also examined the aging of aqueous SOA solutions to investigate the role of cloud

50

processing (e.g., saturated RH conditions). Collectively, these studies have shown that aging can

affect the mass and the chemical composition of SOA, but the experimental conditions only

represent a narrow range of RH conditions found in the atmosphere. Only a limited number of

studies have investigated aging processes under a wide range of RH conditions where the

reactivity of model organic particles was observed to be dependent on RH, as aging by ozone4

and OH radical5 were enhanced under high RH conditions. For complex mixtures such as SOA,

recent work by Zhou et al. observed that the kinetics of the heterogeneous ozonolysis of

benzo[a]pyrene (BaP) within SOA material was suppressed under dry conditions, where the

kinetic behavior was between that expected of solids and liquids. Additionally, it was observed

that the reactivity of BaP was enhanced with increasing RH conditions. These results suggest

that RH affects particle reactivity, as water uptake by hygroscopic SOA under high RH condition

lowers the material’s viscosity, which allows for more rapid diffusion of reactants compared to

dry conditions where SOA is semisolid.6

Indeed, there is growing experimental evidence that SOA can be an amorphous solid with high

viscosity under dry/low RH conditions. With increasing RH, water acts as a plasticizer,

decreasing the viscosity of the organic material (i.e., liquid SOA).7–10

In addition to the study by

Zhou et al., other recent studies have shown that the phase/viscosity of SOA material can affect

gas-particle partitioning11

and aging by heterogeneous OH oxidation.12

Given that past

investigations have shown the importance of RH on the phase/viscosity of SOA, which has

implications on particle reactivity, it is necessary to characterize how variations in RH conditions

affect other aging processes. Of particular interest to the current study is the aging of SOA by

photolysis.

As with other SOA aging experiments, previous laboratory studies have focused on the

photolysis of dry SOA particles, or SOA dissolved in bulk solutions, observing a reduction in

SOA mass and significant photodissociation of compounds containing carbonyl and peroxide

functional groups.13–15

Although there is growing evidence that photolysis may play an important

role in the aging of SOA, the RH effects are still unknown. The objective of the current study is

to investigate the photolysis of particles containing water-soluble α-pinene SOA material and

ammonium sulfate under various RH conditions in an environmental chamber. The changes in

SOA mass and chemical composition were measured using an Aerodyne aerosol mass

spectrometer (AMS). The kinetics of SOA photolysis were examined in order to estimate the

51

atmospheric importance of direct photochemical aging. The use of mixed SOA–ammonium

sulfate particles was chosen for two reasons. In particular, the sulfate mass could be used to

normalize the organic signal under the assumption of no sulfate photochemical reactivity. Also,

the sulfate hygroscopic properties are well characterized ensuring that water was present in the

particles at high relative humidity and not present at the lowest relative humidity.

3.3 Experimental Section

3.3.1 SOA Generation and Collection

SOA was generated and collected separately from the photolysis experiments in order to remove

the effects of gas-phase oxidants and volatile α-pinene oxidation products. The ozonolysis of α-

pinene was used to generate SOA material in a 1 m3 Telfon (FEP) dark chamber, under dry

conditions (RH < 5 %) and in a continuous flow mode. The terpene was introduced into the

chamber via a 6 sccm flow from the headspace of a bubbler containing liquid α-pinene (Sigma-

Aldrich), chilled to −10 °C and mixed with a 9 slpm flow of purified air. Ozone was generated

by passing 0.5 slpm of purified air over a mercury lamp and introduced into the chamber to

initiate SOA formation. The final mixing ratios were 750 ppb of ozone, as monitored by an

ozone analyzer (Thermo Scientific) and 150 ppb of α-pinene, as measured by a proton-transfer-

reaction mass spectrometer (Ionicon). The residence time in the chamber during SOA generation

was 100 min. The resulting SOA particle size distribution was measured using a TSI Scanning

Mobility Particle Sizer (SMPS: DMA TSI-3080 and CPC, TSI-3225). A sample volume

distribution of the resulting SOA is shown in Figure 3.1. The SOA was collected on

polytetrafluoroethylene filters (47 mm, 2 um pore size, Pall Corporation) at 9.5 slpm for 48 h

after the O3 mixing ratio and SOA mass loading were stable. The amount of SOA collected on

filter (∼9 mg) was determined from the difference in mass measured before and after collection.

Immediately after collection, the water-soluble organic compounds in the SOA were extracted in

8.75 mL of purified water (18 mΩ) and stored in a freezer at −20 °C.

52

Figure 3.1. Typical volume distribution of SOA formed during filter collection, measured by

SMPS.

3.3.2 UV/Vis Absorption Spectroscopy

The UV/vis absorption properties of SOA were measured by an UV/vis dual beam spectrometer

(PerkinElmer). A bulk SOA solution was used, using a concentration identical to the solutions

used for generating particles for photolysis experiments (described below). Purified water in a

matching curvette (1 cm, quartz) was used as the reference. From the measured absorbance

(base-10), the effective absorption cross section (base-e) of SOA was determined (see the

Supporting Information for a detailed calculation). Since the UV lamps used in the current study

to initiate photolysis have a sharp emission cutoff at 284 nm, the absorbance below this

wavelength was not considered.

3.3.3 Photolysis of SOA

All photolysis experiments were conducted in the University of Toronto Mobile Concentration

and Aging (MOCA) chamber in batch mode. MOCA is a 1 m3 Teflon (FEP) bag mounted around

a cubic Teflon-coated frame, surrounded by 24 UV lamps (Sylannia) on four of its six sides,

cooled with multiple fans. The temperature and RH inside the chamber were continuously

monitored (Vaisala). Following each experiment, the chamber was flushed with clean air and

irradiated with UV light for a minimum of 12 h for cleaning.

Prior to each experiment, the chamber was conditioned to a specific RH by flowing purified air

through heated water bubblers into the chamber prior to particle introduction. To generate

particles, ammonium sulfate (AS, 0.04 w/v %) was added to the SOA water extract to a total

53

volume of 10 mL, resulting in a 3:2 inorganic-to-organic mass ratio. AS was chosen because its

hygroscopic response to variations in RH is well characterized.16

This bulk solution was

aerosolized into the chamber using a constant output atomizer (TSI 3076) for 20 min. From AMS

measurements (described below) obtained using particle time-of-flight (PTOF) mode, the

particles in the chamber had a typical mass mode of 230 nm prior to UV light exposure (size-

resolved mass distribution shown in Figure 3.2). Prior to photolysis, the typical particle mass

loading in the chamber was 30 μg m-3

. To study the photolysis of SOA material, the AS–SOA

particles were mixed in the dark chamber for 20 min before they were exposed to UV irradiation

for 33 min, following which the chamber was returned to dark conditions. Upon UV light

exposure, the temperature inside the chamber increased by 1° (above room temperature), which

resulted in a decrease of ∼5 % in RH. It is important to note that the effects of UV light exposure

can be due to volatilization (due to heating) and UV irradiation. Since this temperature increase

was identical for all photolysis experiments, the contributions of volatilization to changes in

SOA mass and composition are also identical. That being said, with the temperature increase so

small, we believe the changes are largely due to photolysis.

Figure 3.2. Sized-resolved mass distribution of organics (green) and sulfate (red) measured by

the AMS in PTOF mode for mixed SOA-AS particles before UV light exposure.

Two sets of experiments were conducted to characterize the photolysis of SOA material. For the

first set of experiments, the photolysis of SOA by UVB light centered at 310 nm at three

different RH conditions (85 % (“high RH”), 45 % (“mid RH”), and 5 % (“low RH”)) was

examined to elucidate the effects of RH. The polydisperse particles were fully deliquesced before

entering the chamber, and then equilibrated to each RH condition in the chamber. Given that

previous work has characterized the effects of RH on the particle-phase and viscosity of SOA.9,10

54

we speculate that the SOA material is likely to be liquid at high RH and semisolid at low RH

conditions, possibly having phase separated. At mid RH conditions, the particles were expected

to have intermediate liquid water content, and the resulting phase and viscosity of the SOA

material is likely to be between the values present under high and low RH conditions. For AS, it

is deliquesced at high RH and effloresced at low RH conditions.16

The second set of experiments was conducted to investigate the UV wavelength dependence of

SOA photolysis. Under the same high RH conditions, photolysis using UVA lamps centered at

360 nm was conducted, and compared to results obtained using UVB lamps centered at 310 nm.

The wavelength dependence of the photon flux inside the chamber was measured using a

spectroradiometer (StellaNet Inc.). The measured wavelength dependent photon fluxes from both

UVB and UVA lamps are shown in Figure 3.1, where the actinic flux at solar noon is provided

for comparison. It is important to note that the spectral flux of the UVB and UVA lamps is not

fully representative of that in the atmosphere. Compared to ambient conditions, the UVB lamps

are more intense at lower wavelengths while the UVA lamps are less intense for all wavelengths.

Figure 3.3 Measured photon flux in the chamber from UVB and UVA lamps compared to the

actinic flux for a clear-sky summer day. The actinic flux was obtained from “Quick TUV

Calculator”, available at http://cprm.acd.ucar.edu/Models/TUV/Interactive_TUV/ using the

following parameters: SZA = 0, June 30, 2000, 300 Dobson overhead ozone, surface albedo of

0.1, and 0 km altitude.

55

3.3.4 Particle Measurements

For particle mass and composition measurements, the particles were first dried using a silica gel

diffusion dryer and then sampled online by the time-of-flight aerosol mass spectrometer (ToF-

AMS, Aerodyne Research, Inc.). Experiments were conducted with the unit resolution AMS

(cToF-AMS) unless otherwise stated. The operating and analysis procedure of the AMS has been

previously reviewed in detail.17

Briefly, the AMS provides the size-resolved mass and chemical

composition of the non-refractory component of particles, including organics, sulfate, nitrate,

and ammonium. Since the resulting mass spectra of the sampled particles include signals from

major gases (N2, O2, H2O, Ar, and CO2), their contributions were accounted for by obtaining

multiple filter measurements (e.g., zero particle signal) throughout each experiment.

To understand the changes in organic particle mass due to photolysis, the loss of particle mass to

the chamber walls needs to be accounted for. Since the chamber experiments were conducted in

batch mode, where the particle mass concentration decreases over time due to deposition, the

individual organic fragments and total organic mass were normalized to the total sulfate mass

measured by the AMS in order to determine absolute changes due to photolysis, assuming sulfate

is nonreactive. Additionally, the mass loading of α-pinene SOA has been shown to affect its

chemical composition, as semi-volatile organic compounds will increasingly partition to the

particle at higher mass loadings.18,19

In the current study, where particle mass loading decreases

over time due to wall loss, we expect the evaporation of semi-volatile compounds over the

course of the experiment. Control experiments were conducted where particles were not exposed

to UV irradiation in the chamber, under each RH condition. These control experiments were

conducted at the same temperature as typical photolysis experiments. The observed changes to

the chemical composition of particles due to wall loss were accounted for in order to isolate the

effects due to UV irradiation (Figure 3.4).

56

Figure 3.4. Typical time trace of org:sulfate ratio of the mixed particles due to evaporation in the

dark (control) and exposure to UV irradiation under high RH conditions.


3.4.1 Absorption Cross Section of SOA

While SOA is a complex mixture of many different compounds, the absorption cross section of

the bulk SOA solution (Figure 3.5) provides information about the functional groups that absorb

light in the wavelength range emitted by the UVB and UVA lamps used in this study. The

observed main absorption region corresponds to characteristic absorptions of carbonyl and

peroxy functional groups, which are known products of SOA formed by α-pinene ozonolysis.

The absorption cross section of a known carbonyl product, cis-pinonic acid,20

is also shown in

Figure 3.5 for comparison. The absorption observed at higher wavelengths is likely due to the

presence of peroxide functional groups, as absorption cross sections of peroxides tend to exhibit

a broad tail into the actinic region.21

While it is ideal to measure the absorption cross section of

SOA material in particles, this was not possible with the instrumentation available. However, a

previous measurement of the mass absorption coefficient of SOA particles at RH 70 % observed

a similar spectral shape compared to the spectrum obtained in this study using bulk SOA

solution, suggesting the absorptive properties of SOA material in bulk solutions are similar to

those in particles under higher RH conditions.22

57

Figure 3.5 Absorption cross section of bulk SOA solution (this work) and aqueous cis-pinonic

acid.20

3.4.2 Photolytic Degradation of SOA

Absolute changes to the total organic mass, as indicated by the organic-to-sulfate (org:sulfate)

mass ratio, due to exposure to UVB light under different RH conditions are shown in Figure 3.6.

Org:sulfate ratios were normalized to pre-aging values to guide comparison of the changes for

different experimental conditions. Under all RH conditions, decreases in total organic mass were

observed due to UV light exposure, where the amount of SOA mass loss increases with RH.

Absorption of UV light by organic compounds can initiate photolysis, where the fragmentation

of their carbon skeleton results in the formation of smaller products (e.g., fewer carbons atoms)

that are more volatile.18,23

We speculate that these volatile photolysis products evaporate from

the particle, resulting in the observed loss of total organic mass. In fact, the formation of gas-

phase products, such as CO, CH4, acetone, and other small VOCs, has been observed following

the photodegradation of limonene–SOA on filters and in aqueous solutions.13,14

58

Figure 3.6. Time series profiles of the org:sulfate ratios measured by the AMS for the photolysis

of SOA by UVB radiation under high RH (solid line), mid RH (dashed line), and low RH (dotted

line) conditions. The shaded areas for each RH condition represent the variability (±1σ) between

multiple experiments.

In addition to the total organic mass, the absolute changes in specific organic fragments provide

additional insight into the photolysis of SOA. Due to the evaporation/ionization techniques used

in the AMS, most compounds are subject to extensive fragmentation; however, specific

fragments can be used as tracers for organic compounds with specific functional groups.17

The

CHO+ fragment detected at m/z 29 arises from the fragmentation of compounds containing

carbonyl functional groups,24

while two different fragments can contribute to the signal observed

at m/z 43, with the C2H3O+ fragment originating from functionalities with a single oxygen atom

(e.g., carbonyls, alcohols, and ethers) and the C3H7+ fragment originating from aliphatic

molecules.25

Control experiments using the high-resolution ToF-AMS, which can separate

different ions at the same nominal m/z, indicated that the C2H3O+ ion was the dominant signal at

m/z 43 for this study. The organic ion at m/z 44 (CO2+) arises from the fragmentation of highly

oxidized organic acids (e.g., carboxylic acids and esters)26

and peroxides.27

For all RH

conditions, upon UVB irradiation, loss of the oxidized components of SOA was observed, where

the decays of m/z 29 and m/z 43 (Figure 3.7a,b) were faster than that of m/z 44 (Figure 3.7c).

59

Figure 3.7. Time series profiles of the sulfate normalized (a) m/z 29, (b) m/z 43, and (c) m/z 44

for the photolysis of mixed SOA–sulfate particles by UVB radiation at high RH (solid line), mid

RH (dashed line), and low RH (dotted line) conditions. The shaded areas for each RH condition

represent the variability (±1σ) between multiple experiments.

Given that the absorption spectrum of SOA indicates the presence of carbonyl and peroxide

function groups, both of which absorb UVB radiation, it is not surprising that we observed

decays of m/z 29, 43, and 44, suggesting the photodegradation of these functional groups. To

60

gain further insight of the wavelength dependence of SOA photolysis, experiments under high

RH conditions were conducted using UVA lights. Compared to experiments using UVB

radiation at the same RH condition, SOA exposed to UVA radiation exhibited slower decays for

total organic mass (Figure 3.8a) and for all organic fragments considered (Figure 3.8b–d). In

particular, the loss of m/z 29 and 43 is greatly suppressed for UVA lights, suggesting that the

photolysis of less oxidized components (i.e., carbonyls) contributes significantly to the observed

photochemical aging with the UVB light. It is important to note that the slower decay of m/z 44

compared to m/z 29 and 43 can also arise from secondary reactions of photolysis which result in

the formation of highly oxidized organics. For example, photolysis of peroxides is known to

generate OH radicals, which can oxidize other SOA components, forming highly oxidized

components such as organic acids.28,29

Direct photolysis of aldehydes has also been observed to

generate organic acids as well.13,20

Given that both peroxides and carboxylic acids contribute to

the organic fragment at m/z 44, the combined result of photodissociation of peroxides and the

formation of carboxylic acids may have resulted in the slower decay of m/z 44.

Figure 3.8. Time series profiles of the sulfate normalized (a) total organic mass (org:sulfate

ratio, black), (b) m/z 29, (c) m/z 43, and (d) m/z 44 for the photolysis of mixed SOA–sulfate

particles by UVB (solid lines) and UVA (dashed lines) radiation under high RH conditions. The

shaded areas for each light source represent the variability (±1σ) between multiple experiments.

61

The relative changes in the chemical composition of SOA due to photolysis can be illustrated

using the framework developed by Ng et al. in Figure 3.9, where aging of SOA is described

using the fraction of organic mass at m/z 43 vs 44 (f43 and f44).26

Ambient SOA typically falls

within the triangle denoted by the dashed lines in Figure 3.9c (inset), with less oxidized SOA

mainly residing in the lower half of the triangle and highly oxidized SOA in the upper half. In

general, studies have shown that SOA aging processes, such as OH oxidation, result in

movement toward the upper left of the triangle space.30

The trends observed for this figure are

also consistent with those observed for Figure 3.7, with, for example, a stronger decrease for m/z

43 compared to m/z 44, with the magnitude of the change depending on RH conditions. This

suggests that enhanced loss of less oxidized particulate organics due to photolysis results in a

more oxidized SOA. Note that the composition of the SOA used in the current study resides in

the middle region of the triangle, suggesting that the organics are somewhat oxidized prior to

photolysis.

Considering the experiments discussed above, the loss of organic components and changes in

chemical composition due to SOA photolysis all show RH dependence, with enhanced loss

observed with increasing RH conditions. We speculate that the observed RH dependence for

SOA photolysis is likely due to the effects of viscosity on diffusion, as recent studies have shown

that SOA exhibits liquid-like behavior under high RH and is semisolid/solid under low RH.9,10

In

viscous semisolid/solid material, the slow diffusion of SOA photodissociation products can favor

their recombination, resulting in a reduction in the efficiency of photolysis. For example, the net

quantum yields for the photolysis of certain amines and iodine were previously observed to be

reduced in viscous media.31,32

Additionally, slow diffusion has been shown to kinetically hinder

the evaporation of SOA.11

Thus, the effects of RH on diffusion can lead to the results observed in

the current study, where, under lower RH conditions, slow diffusion of photolysis products

resulted in less mass loss compared to higher RH conditions.

62

Figure 3.9. Aerosol composition described by the fraction of organic mass at m/z 43 and 44 (f43

vs f44) before (open circles), during (closed circles), and after (open circles) UV light exposure

for (a) high RH, (b) mid RH, (c) and low RH conditions. The inset shows the zoomed-out view

of the f43 vs f44 space (with dimensions of Figure 3.9a–c illustrated by the red box), where

ambient SOA is distributed within the triangular area.

63

3.4.3 Kinetics of Photolysis

Figure 3.10. Kinetic plots for the loss of total organic mass due to photolysis of SOA at high RH

(solid line), mid RH (dashed line), and low RH (dotted line). The shaded region for each RH

condition represents the variability (±1σ) between experiments. The mass loss rate constant was

obtained from the linear best fit of the first 10 minutes of photolysis.

To further investigate the role of RH on the reactivity of SOA material on particles, the decay

rates of particulate SOA mass loss due to photolysis were determined. The kinetic plot (shown in

Figure 3.10) indicated that the reaction exhibited first order kinetics at early times, as expected

from direct photolysis. Linear fits to the first 10 min of data yield first-order mass loss rates

(kSOA) for all RH and light conditions, listed in Table 3.1. Assuming that the initial observed

mass loss is only due to the volatilization of small organic compounds from photolysis of SOA,

the observed mass loss rate constants were equated to the effective photolysis rate constant:

Equation 3.1

where σ(λ) is the absorption cross section of SOA, ϕ(λ) is the photolysis quantum yield, and F(λ)

is the photon flux. Using the measured absorption cross section of SOA and the measured photon

flux in the chamber, the quantum yield for the loss of organics due to SOA photolysis can be

estimated. Since the UV lamps used in the current study to initiate photolysis have a sharp

emission cutoff at 284 nm, the absorbance below this wavelength was not considered. We note

that this approach does not take into account the enhancement in photon flux that may occur

inside wet particles relative to values in air.33

64

Table 3.1. Total Organic Mass Loss Rate Constants for Different RH Conditionsa

kSOA (s–1

)

light source high RH mid RH low RH

UVB (1.67 ± 0.20) × 10–4

(1.64 ± 0.09) × 10–4

(7.91 ± 0.13) × 10–5

UVA (4.69 ± 0.14) × 10–5

aThe uncertainties in kSOA represent variability (±1σ) between multiple experiments.

From the observed mass loss rates under high RH conditions for both UVB and UVA lights, the

average effective photolysis quantum yield was determined to be 1.2 ± 0.2. This large quantum

yield suggests that either the primary photolytic process efficiently gives rise to volatile products

or secondary reactions (such as those driven by the photochemical formation of OH in the

particle) may have also contributed to the observed total mass loss rates. The photolysis quantum

yield for cis-pinonic acid, one of the identified compounds in α-pinene SOA, is 0.5 ± 0.3,20

i.e.,

also quite large and similar to our result. The factor of 2 difference could potentially be due to

the presence of other more photolabile compounds in SOA material.

At decreasing RH, the SOA becomes highly viscous and concentrated, where elevated

concentrations can accelerate condensed phase reactions (e.g., polymerization, reactions with

inorganics), thus potentially changing the absorptive properties of the material. Since the

absorption cross section of SOA measured from bulk solutions may not be identical compared to

particles under decreasing RH conditions, the current approach to determine the effective

quantum yield for mid and low RH conditions was not pursued. Nevertheless, the suppression in

mass loss rates with decreasing RH indicates that photolysis is dependent on RH conditions,

where the mass loss rate under low RH is a factor of 2 slower than that at high RH. It is

important to note that the observed mass loss rate at the low RH condition using UVB lights for

the current study (7.91 ± 0.13) × 10–5

s–1

] is similar to the photolytic mass loss rate constant of

dry α-pinene SOA–AS particles for UVA lights (6 × 10–5

s–1

), reported by Henry and Donahue.15

In comparison, for dry isoprene SOA, Kroll et al. also observed mass loss [(1 – 5) × 10–5

s–1

] due

to irradiation with UVA lights.34

However, this mass loss rate might have arisen due to reaction

with gas-phase OH radicals, due to the addition of H2O2 (both UV photons and OH radicals are

present during irradiation).

65

3.5 Conclusions and Atmospheric Implications

In this work, the RH dependence for the photolysis of a chemically complex mixture, water-

soluble SOA, was illustrated. Photolysis resulted in substantial organic mass loss, with the most

rapid loss observed under high RH conditions. In particular, the less oxidized components of

SOA (e.g., carbonyls) exhibited the fastest photodegradation, which resulted in a more oxidized

SOA particle after photolytic aging. We hypothesize that the dependence of photolysis on RH

conditions was due to moisture-induced viscosity changes of SOA material, affecting the

diffusion of reactants/products. In this regard, despite a change in several orders of magnitude in

the viscosity of the SOA material from high to low RH conditions, its photolysis was only slower

by a factor of 2. This suggests that, even at such high viscosity values, photolyzed SOA material

can nevertheless efficiently diffuse out of the particle and evaporate within the experimental time

scale of the current study.8,10

To assess the importance of photolysis as an aging mechanism for SOA in the atmosphere, the

atmospheric lifetime of SOA particles with respect to photolysis is estimated to be between that

observed with the UVB lamps and that for the UVA lamps. In particular, using the

experimentally determined absorption cross section and photolysis quantum yield, and calculated

actinic fluxes, the initial photolysis lifetime is ∼3.7 h at Solar Noon for high RH conditions.

Given that the average lifetime of particles in the atmosphere is approximately 1 week with

respect to deposition, this very rough calculation indicates that photolysis can be an important

sink of SOA material in the atmosphere due to volatilization.

Clearly, there are uncertainties in this estimate, and this initial lifetime should not be equated to

the overall lifetime for loss of SOA particles in the atmosphere. In particular, the contribution of

secondary photolysis to the effective quantum yield was not constrained. Therefore, the average

photolysis quantum yield determined in the current study is likely an upper limit to the primary

photolysis process, corresponding to a lower limit for this calculated lifetime. Nevertheless, the

effects of such secondary reactions of photolysis may be of importance to the fate of SOA. For

example, it is known that photolysis of organic compounds in cloudwater is an important source

of H2O2, which is an important oxidant.35

Also, we note that SOA reactivity may be dependent on its atmospheric age and the degree of

prior processing. In particular, the slopes of the mass loss decay curves (e.g., Figure 3.10)

66

decreased with time, indicating depletion of the most photochemically active species (i.e.,

carbonyls and peroxides) and formation of more stable compounds, such as carboxylic acids. On

the other hand, Henry and Donahue observed enhanced mass loss for the photolysis of SOA that

was aged via OH oxidation, as oxidative aging by OH may lead to the formation of more

carbonyl and peroxide compounds compared to photolytic aging.15

Additionally, brown carbon

formed via condensed phase reactions in aerosols has been observed to be photolabile.36

As such,

it is important to understand how the reactivity of SOA changes with atmospheric age.

Also, we point out that this work has implications to photooxidation SOA chamber and flow tube

studies, given that many of these reactors use UV lamps similar to those in this experiment.

Thus, the changes in SOA mass and chemical composition due to photolytic aging observed in

this study may also occur in other experiments.

Finally, the formation mechanism of the mixed SOA–AS is not necessarily the same as that in

the atmosphere, probably resembling the process by which particles are formed by cloud

processing more than that from condensation of terpene oxidation products on an ammonium

sulfate particle.

Overall, this is one of the first studies to quantify the changes in SOA that occur upon exposure

to ultraviolet light, and the first study to look at the effect of RH on this process. This RH

dependence needs to be further investigated with other aging processes, SOA types, and various

conditions (e.g., peroxides are more likely to be formed under low NOx conditions during SOA

formation), in order to understand the processing of such species under a wide range of

atmospherically relevant conditions.

Acknowledgements

The authors thank Natural Sciences and Engineering Research Council and Canada Foundation

for Innovation for funding.

67


3.6.1 Absorption Cross Section Calculations

The effective absorption cross section ( base-e) of SOA was calculated from the measured

absorbance ( , base-10) using the following equation:

σ = ln (10) 1000

Equation 3.2

where is the cuvette path length in cm, is the concentration of the SOA solution in mol L-1

,

and is Avogrado’s number. In order to calculate the molar concentration of the SOA solution,

the density of the SOA solution was calculated from the mass of SOA material collected on the

filter dissolved in a known volume of purified water. The average molecular weight of this is

estimated to be 200 g mol-1

.14,37

3.7 References

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M. R.; Takami, A.; Middlebrook, A. M.; Sun, Y. L.; et al. Ubiquity and Dominance of

Oxygenated Species in Organic Aerosols in Anthropogenically-Influenced Northern

Hemisphere Midlatitudes. Geophys. Res. Lett. 2007, 34, L13801.

(2) Heald, C. L.; Ridley, D. A.; Kreidenweis, S. M.; Drury, E. E. Satellite Observations Cap

the Atmospheric Organic Aerosol Budget. Geophys. Res. Lett. 2010, 37, L24808.




Atmos. Chem. Phys. 2009, 9, 5155–5236.

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Pope, F. D.; Kalberer, M. The Effect of Humidity on the Ozonolysis of Unsaturated

Compounds in Aerosol Particles. Phys. Chem. Chem. Phys. 2012, 14, 8023–8031.

(5) Nguyen, T. B.; Coggon, M. M.; Flagan, R. C.; Seinfeld, J. H. Reactive Uptake and Photo-

Fenton Oxidation of Glycolaldehyde in Aerosol Liquid Water. Environ. Sci. Technol.

2013, 47, 4307–4316.




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(7) Virtanen, A.; Joutsensaari, J.; Koop, T.; Kannosto, J.; Yli-Pirilä, P.; Leskinen, J.; Mäkelä,

J. M.; Holopainen, J. K.; Pöschl, U.; Kulmala, M.; et al. An Amorphous Solid State of

Biogenic Secondary Organic Aerosol Particles. Nature 2010, 467, 824–827.




19238–19255.

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Pedernera, D. A.; Onasch, T. B.; Laaksonen, A.; Davidovits, P.; et al. Humidity-

Dependent Phase State of SOA Particles from Biogenic and Anthropogenic Precursors.

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(10) Renbaum-Wolff, L.; Grayson, J. W.; Bateman, A. P.; Kuwata, M.; Sellier, M.; Murray, B.

J.; Shilling, J. E.; Martin, S. T.; Bertram, A. K. Viscosity of α-Pinene Secondary Organic

Material and Implications for Particle Growth and Reactivity. Proc. Natl. Acad. Sci. 2013,

110, 8014–8019.

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Implications for OA Phase and Partitioning Behavior. Atmos. Chem. Phys. 2011, 11,

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(12) Kolesar, K. R.; Buffaloe, G.; Wilson, K. R.; Cappa, C. D. OH-Initiated Heterogeneous

Oxidation of Internally-Mixed Squalane and Secondary Organic Aerosol. Environ. Sci.

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Nizkorodov, S. A. Contribution of Carbonyl Photochemistry to Aging of Atmospheric

Secondary Organic Aerosol. J. Phys. Chem. A 2008, 112, 8337–8344.



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(15) Henry, K. M.; Donahue, N. M. Photochemical Aging of Α-Pinene Secondary Organic

Aerosol: Effects of OH Radical Sources and Photolysis. J. Phys. Chem. A 2012, 116,

5932–5940.

(16) Tang, I. N.; Fung, K. H.; Imre, D. G.; Munkelwitz, H. R. Phase Transformation and

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(18) Donahue, N. M.; Robinson, A. L.; Stanier, C. O.; Pandis, S. N. Coupled Partitioning,

Dilution, and Chemical Aging of Semivolatile Organics. Environ. Sci. Technol. 2006, 40,

2635–2643.

(19) Shilling, J. E.; Chen, Q.; King, S. M.; Rosenoern, T.; Kroll, J. H.; Worsnop, D. R.;

DeCarlo, P. F.; Aiken, A. C.; Sueper, D.; Jimenez, J. L.; et al. Loading-Dependent

Elemental Composition of Α-Pinene SOA Particles. Atmos. Chem. Phys. 2009, 9, 771–

782.

(20) Lignell, H.; Epstein, S. A.; Marvin, M. R.; Shemesh, D.; Gerber, B.; Nizkorodov, S.

Experimental and Theoretical Study of Aqueous Cis-Pinonic Acid Photolysis. J. Phys.

Chem. A 2013, 117, 12930–12945.

(21) Stephen A. Mang; Maggie L. Walser; Xiang Pan; Jia-Hua Xing; Adam P. Bateman; Joelle

S. Underwood; Anthony L. Gomez; Jiho Park; Sergey A. Nizkorodov. Photochemistry of

Secondary Organic Aerosol Formed from Oxidation of Monoterpenes. In Atmospheric

Aerosols; ACS Symposium Series; American Chemical Society, 2009; Vol. 1005, pp. 91–

109.

(22) Zhong, M.; Jang, M. Light Absorption Coefficient Measurement of SOA Using a UV–

Visible Spectrometer Connected with an Integrating Sphere. Atmos. Environ. 2011, 45,

4263–4271.

(23) Calvert, J. G.; Pitts, J. N. J. Photochemistry; John Wiley & Sons, Inc.: New York, 1966.

(24) Lee, A. K. Y.; Hayden, K. L.; Herckes, P.; Leaitch, W. R.; Liggio, J.; Macdonald, A. M.;

Abbatt, J. P. D. Characterization of Aerosol and Cloud Water at a Mountain Site during

WACS 2010: Secondary Organic Aerosol Formation through Oxidative Cloud Processing.

Atmos. Chem. Phys. 2012, 12, 7103–7116.

(25) Zhang, Q.; Rami Alfarra; Worsnop, D. J.; Allan, J. D.; Coe, H.; Canagaratna, M. R.;

Jimenez, J. L. Deconvolution and Quantification of Hydrocarbon-like and Oxygenated

Organic Aerosols Based on Aerosol Mass Spectrometry. Env. Sci Technol 2005, 39, 4938-

4952.

(26) Ng, N. L.; Canagaratna, M. R.; Zhang, Q.; Jimenez, J. L.; Tian, J.; Ulbrich, I. M.; Kroll, J.

H.; Docherty, K. S.; Chhabra, P. S.; Bahreini, R.; et al. Organic Aerosol Components

Observed in Northern Hemispheric Datasets from Aerosol Mass Spectrometry. Atmos.

Chem. Phys. 2010, 10, 4625–4641.

(27) Aiken, A. C.; DeCarlo, P. F.; Jimenez, J. L. Elemental Analysis of Organic Species with

Electron Ionization High-Resolution Mass Spectrometry. Anal. Chem. 2007, 79, 8350–

8358.



10521–10539.

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(29) Ng, N. L.; Canagaratna, M. R.; Jimenez, J. L.; Chhabra, P. S.; Seinfeld, J. H.; Worsnop, D.

R. Changes in Organic Aerosol Composition with Aging Inferred from Aerosol Mass

Spectra. Atmos. Chem. Phys. 2011, 11, 6465–6474.

(30) Slowik, J. G.; Wong, J. P. S.; Abbatt, J. P. D. Real-Time, Controlled OH-Initiated

Oxidation of Biogenic Secondary Organic Aerosol. Atmos. Chem. Phys. 2012, 12, 9775–

9790.

(31) Ratcliff, M. A.; Kochi, J. K. Cage Effects and the Viscosity Dependence of the Photolysis

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(33) Nissenson, P.; Knox, C. J. H.; Finlayson-Pitts, B. J.; Phillips, L. F.; Dabdub, D. Enhanced

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1877.

(35) Faust, B. C. Photochemistry of Clouds, Fogs, and Aerosols. Environ. Sci. Technol. 1994,

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(36) Sareen, N.; Moussa, S. G.; McNeill, V. F. Photochemical Aging of Light-Absorbing

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(37) Asa-Awuku, A.; Nenes, A.; Gao, S.; Flagan, R. C.; Seinfeld, J. H. Water-Soluble SOA

from Alkene Ozonolysis: Composition and Droplet Activation Kinetics Inferences from

Analysis of CCN Activity. Atmos. Chem. Phys. 2010, 10, 1585–1597.

71

Chapter 4

4 Oxidation of ambient biogenic secondary organic aerosol

by hydroxyl radicals: Effects on cloud condensation nuclei

activity

Oxidation of ambient biogenic secondary organic

aerosol by hydroxyl radicals: Effects on cloud

condensation nuclei activity

J. P. S. Wong, A.K. Y. Lee, J.G. Slowik, D.J. Cziczo, W.R. Leaitch, A. MacDonald, J.P. D.

Abbatt, Geophysical Research Letters, 2011, 38, L22805, doi:10.1029/2011GL049351.

Reprinted (adapted) with permission from Geophysical Research Letters

Copyright 2014 American Geophysical Union

72

4.1 Abstract

Changes in the hygroscopicity of ambient biogenic secondary organic aerosols (SOA) due to

controlled OH oxidation were investigated at a remote forested site at Whistler Mountain, British

Columbia during July of 2010. Coupled photo-oxidation and cloud condensation nuclei (CCN)

experiments were conducted on: i) ambient particles exposed to high levels of gas-phase OH,

and ii) the water-soluble fraction of ambient particles oxidized by aqueous-phase OH. An

Aerodyne Aerosol Mass Spectrometer (AMS) monitored the changes in the chemical

composition and degree of oxidation (O:C ratio) of the organic component of ambient aerosol

due to OH oxidation. The CCN activity of size-selected particles was measured to determine the

hygroscopicity parameter (κorg,CCN) for particles of various degrees of oxygenation. In both cases,

the CCN activity of the oxidized material was higher than that of the ambient particles. In

general, κorg,CCN of the aerosol increases with its O:C ratio, in agreement with previous laboratory

measurements.

4.2 Introduction

One of the largest uncertainties in global radiative forcing assessment is the cooling effect of

atmospheric particulates via their ability to act as CCN, which is dependent on their size and

composition. Organic compounds constitute a large fraction of ambient aerosol, as demonstrated

at various locations in the Northern Hemisphere.1 Characterization of organic aerosol

hygroscopicity is thus crucial for reducing uncertainties in current climate models.

The earliest studies of organic aerosol CCN properties used model compounds, mainly chosen by

their solubility properties.2–5

These studies illustrated that highly water soluble compounds give

rise to good CCN – indicating that organic constituents could be of comparable importance to

cloud droplet formation as highly soluble, inorganic components. CCN closure studies were also

performed.6,7

Recent lab work has demonstrated that SOA, formed by photo-oxidation processes

from specific precursors, is quite hygroscopic.8–15

Given that a complete chemical characterization of ambient particles is not yet achievable, their

CCN properties cannot be calculated from first-principles. As a result, the particle hygroscopicity

is now typically described using the κ-Köhler method, where one parameter (κ) describes the

average properties (e.g., molecular weight and solubility) of all compounds present.16

This

73

eliminates the need for the specific chemical identities of the constituents found in ambient

aerosol and allows for a simple representation of CCN activation in global climate models.

In this context, a highly oxygenated organic particle is more likely to be a better CCN due to the

increased polarity and solubility of its constituents. Given that oxidative aging leads to more

functionalized solute molecules,17

it is reasonable to inquire whether such aging increases the

hygroscopicity of these particles. For laboratory data, a simple trend of increasing CCN activity

and sub-saturated hygroscopicity with degree of SOA oxidation (approximated by O:C atomic

ratios) has been reported.12,14,18,19

However, since the chemical composition of laboratory SOA is

different than that measured in the atmosphere,20

it is necessary to determine how oxidation

processes affect the CCN activation of ambient SOA. In particular, Chang et al.21

conducted a

CCN closure study using ambient aerosol measurements, where a direct relationship between the

organic aerosol's degree of oxygenation and its hygroscopicity parameter determined from CCN

activity (κorg,CCN) was proposed. An objective of the current study was to investigate the

postulate made by Chang et al.21

that there is a simple linear relationship between κorg,CCN of an

organic aerosol and its degree of oxidation.

Here we report the degree to which OH-initiated gas- and aqueous-phase oxidation affect the

CCN activity of ambient organic aerosol sampled in a biogenically-rich region. This marks the

first deployment of a portable flow tube apparatus that can generate controlled levels of gas-

phase OH, and it demonstrates the relationship between the CCN activity of ambient particles

and their O:C ratios due to OH oxidation. The study of ambient particles, in which their O:C

ratios were changed by OH exposure on-site, further supports the observations from earlier

laboratory experiments14,15,18,19,22

of increasing CCN activity with oxidation, and the relationship

of κorg,CCN vs. O:C postulated by Chang et al.21

and Lambe et al.15

4.3 Experiment

Experiments were conducted at the Raven's Nest site at Whistler, BC, Canada (1300 m ASL,

50°N 122°W) as part of Environment Canada's Whistler Aerosol and Cloud Study (WACS)

2010. The site is situated in a coniferous forest that produces high levels of biogenic organic

aerosol23

during warm summertime conditions, with minimal contributions from local pollution

and no evidence for biomass burning impact during the periods described below that had daytime

temperatures ranging from 20 to 27°C.

74

4.3.1 Gas-phase OH Oxidation

Gas-phase OH oxidation of ambient particles was conducted in the Toronto Photo-Oxidation

Tube (TPOT) (Figure 4.1), which is a modified version of a system15,24

that produces high OH

exposures (1.0 – 1.3 × 1012

molecules cm−3

s) via the photolysis (λ = 254 nm) of gas-phase O3

(1.3 – 2.7 × 1013

molecules cm−3

) in the presence of water vapour (relative humidity = 40 %) for

a residence time of 2.2 minutes in the oxidation region. Modifications from the technique

described in previous publications15,24

include the use of a wider diameter, stainless steel flow

reactor that was electrochemically coated with amorphous silicon material (SilcoTek Corp.) to

reduce gas-phase adsorption and the use of multiple point aerosol injection to accelerate the

mixing of particles with the gas flows. The sample (1100 sccm) was introduced through 3 m of

electrochemically coated 6 mm o.d. stainless steel tubing (no cyclone) that extended 0.5 m above

the roof of the sampling site. The UV lamp was placed in the centre of the flow tube, and was

surrounded by a quartz tube cooled with a flow of room air (∼20 lpm). The OH concentration

was determined by monitoring the decay of methyl-ethyl-ketone by reaction with OH using the

Ionicon PTR-MS. The OH exposures were equivalent to 8 – 10 days (24-h) of ambient OH

exposure of 1.5 × 106 molecules cm

−3.

Figure 4.1. Toronto Photo-Oxidation Tube (TPOT) setup for the study of the gas-phase OH

oxidation of ambient organic aerosol and their CCN activity.

75

During normal operation, ambient aerosol was sampled continuously. Aerosol is mixed with a

humidified flow (700 sccm). An automated switching valve alternately injects either an ozone-

containing flow (200 sccm) or a blank flow of O2 and N2 (200 sccm) with a period of 12 min.

These flows are allowed to mix and then pass into the reaction tube, where they are irradiated

with UV light. If the ambient aerosol was mixed with ozone, OH radicals are produced;

otherwise unreacted particles are measured. In addition to reaction with OH, ambient aerosol can

potentially react with ozone and/or be volatilized by heat and UV light produced from the lamp.

Ozone control experiments were conducted by comparing organic mass concentrations and mass

spectra with and without added ozone under dark conditions using a bypass tube. No reaction

with ozone was observed. Volatilization control experiments were conducted by comparing

ozone-free organic AMS mass spectra and concentrations in light (i.e., UV lamp on) vs. dark

conditions. With the lamp on, the temperature increased by approximately 4°C (25°C to 29°C).

In the presence of UV light, an 18 % decrease in total organic mass was observed, which can be

due to both increased temperature and photolysis of aerosol compounds leading to their

volatilization. More details on these control experiments are in the auxiliary material. Ambient

particles were subjected to OH and no OH oxidation alternatively for 12 minutes each, for the

total time period required to complete a CCN activation curve scan (described below). Three

time periods from each of two days of experiments are analyzed here.

4.3.2 Aqueous-phase OH Oxidation

A sampling inlet for particle filter samples was situated beside that for the gas-phase OH

oxidation studies. The particle samples passed through a cyclone with PM1 cut size (UGR,

Model 463) and were collected on 47 mm Teflon filters (2.0 μm pores) for 24 hour periods,

following which water extraction was conducted. The aqueous solutions were oxidized on-site

using our custom photoreactor,25

with a final H2O2 concentration of 70 mM (continuously

stirred) and then irradiated by a UV lamp (UVP, 254 nm) inserted into the solution. The

solutions were oxidized for 10 minutes, then nebulized (TSI 3076 atomizer) with compressed air

for the detection by particle instruments. Solutions without H2O2 were also nebulized under dark

conditions (i.e., UV lamp off). Four samples are considered for the current analysis.

76

4.3.3 Particle Composition and CCN Measurements

The non-refractory particle composition was measured by an Aerodyne Time-of-Flight aerosol

mass spectrometer (C-ToF-AMS). For the CCN measurements, particles were first dried using a

silica gel diffusion dryer and then size-selected at a mobility diameter of 75 nm (TPOT output)

or 100 nm (aqueous reactor output) by a TSI-3081 DMA before being measured by the CCNC.

The CCN active number fraction was determined from the ratio of the number of CCN-active

particles measured by one channel of a DMT-200 CCN counter to the total particle number

concentration, measured by a TSI-3010 Condensation Particle Counter (CPC).

To obtain a CCN activation curve, multiple supersaturations between 0.07 – 1.0 % were scanned

for a period of 24 minutes on each supersaturation for the gas-phase oxidation experiments, and

10 minutes for the aqueous-phase oxidation. CCN activation curves were constructed by

calculating the fraction of activated particles, determined from the CCN/CPC ratio at the various

supersaturation values. Using the dry mobility diameter (Di), and the critical supersaturation (Sc),

defined as the supersaturation in which 50 % of the size-selected particles act as CCN, the

kappa-parameter for the total particles was calculated as described by Petters and Kreidenweis,16

Equation 4.1

where D is the droplet wet diameter, Mw, ρw and σw are the molecular weight, density, and

surface tension of water.

The overall κ of the particles (κtotal,CCN) is calculated as the sum of the volume-fraction weighted

average κCCN's of the individual components. To arrive at the kappa-parameter for the organic

component of the particles, the following equation was used:16

Equation 4.2

where ɛorg is the volume fraction of the organic component determined from the C-ToF AMS,

assuming an initial organic density of 1200 kg m−3. Only samples with ɛorg ≥ 67 % (AMS mass

fraction, fOrg ≥ 0.63) are considered for the current analysis, so that the changes in κorg,CCN due to

oxidation are statistically significant and not dominated by the inorganic fraction. For example,

the estimated uncertainty in κorg,CCN is 25 % when ɛorg = 0.8, with smaller values for larger ɛorg

77

values. For the gas-phase oxidation experiments, the increase in the organic component density

due to oxidation was accounted for, using results obtained from George and Abbatt22

where they

determined changes in ρorg of laboratory α-pinene SOA as a function of OH exposure. This

amounts to a relative change in the κorg,CCN of only 0.5 % for the current analysis. For the

aqueous oxidation experiments, changes in organic density were not corrected for since the OH

exposures used were unknown. The aerosol inorganics measured by the AMS were considered in

the calculation as ammonium nitrate (κ = 0.72, density = 1730 kg m−3

kg) and ammonium sulfate

(κ = 0.59, density = 1770 kg m−3

).26–28

During the periods studied, ammonium sulfate accounted

for more than 80 % of the measured inorganic mass. At times the particle was not fully

neutralized but the sensitivity of calculated κorg,CCN to particle acidity was shown to be very low.

An assumption made in the analysis was that all particles were internally mixed. Molecular O:C

ratios were estimated from the fraction of the total organic signal measured at m/z 44 (f44), using

the empirical relationship formulated by Aiken et al.29

The AMS measurements were averaged

over the period of time required to complete the corresponding CCN activation scans.


Sample CCN activation curves from both gas- and aqueous-phase OH oxidation experiments are

shown in Figure 4.2. For all experiments, the critical superstaturation (Sc) decreased for samples

subjected to oxidation, compared to the control samples. The average κorg,CCN values for the

TPOT experiments were smaller than those for the aqueous-phase samples as there may be some

larger, less soluble organic species in the ambient particles compared to the water-soluble

component. The CCN activation curves for particles subjected to aqueous-phase OH oxidation

are steeper compared to those from gas-phase OH oxidation for two reasons. First, the aqueous

reactor is expected to produce more uniform OH exposures as the solution was constantly stirred.

Second, the extraction and dissolution process is likely to homogenize the samples.

78

Figure 4.2. Activated particle fraction (CCN/CN) plotted as a function of the water

supersaturation (SS) for unoxidized and oxidized ambient particles by gas-phase OH oxidation

(Dm = 75 nm; July 22, 2010, 00:02–02:50) (red circle) and aqueous-phase OH oxidation (Dm =

100 nm; July 8, 2010) (blue square).

For all the experiments considered, the O:C ratios increased after oxidation (see auxiliary

material for detailed results on all experiments, including sample aerosol mass spectra). The

mass fraction of m/z 44 (predominantly CO2+) to the total organic component (f44), an indicator

of highly oxidized species, increased while the mass fraction of m/z 43 (predominantly C2H3O+)

to the total organic component (f43), an indicator of less oxidized species, decreased slightly.30

For the gas-phase OH oxidation experiments, the changes in hygroscopicity can arise from

conversion of insoluble organic species to more soluble species, changes in the properties of

dissolved constituents, the loss of volatile components that are less hygroscopic, and the

condensation of oxidation products onto the aerosol. While it is possible there was some

condensation of oxidation products, an average decrease in the total organic mass of 10 % was

observed and so changes in hygroscopicity due to this process were likely to be minor. Note that

the small increase in the mass fraction of sulfate to total particle mass (fSO42−

) due to oxidation

are not large enough to explain the overall increase in κtotal,CCN observed.

Figure 4.3 shows the relationship between O:C to κorg,CCN. Literature values of other oxidation-

CCN measurements, mostly from lab studies, are included for comparison. We note that other

79

studies have examined this relationship through the hygroscopic growth factor (HGF 95

%).14,18,19

The κorg,CCN and O:C values from a laboratory oxidative study of O3-α-pinene SOA are

included; specifically, the “lamp” and “OH” flow tube conditions from Figure 9a of George and

Abbatt22

provided the best comparison of experimental conditions to the no OH and OH

oxidation conditions employed in this study. The κorg,CCN values from oxidation of bis-2-

ethylhexyl sebacate (BES) were also included as a model of primary organic aerosol,31

although

we recognize that this is a different starting material from the monoterpene. The corresponding

O:C ratios for BES were calculated from the measured f44 values, using the relationship in

Figure A1 from Lambe et al.15

Results from other lab CCN studies of α-pinene SOA are included

to further evaluate the generality of the relationship of κorg,CCN and O:C.14,32

Also included in the

plot are two relationships between κorg,CCN and O:C, i.e., from Chang et al.,21

which was derived

from CCN closure at a rural field site under the postulate of a direct linear relationship between

the two quantities (solid green line), and from Lambe et al.,15

which was derived from various

laboratory generated organic aerosol (solid blue line).

The uncertainties in the calculated values of O:C and κorg,CCN are mainly influenced by the

systematic errors associated with each measurement, including the aerosol's chemical

composition (AMS)33

, instrument supersaturation (CCNC), particle size selection (DMA), and

aerosol number (CPC). The uncertainties reported for the empirical relationship formulated by

Aiken et al.29

to calculate O:C ratios from f44 were also considered. For this study, the average

uncertainty was estimated to be 15 % for κorg,CCN and 10 % for O:C ratios. The results obtained

from the gas- and aqueous-phase OH oxidation of ambient organic aerosol follow a direct

positive relationship between O:C and κorg,CCN, as observed in previous lab studies. Also, the

CCN closure result from Chang et al.21

is consistent with the other measurements. Given the

range of potential organic aerosol precursors in the field measurements, it is not surprising that

there is some scatter in this plot that combines both lab and field data. Also, to match best with

the field data, we note that only the data from Massoli et al.14

and Lambe et al.32

that arise from

the α-pinene precursor are included (as single points). Nevertheless, correspondence between the

two quantities appears clear, extending down to low values of O:C. Thus, this study validates the

postulate made earlier by Chang et al.21

that there is a connection between the two quantities.

Indeed, the correspondence to the results from Whistler to those from Chang et al.21

may arise

because biogenic SOA from monoterpene precursors was present at the CCN closure field site.34

80

It is important to note, as mentioned by Jimenez et al.,18

the κorg,CCN and overall composition

from a variety of precursors may become increasingly similar with increasing O:C. Also, a

similar molecular weight for many of the SOA precursors in Figure 4.3 will tend to lead to

oxidation products of similar molecular weight, and thus similar hygroscopicities for the same

O:C values.

Figure 4.3. Relationship between O:C and κorg,CCN from the current study and from several

laboratory studies of model POA and biogenic SOA,14,22,31,32

and a field CCN closure study21

for

comparison. The Lambe et al.32

solid line represents a fit for SOA from a number of precursors.

For each pair of points, the samples of the lowest and highest O:C values are shown from each

study. Dashed lines are included only to indicate the relationship between these two points. The

following were the OH exposures used for the laboratory studies of: BES (3.0 × 1012

molecules

cm−3

s),31

O3-α-pinene SOA (1.3 × 1012

molecules cm−3

s),22

OH-α-pinene SOA (1.2 × 1012

molecules cm−3

s)14

and OH-α-pinene SOA (1.7 × 1012

molecules cm−3

s).32

The shaded region

illustrates the uncertainty of the κorg,CCN and O:C relationships derived from the CCN closure

study21

and laboratory generated organic aerosol.32

The uncertainties in the calculated values of

O:C and κorg,CCN are mainly influenced by the systematic errors associated with each

measurement, as discussed in the main text. Also, daily averages are shown for the gas-phase OH

oxidation experiments, and the average of 4 one-day aqueous-phase OH oxidation experiments

(results from individual experiments included in the supporting material).

81

4.5 Summary

This study is the first direct measurement of the relationship between the degree of oxygenation

and the CCN hygroscopicity of the organic component of ambient biogenic aerosol (κorg,CCN)

subject to OH oxidation, using novel methods for exposure to both gas- and aqueous-phase OH.

When combined with lab measurements, the results support a general linear relationship between

the κorg,CCN of biogenic organic aerosols and their degree of oxygenation, as postulated by Chang

et al.21

and shown previously to prevail for hygroscopic growth factors18

. The results emphasize

that OH is an important aerosol oxidant. Additional field data with onsite OH processing should

provide an indication of whether OH oxidation is the main organic aerosol aging process that

drives the increases in hygroscopicity. While empirical in nature, this relationship provides a

simple method for relating organic aerosol hygroscopicity to its composition, one that could

potentially be included in atmospheric models. This trend needs to be further explored with

ambient aerosol arising from a variety of sources, especially in urban and biomass burning

regions, given that not all lab SOA precursors follow this relationship as closely.14,32

Acknowledgements

The authors would like to thank Environment Canada, NSERC and CFCAS-CAFC for funding.


Table 4.1. Control experiments: particle properties of ambient organic aerosol. Absolute mass

(µg m-3

) of the various components is noted in the brackets. Date/Time

(GMT+8) Condition fOrg fSO4

2- fNO3

- fNH4

+ f43 f44 O:C

07/15/2010

14:21-15:00

Dark 0.75

(0.99)

0.20

(0.26)

0.03

(0.04)

0.02

(0.03) 0.1 0.1 0.47

hv 0.70

(0.78)

0.24

(0.27)

0.02

(0.02)

0.04

(0.04) 0.1 0.12 0.53

07/19/2010

09:05-09:38

Dark 0.72

(1.41)

0.20

(0.39)

0.04

(0.07)

0.05

(0.00) 0.1 0.1 0.46

Dark (O3) 0.71

(1.40)

0.20

(0.39)

0.04

(0.07)

0.05

(0.10) 0.1 0.1 0.47

82

Figure 4.4. AMS mass spectrum of organic and inorganic component of particles subjected to

“hv light” condition.

Figure 4.5. AMS mass spectrum of organic and inorganic component of particles subjected to

“OH Oxidation” condition.

83

Table 4.2. Particle properties, and CCN activity of ambient organic aerosol. Absolute mass (µg m-3

)

of the various components is noted in the brackets for the gas-phase OH oxidation experiments.

For aqueous OH oxidation experiments, 24-hr particle filter sample was obtained from 08:00 to

08:00 of the following day.

Date/Time

(GMT+8) Sample fOrg fSO4

2- fNO3

- fNH4

+ O:C

Di

(nm) Sc %

κtotal,

CCN

κorg,

CCN

Gas-phase OH Oxidation

07/21/2010

18:23-21:13

Unoxd. 0.83

(1.84)

0.12

(0.25)

0.02

(0.03)

0.04

(0.08) 0.52 75 0.425 0.18 0.12

Oxd. 0.79

(1.63)

0.15

(0.30)

0.01

(0.03)

0.05

(0.10) 0.69 75 0.369 0.24 0.17

07/21/2010

21:14-00:01

Unoxd. 0.81

(1.51)

0.13

(0.25)

0.02

(0.03)

0.04

(0.08) 0.53 75 0.421 0.18 0.11

Oxd. 0.78

(1.30)

0.16

(0.27)

0.01

(0.02)

0.05

(0.09) 0.7 75 0.374 0.23 0.15

07/22/2010

00:02-02:50

Unoxd. 0.78

(1.38)

0.15

(0.25)

0.03

(0.04)

0.04

(0.07) 0.52 75 0.397 0.21 0.12

Oxd. 0.74

(1.17)

0.19

(0.29)

0.02

(0.03)

0.05

(0.09) 0.72 75 0.334 0.29 0.21

07/25/2010

22:43-01:31

Unoxd. 0.87

(1.83)

0.08

(0.17)

0.02

(0.04)

0.02

(0.06) 0.48 75 0.43 0.17 0.13

Oxd. 0.84

(1.61)

0.10

(0.20)

0.02

(0.03)

0.02

(0.08) 0.64 75 0.397 0.21 0.16

07/25/2010

01:32-04:20

Unoxd. 0.88

(1.83)

0.08

(0.17)

0.02

(0.04)

0.02

(0.05) 0.48 75 0.43 0.17 0.13

Oxd. 0.85

(1.61)

0.10

(1.61)

0.02

(0.20)

0.04

(0.07) 0.65 75 0.397 0.22 0.16

07/26/2010

04:21-07:09

Unoxd. 0.88

(1.89)

0.08

(0.17)

0.02

(0.04)

0.02

(0.04) 0.49 75 0.416 0.19 0.14

Oxd. 0.85

(1.67)

0.10

(0.19)

0.02

(0.03)

0.02

(0.06) 0.66 75 0.383 0.22 0.17

Aqueous-phase OH Oxidation

07/06/2014 Unoxd. 0.87 0.05 0.06 0.02 0.52 100 0.24 0.25 0.2

Oxd. 0.81 0.07 0.07 0.05 0.98 100 0.33 0.34 0.29

07/08/2010 Unoxd. 0.9 0.01 0.03 0.06 0.47 100 0.15 0.16 0.13

Oxd. 0.89 0.02 0.03 0.06 0.97 100 0.24 0.25 0.23

07/10/2010 Unoxd. 0.88 0.03 0.07 0.03 0.41 100 0.24 0.23 0.18

Oxd. 0.69 0.06 0.16 0.08 0.86 100 0.2 0.36 0.26

07/19/2010 Unoxd. 0.73 0.18 0.04 0.06 0.49 100 0.23 0.27 0.17

Oxd. 0.63 0.23 0.05 0.09 0.84 100 0.2 0.34 0.22

84

4.7 References

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M. R.; Takami, A.; Middlebrook, A. M.; Sun, Y. L.; et al. Ubiquity and Dominance of

Oxygenated Species in Organic Aerosols in Anthropogenically-Influenced Northern

Hemisphere Midlatitudes. Geophys. Res. Lett. 2007, 34, L13801.



(3) Corrigan, C. E.; Novakov, T. Cloud Condensation Nucleus Activity of Organic

Compounds: A Laboratory Study. Atmos. Environ. 1999, 33, 2661–2668.



Chem. Phys. 2003, 3, 509–520.

(5) Shantz, N. C.; Leaitch, W. R.; Caffrey, P. F. Effects of Organics of Low Solubility on the

Growth Rate of Cloud Droplets. J. Geophys. Res. Atmos. 2003, 108, AAC 5–1 AAC 5–9.

(6) VanReken, T. M.; Rissman, T. A.; Roberts, G. C.; Varutbangkul, V.; Jonsson, H. H.;

Flagan, R. C.; Seinfeld, J. H. Toward Aerosol/cloud Condensation Nuclei (CCN) Closure

during CRYSTAL-FACE. J. Geophys. Res. Atmos. 2003, 108, D20.

(7) Chang, R. Y.-W.; Liu, P. S.K.; Leaitch, W. R.; Abbatt, J. P. D. Comparison between

Measured and Predicted CCN Concentrations at Egbert, Ontario: Focus on the Organic

Aerosol Fraction at a Semi-Rural Site. Atmos. Environ. 2007, 41, 8172–8182.

(8) Huff Hartz, K. E.; Rosenørn, T.; Ferchak, S. R.; Raymond, T. M.; Bilde, M.; Donahue, N.

M.; Pandis, S. N. Cloud Condensation Nuclei Activation of Monoterpene and

Sesquiterpene Secondary Organic Aerosol. J. Geophys. Res. Atmos. 2005, 110, 1–8.

(9) VanReken, T. M.; Ng, N. L.; Flagan, R. C.; Seinfeld, J. H. Cloud Condensation Nucleus

Activation Properties of Biogenic Secondary Organic Aerosol. J. Geophys. Res. Atmos.

2005, 110, D07206.

(10) King, S. M.; Rosenoern, T.; Shilling, J. E.; Chen, Q.; Martin, S. T. Cloud Condensation

Nucleus Activity of Secondary Organic Aerosol Particles Mixed with Sulfate. Geophys.

Res. Lett. 2007, 34, L24806.

(11) Prenni, A. J.; Petters, M. D.; Kreidenweis, S. M.; DeMott, P. J.; Ziemann, P. J. Cloud

Droplet Activation of Secondary Organic Aerosol. J. Geophys. Res. Atmos. 2007, 112.

(12) Duplissy, J.; Gysel, M.; Alfarra, M. R.; Dommen, J.; Metzger, A.; Prevot, A. S. H.;

Weingartner, E.; Laaksonen, A.; Raatikainen, T.; Good, N.; et al. Cloud Forming Potential

of Secondary Organic Aerosol under near Atmospheric Conditions. Geophys. Res. Lett.

2008, 35, L03818.

85

(13) Engelhart, G. J.; Asa-Awuku, A.; Nenes, A.; Pandis, S. N. CCN Activity and Droplet

Growth Kinetics of Fresh and Aged Monoterpene Secondary Organic Aerosol. Atmos.

Chem. Phys. 2008, 8, 3937–3949.

(14) Massoli, P.; Lambe, A. T.; Ahern, A. T.; Williams, L. R.; Ehn, M.; Mikkilä, J.;

Canagaratna, M. R.; Brune, W. H.; Onasch, T. B.; Jayne, J. T.; et al. Relationship between

Aerosol Oxidation Level and Hygroscopic Properties of Laboratory Generated Secondary

Organic Aerosol (SOA) Particles. Geophys. Res. Lett. 2010, 37, L24801.

(15) Lambe, A. T.; Ahern, A. T.; Williams, L. R.; Slowik, J. G.; Wong, J. P. S.; Abbatt, J. P.

D.; Brune, W. H.; Ng, N. L.; Wright, J. P.; Croasdale, D. R.; et al. Characterization of

Aerosol Photooxidation Flow Reactors: Heterogeneous Oxidation, Secondary Organic

Aerosol Formation and Cloud Condensation Nuclei Activity Measurements. Atmos. Meas.

Tech. 2011, 4, 445–461.



1971.

(17) George, I. J.; Abbatt, J. P. D. Heterogeneous Oxidation of Atmospheric Aerosol Particles

by Gas-phase Radicals. Nat. Chem. 2010, 2, 713–722.




(19) Duplissy, J.; DeCarlo, P. F.; Dommen, J.; Alfarra, M. R.; Metzger, A.; Barmpadimos, I.;

Prevot, A. S. H.; Weingartner, E.; Tritscher, T.; Gysel, M.; et al. Relating Hygroscopicity

and Composition of Organic Aerosol Particulate Matter. Atmos. Chem. Phys. 2011, 11,

1155–1165.




Atmos. Chem. Phys. 2009, 9, 5155–5236.





(22) George, I. J.; Abbatt, J. P. D. Chemical Evolution of Secondary Organic Aerosol from

OH-Initiated Heterogeneous Oxidation. Atmos. Chem. Phys. 2010, 10, 5551–5563.

(23) Schwartz, R. E.; Russell, L. M.; Sjostedt, S. J.; Vlasenko, A.; Slowik, J. G.; Abbatt, J. P.

D.; MacDonald, A. M.; Li, S. M.; Liggio, J.; Toom-Sauntry, D.; et al. Biogenic Oxidized

Organic Functional Groups in Aerosol Particles from a Mountain Forest Site and Their

Similarities to Laboratory Chamber Products. Atmos. Chem. Phys. 2010, 10, 5075–5088.

86

(24) George, I. J.; Vlasenko, A.; Slowik, J. G.; Broekhuizen, K.; Abbatt, J. P. D.

Heterogeneous Oxidation of Saturated Organic Aerosols by Hydroxyl Radicals: Uptake

Kinetics, Condensed phase Products, and Particle Size Change. Atmos. Chem. Phys. 2007,

7, 4187–4201.

(25) Lee, A. K. Y.; Herckes, P.; Leaitch, W. R.; Macdonald, A. M.; Abbatt, J. P. D. Aqueous

OH Oxidation of Ambient Organic Aerosol and Cloud Water Organics: Formation of

Highly Oxidized Products. Geophys. Res. Lett. 2011, 38, L11805.

(26) Windholz, M. The Merck Index; 10th ed.; Merck: Rahway, N.J., 1983.

(27) Clegg, S. L.; Milioto, S.; Palmer, D. A. Osmotic and Activity Coefficients of Aqueous

(NH4)2SO4 as a Function of Temperature, and Aqueous (NH4)2SO4−H2SO4 Mixtures at

298.15 K and 323.15 K. J. Chem. Eng. Data 1996, 41, 455–467.

(28) Wexler, A. S.; Clegg, S. L. Atmospheric Aerosol Models for Systems Including the Ions

H+, NH4

+, Na

+, SO4

2-, NO3

-, Cl

-, Br

-, and H2O. J. Geophys. Res. Atmos. 2002, 107, D14.

(29) Aiken, A. C.; Decarlo, P. F.; Kroll, J. H.; Worsnop, D. R.; Huffman, J. A.; Docherty, K.

S.; Ulbrich, I. M.; Mohr, C.; Kimmel, J. R.; Sueper, D.; et al. O/C and OM/OC Ratios of

Primary, Secondary, and Ambient Organic Aerosols with High-Resolution Time-of-Flight

Aerosol Mass Spectrometry. Environ. Sci. Technol. 2008, 42, 4478–4485.

(30) Ng, N. L.; Canagaratna, M. R.; Jimenez, J. L.; Chhabra, P. S.; Seinfeld, J. H.; Worsnop, D.

R. Changes in Organic Aerosol Composition with Aging Inferred from Aerosol Mass

Spectra. Atmos. Chem. Phys. 2011, 11, 6465–6474.

(31) George, I. J.; Chang, R. Y.-W.; Danov, V.; Vlasenko, A.; Abbatt, J. P. D. Modification of

Cloud Condensation Nucleus Activity of Organic Aerosols by Hydroxyl Radical

Heterogeneous Oxidation. Atmos. Environ. 2009, 43, 5038–5045.

(32) Lambe, A. T.; Onasch, T. B.; Massoli, P.; Croasdale, D. R.; Wright, J. P.; Ahern, A. T.;

Williams, L. R.; Worsnop, D. R.; Brune, W. H.; Davidovits, P. Laboratory Studies of the

Chemical Composition and Cloud Condensation Nuclei (CCN) Activity of Secondary

Organic Aerosol (SOA) and Oxidized Primary Organic Aerosol (OPOA). Atmos. Chem.

Phys. 2011, 11, 8913–8928.

(33) Bahreini, R.; Ervens, B.; Middlebrook, A. M.; Warneke, C.; De Gouw, J. A.; DeCarlo, P.

F.; Jimenez, J. L.; Brock, C. A.; Neuman, J. A.; Ryerson, T. B.; et al. Organic Aerosol

Formation in Urban and Industrial Plumes near Houston and Dallas, Texas. J. Geophys.

Res. Atmos. 2009, 114, D00F16.

(34) Slowik, J. G.; Stroud, C.; Bottenheim, J. W.; Brickell, P. C.; Chang, R. Y.-W.; Liggio, J.;

Makar, P. A.; Martin, R. V.; Moran, M. D.; Shantz, N. C.; et al. Characterization of a

Large Biogenic Secondary Organic Aerosol Event from Eastern Canadian Forests. Atmos.

Chem. Phys. 2010, 10, 2825–2845.

87

Chapter 5

5 Suppression in droplet growth kinetics by the addition of

organics to sulfate particles

Suppression in Droplet Growth Kinetics by the

Addition of Organics to Sulfate Particles

J. P. S. Wong, J. Liggio, S.-M. Li, A. Nenes, J.P. D. Abbatt, Journal of Geophysical Research

Atmospheres, 2014, 119, 12 222 – 12 232, doi:10.1002/2014JD021689.

Reprinted (adapted) with permission from Journal of Geophysical Research Atmospheres

Copyright 2014 American Geophysical Union

88

5.1 Abstract

Aerosol-cloud interactions are affected by the rate at which water vapor condenses onto particles

during cloud droplet growth. Changes in droplet growth rates can impact cloud droplet number

and size distribution. The current study investigated droplet growth kinetics of acidic and neutral

sulfate particles which contained various amounts and types of organic compounds, from model

compounds (carbonyls) to complex mixtures (α-pinene secondary organic aerosol and diesel

engine exhaust). In most cases, the formed droplet size distributions were shifted to smaller sizes

relative to control experiments (pure sulfate particles), due to suppression in droplet growth rates

in the cloud condensation nuclei counter. The shift to smaller droplets correlated with increasing

amounts of organic material, with the largest effect observed for acidic seed particles at low

relative humidity. For all organics incorporated onto acidic particles, formation of high

molecular weight compounds was observed, probably by acid-catalyzed Aldol condensation

reactions in the case of carbonyls. To test the reversibility of this process, carbonyl experiments

were conducted with acidic particles exposed to higher relative humidity. High molecular weight

compounds were not measured in this case and no shift in droplet sizes was observed, suggesting

that high molecular weight compounds are the species affecting the rate of water uptake. While

these results provide laboratory evidence that organic compounds can slow droplet growth rates,

the modeled mass accommodation coefficient of water on these particles (α > 0.1) indicates that

this effect is unlikely to significantly affect cloud properties, consistent with infrequent field

observations of slower droplet growth rates.

5.2 Introduction

Atmospheric particles affect the radiative budget of the Earth directly by absorbing and

scattering incoming radiation, or indirectly by influencing the microphysical properties,

abundance, and lifetimes of clouds. This indirect effect represents one of the largest uncertainties

in the assessment of global radiative forcing.1 The activation of aerosol particles as cloud

condensation nuclei (CCN) and their subsequent growth into cloud droplets by the condensation

of water vapor are key processes that determine the magnitude of the cloud indirect effect.2,3

The ability of particles to act as CCN depends on their size and composition.4 Given that organic

compounds constitute a large fraction of ambient particles, many laboratory studies have focused

on characterizing hygroscopic properties of model organic compounds. These studies have

89

illustrated that organic constituents are quite hygroscopic and can be of importance to cloud

formation.5–8

Since the chemical composition of ambient organic particles is highly complex,

recent studies have shown that the influence of particle composition on CCN activity can be

represented by a single hygroscopicity parameter, κ.9 Additionally, studies have confirmed that

the hygroscopicity of organics (κorg) can contribute to the CCN activity of ambient particles.10–16

While the thermodynamic properties of organic-bearing particles are becoming well understood,

much less is known regarding how organic compounds affect the rate of water vapor transport

during droplet growth (i.e., kinetic effects). Changes to the growth rate will affect cloud droplet

size distribution, thus influencing the radiative properties as well as the lifetime of clouds.3,17,18

In fact, the assumption that the activation of particles to CCN can be modeled only as an

equilibrium process has been shown to be invalid in some conditions, leading to overestimates in

cloud radiative forcing.2,3

Being highly uncertain, such kinetic limitations may arise due to slow

solute dissolution associated with highly viscous organic particle-phase19–21

or the formation of

organic films at the droplet surface.18,22

To investigate whether kinetic limitations in droplet growth rate arise due to differences in

chemical composition, previous studies compared the droplet size from the particle being studied

to that grown from calibration inorganic particles (such as ammonium sulfate) with known rapid

growth kinetics.11,23,24

Studies have also coupled these measurements with a computational fluid

dynamics model, where the effects on the droplet size can be parameterized to an effective mass

accommodation coefficient (α), to which droplet growth is highly sensitive.25–27

The effective

mass accommodation coefficient accounts for all processes that determine the probability for a

water molecule striking the particle surface being taken up by the growing droplet, including

gas-phase diffusion, interfacial transport, and bulk phase diffusion. In this regard, smaller droplet

sizes arise due to the sample particles having lower values of α (i.e., slow droplet growth

kinetics) than the standard (e.g., ammonium sulfate, α > 0.2). On the other hand, if the droplet

size is indistinguishable from the standard, they are considered to have similar α values (i.e.,

rapid growth kinetics). It is generally assumed that only particles with values of α smaller than

0.1 will significantly impact cloud formation and shortwave cloud forcing.27,28

Slow droplet growth rates have been observed in both laboratory and ambient measurements, yet

it remains unclear the extent to which suppression in growth rates occur as only a limited number

90

of studies have investigated this process. On the one hand, from the analysis of 10 ambient data

sets, Raatikainen et al.27

constrained the effective mass accommodation coefficient between the

values of 0.1 to 1, indicating that rapid growth kinetics is globally prevalent. On the other hand,

lab-generated carboxylic acid particles7 highly viscous sucrose/sodium chloride particles

29,

mineral dust particles30, and secondary organic aerosol from β-caryophylene ozonolysis

26

exhibited slow droplet growth kinetics. There are also ambient observations of kinetic limitations

where slow droplet growth rates were observed arising from different particles sources, such as

bovine emissions23

and continental particles containing anthropogenic organic components.24

These contrasting results, both from the laboratory and in the field, pose a challenge to determine

whether the kinetic limitation of droplet growth is an important process. While previous studies

have identified certain compounds and particle sources that can result in such kinetic limitations,

more work is required to further explore the droplet growth of a wider range of organic particles;

in particular, biogenic secondary organic aerosol (SOA) material, which is abundant in the

atmosphere.31

Additionally, given ambient observations that organic particles are chemically

reactive32

, it is necessary to investigate how the chemical evolution of organics affects their

droplet growth kinetics.

Given that the effects of many atmospherically relevant organic compounds on droplet growth

are currently unknown, the objective of the current study is to investigate the droplet growth

kinetics of particles composed of both inorganic hygroscopic components and various organic

compounds with different functionalities: α-pinene SOA, carbonyls, and diesel engine exhaust.

The size of droplets arising from organic-bearing sulfate particles was measured using a CCN

counter, from which droplet growth was inferred. The effects of chemical evolution of organic-

bearing particles on growth rates were also investigated. Finally, the mass accommodation

coefficient of water onto growing droplets was modeled to assess the potential atmospheric

impacts of delayed growth kinetics.

5.3 Experiment

To investigate whether the addition of organic compounds to particles affects their droplet

growth, size-selected sulfate seed particles were exposed to various organics in an environmental

chamber or flow tube. The chemical composition and droplet growth of these resulting particles

were monitored using online measurements. α-pinene, 2-pentanone, and 1-octanal (Sigma-

91

Aldrich) were introduced into a 2 m3 dark Teflon chamber (batch mode) by slowly injecting a

small volume of the liquid organic into 10 L min−1

of zero air for 10 min (final concentrations:

34–78 ppb of α-pinene, 600 ppb of 2-pentanone, and 600 ppb of 1-octanal). Sulfuric acid (SA, 10

% w/v) and ammonium sulfate (AS, 0.1 % w/v) seed particles were generated using a TSI

atomizer (Model 3076), dried using a silica gel diffusion dryer, then size selected at a mobility

diameter of 200 nm by a TSI-3081 Differential Mobility Analyzer (DMA). Size-selected

particles were introduced into the chamber until sufficient number concentrations and mass

loading for particle measurements were achieved. SA and AS were chosen as they are ubiquitous

in the atmosphere and are commonly used seed particles in laboratory chamber studies.

To generate SOA, pulses of a 4 L min−1

ozone (2 ppm) flow were introduced into the chamber

for a time period of 30 s to 2 min. Multiple pulses of ozone were injected to generate increasing

amounts of SOA material (monitored by increasing organic-to-sulfate mass ratio by the Aerosol

Mass Spectrometer, described below) and to prevent homogeneous nucleation. Particle size

distribution measurements by a Scanning Mobility Particle Sizer (SMPS) (DMA, TSI-3080, and

Condensation Particle Counter (CPC) TSI-3076) during control experiments indicated no

homogeneous nucleation occurred upon the reaction of α-pinene and ozone. Unless specified, the

relative humidity (RH) for all experiments was under 12 % in the Telfon chamber (Vaisala

HMP60 probe), i.e., the SA particles were liquid and very acidic, whereas the AS particles were

solid and neutral. Multiple experiments were conducted to ensure reproducibility.

Similar experiments utilizing diesel engine exhaust were conducted at Environment Canada's

Environmental Technology Centre in Ottawa, ON, Canada, as part of the Diesel Engine Exhaust

Research Experiment (DEERE) 2012. Specific details of the engine conditions and flow tube

setup are discussed elsewhere.33

A Jetta Turbocharged Direct Injection engine equipped with a

diesel oxidation catalyst was operated under steady state driving conditions with the engine

exhaust directly vented into a constant volume sampling system, where it was mixed with

filtered laboratory air to maintain a constant volume flow. The diluted exhaust flow was then

filtered to remove particles prior to injection into an electropolished stainless steel flow tube with

an inner diameter of 0.2 m and 2.5 m in length. Five sampling ports were located along the length

of the flow tube at equal intervals. Additional details of the flow tube are discussed elsewhere.34

Acidic sulfate particles with 1:4 and 2:5 ammonium:sulfate molar ratio were generated (TSI

3081 atomizer) and size selected (Dm = 300 nm, TSI 3081 DMA) prior to injection into the flow

92

tube. The total flow rate in the flow tube was 20 L min−1

, resulting in a total residence time of

10 min. For all the experiments considered for this study, the RH was below 20 % in the flow

tube, and samples were measured from the fifth sampling port, resulting in a residence time of

8 min.

For experiments conducted in both the Teflon chamber and the flow tube, the chemical

composition of the particles was measured online using the time-of-flight Aerosol Mass

Spectrometer (ToF-AMS, Aerodyne Research, Inc.), providing chemical composition of the non-

refractory component of particles.35,36

Both high-resolution (HR-ToF) and compact (C-ToF)

AMS instruments were used in this study; however, while the HR-ToF-AMS was operated by

alternating between V and W mode, only V mode data were analyzed from the high-resolution

version for comparable limit of detection to the C-ToF-AMS. The mode of operation of the HR-

AMS refers to the different ion paths in the ToF mass spectrometer, where V mode is more

sensitive, but offers lower mass resolution than W mode. Details of the AMS principles of

operation and typical data analysis procedures are reviewed in the literature.37

During these experiments, the CCN properties of particles were continuously measured by first

drying the particles using a silica gel diffusion dryer (residence time of 20 s), then sampling by a

Condensation Particle Counter (CPC, TSI-3075), and a Cloud Condensation Nuclei Counter

(CCNC, Droplet Measurement Technologies (DMT)-100). Particles were exposed to a maximum

water supersaturation of 0.3 % in the CCNC for the Teflon chamber experiments, and 1.0 % for

the DEERE experiments. For all experiments, 100 % of particles activated into droplets, as

measured by the ratio of the number of CCN (measured by the CCNC) to the total particle

number concentration (measured by the CPC).

To investigate droplet growth kinetics, the geometric mean diameters were calculated from the

measured water droplet size number distributions measured by the optical particle counter in the

CCNC. The calculated droplet sizes from inorganic particles with organics were compared to

those arising from bare inorganic particles (i.e., no organics). The addition of organic material

onto the sulfate seed particles increased the particle geometric mean diameter up to 10 %, as

measured by an SMPS (DMA, TSI-3080, and CPC, TSI-3076). This additional hygroscopic

material may affect droplet sizes by lowering the critical supersaturation of the particle due to

increased solute amount, whereby it activates into a droplet earlier in time, allowing for more

93

growth in the CCNC, thus resulting in larger droplets. To evaluate the effects an increased

amount of hygroscopic material would have on droplet size, the droplet sizes of particles with

varying amounts of a highly hygroscopic material were compared. This was conducted by

comparing droplet sizes arising from pure sulfuric acid seed particles with different mobility

diameters (200 – 220 nm), at similar particle number concentrations (1100 – 1250 number cm-3

).

Similar particle concentrations were used to account for water vapor depletion in the CCNC, as

discussed below. No significant differences in the resultant droplet diameters were observed due

to increased amounts (up to 30 %) of sulfuric acid. Since sulfuric acid is more hygroscopic than

the organic compounds used in this study, we infer from these control experiments that the

addition of organic material onto sulfate seed particles does not affect droplet size due to changes

in particle hygroscopicity (i.e., thermodynamic processes). From these control experiments, we

know that if the addition of hygroscopic organic material resulted in smaller droplets, then a

suppression in droplet growth rates is implied.

We note that Lathem and Nenes38

observed that there is a small dependence of droplet size on

particle number concentration in the CCNC. When particle number concentration is large

enough, the rate of water uptake increases to a point where water vapor depletion in the CCNC

occurs, affecting the supersaturation and the size growing droplets can reach. Since chamber

experiments were done in batch mode, where particle concentration decreases over time due to

wall loss, this effect must be accounted for in order to study droplet growth kinetics. This

relationship was empirically measured for pure inorganic particles (e.g., dashed lines in Figure

5.1), and this is the reference state to which the sizes of droplets grown from particles with

organics are compared. This water vapor depletion effect is consistent with the observations by

Lathem and Nenes.38

Accounting for this effect, and that all particles were exposed to identical

supersaturations and residence times in the CCNC, differences in droplet sizes were inferred to

be due to differences in droplet growth kinetics. It is important to note that we cannot distinguish

whether differences in droplet size arose due to differences in overall or initial water uptake rate.

94

Figure 5.1. Geometric mean diameter of droplets plotted as a function of seed particle number

concentration (number cm-3) for α-pinene SOA condensed onto (a) sulfuric acid and (b)

ammonium sulfate particle. Each data point and the corresponding error bars for seed particle

number concentration and droplet diameters reflect the 5 min averages of the measurements and

±1σ of measurement variability. The results for controls (bare inorganic particles) are indicated

by the dashed lines (grey area, ±1σ).


5.4.1 α-Pinene SOA

The comparison of the droplet sizes arising from bare inorganic particles (dashed line) with those

with α-pinene SOA material (colored square and circle points) is shown in Figure 5.1. From

these plots, at the same particle concentration, differences in droplet size between the bare

inorganic particle and those with organic material arise due to differences in droplet growth

kinetics. The color scale indicates the organic-to-sulfate (org:sulfate) mass ratio from AMS

measurements, as an indication of the amount of organics added to the sulfate seed particles. For

both SA and AS, the addition of SOA material (indicated by the increasing org:sulfate ratio)

resulted in smaller droplets than those of the bare inorganic particle, where this decrease in

droplet size is greater for SA than AS. The differences in droplet size arising from SA and AS

may have arisen due to differences in the amount of organics in the particles.

95

At the RH employed in these experiments (RH < 12 %), SA and AS have different liquid water

content, resulting in large differences in particle acidity as well as different physical states (i.e.,

liquid SA and effloresced AS). Previous studies have shown that particle acidity affects the

uptake of organics, where acidic particles enhanced the uptake of organics.39–41

The physical

state of particles can also affect the uptake of organic compounds, by changing their gas-particle

partitioning.42

These differences in particle properties can result in differences in the amount of

organics taken up by SA and AS. Note that for SA, the particles have an org:sulfate ratio of

approximately 0.3 prior to ozone addition, suggesting uptake of α-pinene, which was previously

observed by Liggio et al.,43

also resulted in smaller droplets in one case. For AS, slower droplet

growth was only observed following addition of ozone into the chamber, initiating the ozonolysis

of α-pinene to generate SOA material that condenses onto the particle.

Figure 5.2 further illustrates the role of organics in affecting droplet growth for the condensation

of SOA material onto SA and AS. Here droplet growth suppression was calculated as the fraction

of the droplet geometric mean diameter with (Dd) and without organics present (Dd, control), at the

same particle number concentration. For both types of particles, increasing amounts of SOA

material result in a greater suppression in droplet growth kinetics, as indicated by the decreasing

droplet size with increasing org:sulfate ratio. Given the same amount of organic material (i.e.,

same org:sulfate ratio), the suppression in droplet growth is greater when SOA material

condensed onto SA, suggesting that this enhancement in droplet growth suppression was due to

differences in the nature of the organics in the particles.

Figure 5.2. Fraction of the geometric mean diameter of droplets with (Dd) and without (Dd, control)

α-pinene SOA on sulfuric acid (red circle) and ammonium sulfate (blue square) seed particles,

plotted as a function of the org:sulfate mass ratio. The dashed line indicates no change in droplet

diameter, drawn to guide the eye.

96

A different particle composition for SA seed particles is demonstrated by the aerosol mass

spectra of the organic component during SA and AS experiments. The mass loading at each m/z

is normalized to the total organic mass to account for particle wall loss in the Telfon chamber.

Over the course of the experiments, organic fragments at higher m/z increased for SA seeds. This

can also be observed in the time series of the various particle components measured by the AMS

(Figure 5.3), where the fraction of organic mass at high m/z (forg, high m/z) increased over the course

of the experiment only for SA. The decrease in the mass loading for the various components is

due to particle loss to the chamber walls. Given the harsh ionization and fragmentation

associated with the AMS, organic fragments at high m/z likely result from the fragmentation of

even higher molecular weight species. Here higher molecular weight products are defined as

organics that resulted in fragments at m/z higher than the molecular weight of pinic acid, a

common oxidation product of α-pinene ozonolysis.44

Figure 5.3. Time series of various particle components measured by the AMS for α-pinene SOA

condensed onto (left) sulfuric acid and (right) ammonium sulfate seed particles. The decrease in

the mass loadings for the various components is due to particle loss to the chamber walls.

The formation of high molecular weight products observed in the current study can occur in both

the gas-phase45

and the particle-phase. Given that the oxidant levels and the reaction time for α-

pinene ozonolysis are similar for SA and AS, we speculate that the formation of high molecular

weight compounds occurred due to differences in particle acidity. In fact, previous studies have

shown that in acidic particles, organic compounds can undergo various condensed phase

97

reactions, generating higher molecular weight compounds/oligomers.39–41,43,46,47

In particular,

organosulfate formation has been observed following the uptake of α-pinene SOA material onto

acidic particles40,41

, and these compounds are likely to be surface active.48

Surface-active species

can form an organic film, which can potentially inhibit the uptake of water vapor, resulting in

suppression in droplet growth as observed in the current study.18,49,50

The observed slower

growth kinetics may also have arisen from the SOA material being in a glassy, highly viscous

state, which has been reported for SOA particles originating from terpene oxidation at ambient

temperature and lower RH conditions comparable to those in the current study.51,52

The exact

mechanism that resulted in the observed slow droplet growth is difficult to assess at this time.

5.4.2 Carbonyls

To further investigate the effects of acid-catalyzed reactions in the particle-phase on droplet

growth kinetics, the growth of SA and AS seeds exposed to two organic compounds with ketone

(2-pentanone) and aldehyde (1-octanal) functionalities were examined. In the presence of acids,

ketones, and aldehydes undergo Aldol condensation reactions in bulk solutions, generating

higher molecular weight compounds.53

Such reactions forming oligomers and/or high molecular

weight species have been observed previously.39,54–56

As shown in Figure 5.4, under the same

relative humidity conditions, slower droplet growth only arises when the two carbonyl

compounds were exposed to SA. For AS, since negligible amounts of particulate organic

material were measured by the AMS, the time series and the organic mass spectrum are not

shown, and the org:sulfate ratio was taken to be zero. The differences in the amount of

particulate organic were due to the fact that the uptake of these volatile carbonyl compounds is

enhanced by acid-catalyzed reactions.55,57,58

These results suggest that the presence of organic

material results in the suppression of droplet growth rates.

98


2-pentanone and 1-octanal on sulfuric acid at RH 11% (red circle) and RH 40 % (open circle), as

well as ammonium sulfate (blue square) seed particles, plotted as a function of the org:sulfate

ratio.

Since Aldol condensation reactions are reversible in the presence of water53

, the RH in the

chamber was increased to 40 % to demonstrate the effects of this reaction on droplet growth. At

higher RH, the liquid water content increases for SA particles,59

which will reverse the reaction,

decreasing the amount of high molecular weight products. As well, the acidity will be lower. As

demonstrated in Figure 5.4, for a given amount of organic material in the particles, the

suppression of droplet growth was diminished at higher RH, suggesting that the Aldol

condensation reaction forms compounds that contribute to the suppression in droplet growth

kinetics for carbonyls on SA. However, we note that it is possible that the role of water as a

plasticizer would have also given rise to the results observed at higher RH, if the particles were

less solid, leading to faster dissolution of organics compared to lower RH conditions. The AMS

time series shown in Figure 5.5 indicated that at similar org:sulfate ratios, the fraction of higher

molecular weight products is higher for SA under lower RH conditions, consistent with previous

observations.60,61

For carbonyls, higher molecular weight compounds are defined as organic

fragments having m/z higher than 1-octanal. These observations suggest that formation of higher

molecular weight compounds by acid-catalyzed reactions give rise to the suppression in droplet

growth kinetics and that the extent of suppression depends on RH.

99

Figure 5.5. Time series of various particle components measured by the AMS for carbonyls

condensed onto sulfuric acid seed particles at (left) RH 11 % and (right) 40 %.

5.4.3 Engine Exhaust

Previous ambient observations reported that particles containing organics of anthropogenic

origin exhibited suppressed droplet growth kinetics.24

The current study examined in a more

systematic manner the effects of organics on droplet growth from a complex source, i.e., vehicle

engine exhaust. In particular, the acidity of the sulfate particles was varied to further investigate

whether acid-catalyzed reactions of engine exhaust organics affect droplet growth. Following

exposure to diesel engine exhaust, suppression in droplet growth was observed for acidic sulfate

particles (Figure 5.6). Increased particle acidity did not give rise to larger suppression in droplet

growth. Comparison of the mass spectra (Figure 5.7) also suggests differences in the observed

organic components, where more organic mass at m/z > 150 was observed for sulfate particles of

lower acidity. The variability in the observations may have arisen due to variations in engine

operation conditions, which affects the composition of the exhaust62

. Nevertheless, particles with

a higher fraction of organics at high m/z (forg, high m/z) correlated with greater growth kinetics

suppression, where forg, high m/z for acidic sulfate (1:4 NH4+:SO4

2−) is 0.016 and 0.098 for acidic

sulfate (2:5 NH4+:SO4

2−). Here high molecular weight compounds are defined as giving rise to

organic fragments higher than m/z 150. These observations further suggest that higher molecular

weight organic compounds can lead to the suppression of droplet growth.

100


engine exhaust on acidic sulfate with NH4+:SO4

2− ratio of 1:4 (red circle), and NH4

+:SO4

2− ratio

of 2:5 (green circle), plotted as a function of the org:sulfate ratio.

Figure 5.7. Mass spectra of the organic component measured by the AMS for engine exhaust

condensed on acidic sulfate particles with NH4+:SO4

2- ratio of 1:4 (red) and 2:5 (green).

5.5 Interpreting the Degree of Droplet Growth Suppression

Using the coupled microphysical fluid dynamical model of the DMT CCNC63

, we modeled the

experimental results with the goal being to determine the effective mass accommodation

coefficient of water vapor onto the particles as they activate and grow into droplets. While the

data indicated clearly that smaller droplets were formed when organics were condensed onto the

sulfate cores, the degree of growth kinetics suppression will determine the overall atmospheric

impact of the results. SOA material condensed onto sulfuric acid seed particles was chosen for

modeling because the largest suppression in droplet growth kinetics was observed using this

system. The input to the model included the total pressure, temperatures, and flows of the

CCNC, as well as dry particle size distributions and assumed hygroscopicities. κ = 0.1 was used

for SOA material, although the mass accommodation coefficient was insensitive to the κ value,

consistent with the results from Raatikainen et al.27,63

Water depletion effects were fully

accounted for in the model. According to Raatikainen et al.,27,63

the value of the mass

101

accommodation coefficient, inferred from CCN size distributions, cannot be precisely

determined when shifts in droplet size are less than 0.3 µm. This was referred to as the

“unresolvable range” of mass accommodation coefficient by Raatikainen et al.63

For the system considered, the variations in droplet sizes were at most 0.3 µm. Given this, for the

system considered, the mass accommodation coefficient for water onto the growing droplets

ranges between the values of 0.1 and 1.0 – comparable to that of the standard reference particles

(ammonium sulfate). We note that the model predicts droplet size changes of 0.9 and 1.8 µm for

mass accommodation coefficients of 0.05 and 0.02, respectively. Changes of this magnitude are

clearly much larger than the measurement precision, which as shown in Figure 5.1, is very high.

As a result the data rule out mass accommodation coefficients this small.

A decrease in the mass accommodation coefficient from 1.0 to 0.1 has been modeled to result in

a relatively small (10 – 15 %) increase in the number of cloud droplets globally.27

This

observation is also consistent with laboratory measurements which show that the kinetics of

evaporation and condensation of water was negligibly influenced by the presence of an organic

film except for specific conditions (i.e., films of hydrocarbons with carbon chain length larger

than 12).50

5.6 Conclusions and Atmospheric Implications

Organic material, such as SOA, carbonyls, and diesel engine exhaust, when condensed onto

hygroscopic particles, resulted in the suppression of droplet growth kinetics, which was only

observed under low relative humidity conditions. We speculate that this was due to the formation

of higher molecular weight organic products in the particle-phase. While these experimental

observations show that the addition of organics to a highly hygroscopic core can slow the rate of

growth of particles into droplets, the modeled mass accommodation coefficient lower limit of 0.1

suggests that the observed suppression in droplet growth are not large enough that this process

will become rate limiting in the environment. In this manner, the results are fully consistent with

the analysis of field data by Raatikainen et al.27

which showed that kinetic effects larger than this

limit were infrequently observed in the environment. This conclusion is strengthened by the

observation that the strongest effects were only observed under the conditions of very high

particle acidity and low relative humidity, which only rarely prevail in the atmosphere.

102

Despite the fact that suppression in water uptake by the addition of organics to sulfate particles

appears to not be an important process governing cloud properties, the kinetic limitations of

transport driven by the presence of organic compounds observed in the current study may also

occur for the transport of other important atmospheric species. Such kinetic limitations of gas

uptake can influence the reactivity, and thus, properties of organic-containing particles. For

example, Liggio et al.34

observed that the uptake of ammonia onto acidic sulfate particles was

reduced in the presence of ambient organic gases, suggesting that ambient organic particles can

remain more acidic for longer than expected in the atmosphere. From flow tube experiments,

Shiraiwa et al.64

and Zhou et al.65,66

have shown that ozone uptake onto particles with organic

compounds can be kinetically hindered due to slow diffusion, which has implications for the

lifetime of organic compounds in the atmosphere. As such, it is important to understand whether

the uptake of other important atmospheric species, such as OH radicals, is affected by organic

compounds.

As well, we point out that this work has direct implications to smog chamber studies that study

SOA formation yields and aerosol processing given that many of these studies are performed

with aerosol seeds similar to those used in this experiment, i.e., either pure sulfuric acid or solid

ammonium sulfate, at low relative humidity. Thus, the high molecular weight organics species

and kinetics limitations in gas-phase uptake observed here may also occur in those experiments,

at least at early times.

Overall, the current study demonstrated that under specific conditions (e.g., low RH), organics

and their reactions in the particle-phase can affect the rate of water droplet growth but not to the

degree such that the process will be limiting to the growth of particles intro droplets formed in

atmospheric warm clouds. However, we note that this process was only studied for a limited

range of conditions (e.g., temperature, RH, and particle acidity) and that there may be other

atmospherically relevant conditions where droplet growth could be suppressed. Additionally,

likely arising via similar mechanisms, such kinetic limitations can be important to the gas-

particle transport for other atmospherically relevant species.

Acknowledgements

The authors thank Environment Canada, Natural Sciences and Engineering Research Council,

and Canada Foundation for Innovation for funding. We would also like to thank the Particles and

103

Related Emission Project (Project C12.007), a program administered by the Natural Resources

Canada for funding and logistical support from the staff of Emissions Measurement Research

Section of Environment Canada during DEERE2012 campaign.


Figure 5.8. Mass spectra of the organic component measured by the AMS for carbonyls

condensed onto sulfuric acid seed particles at RH 11 % (red) and 40 % (pink).

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the Evaporation and Condensation of Water in Aerosol. Proc. Natl. Acad. Sci. U. S. A.

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Chapter 6

6 Conclusions and Future Research

Conclusions and Future Research

6.1 Summary and Future Directions of SOA Formation Studies

As discussed in Section 1.2, the discrepancy in modeled and predicted SOA mass loadings may

result from numerous effects: i) SOA yields measured in the laboratory under-represent actual

yields in the atmosphere; ii) existence of unidentified precursors; and iii) unidentified formation

110

mechanisms from known precursors. One of the objectives of this thesis is to better understand

the impacts sulfate seed particle properties have on isoprene SOA formation. Using a flow tube

setup that has the capacity to control the phase of sulfate particles while maintaining all other

experimental conditions constant, we found that particle-phase water was the dominant factor

controlling the amount of SOA formed, giving rise to 60 % more mass. While we have identified

that organic acids likely contributed to the additional SOA mass formed, we were unable to

determine their molecular identity. It is critical to characterize the molecular identity of these

organic acids and their gas-phase yields from the photooxidation of isoprene in order to fully

assess their potential to contribute to ambient SOA mass. Additionally, the results from our

study suggest that sulfate particle acidity affects the chemical composition of the resulting SOA

due to acid-catalyzed condensed phase reactions. The products of these reactions, high molecular

weight compounds, have been previously shown to degrade due to aqueous OH oxidation.1

However, the kinetics of such degradation processes are uncertain, hence the atmospheric

lifetimes of these high molecular weight compounds remain unknown.

It is important to note that our results represent the SOA formation potential from the oxidation

products of isoprene under high OH exposures and low NOx conditions on sulfate seed particles

(i.e., characteristic of gas-phase oxidation products further away from isoprene sources, where

IEPOX has been degraded). The process by which IEPOX contributes to SOA mass remains

unclear. While recent studies have shown that particle acidity is the controlling factor for the

reactive uptake of IEPOX,2 its contribution to SOA mass was largely independent of acidity.

3

Whereas IEPOX is an important intermediate for isoprene formation under low NOx conditions,

it has been identified that methacryloyperoxynitrate (MPAN) is the key reactive intermediate for

isoprene SOA formation under high NOx conditions.4 The effects of sulfate seed particle

properties on the isoprene SOA formation under high NOx conditions remain unexplored.

While our results have indicated that particle acidity can be an important factor controlling SOA

chemical composition, the pH of ambient aerosols is, however, highly variable and poorly

characterized. This is largely due to the fact that one of the most common techniques used to

discern particle pH, ion balancing, has recently been demonstrated to not accurately estimate pH

levels under certain conditions.5–7

Accurate measurements of ambient particle acidity are

necessary in order to assess the extent to which particle acidity will actually control SOA

chemical composition in the atmosphere.

111

Finally, while our study focused on isoprene SOA, the effects of sulfate seed particle properties

on SOA formation from other precursors, both biogenic (e.g., α-pinene) and anthropogenic (e.g.,

toluene) remain unknown. In fact, recent work has shown that large multifunctional gas-phase

oxidation products of α-pinene may be sufficiently soluble such that they can partition

significantly to particle-phase water, contributing to SOA mass.8 Additionally, the oxidation of

other SOA precursors has previously been shown to generate organic acids,9 which as shown

from our study, can experience enhanced uptake due to the presence of particle-phase water. It is

important to note that while isoprene is the dominant biogenic SOA precursor in tropical regions,

monoterpenes (e.g., α-pinene) can contribute significantly to biogenic SOA in boreal regions.10

Also, emissions associated with fossil fuel combustion and biomass burning are likely to be the

important SOA precursors in countries such as China and India.11,12

Given the ubiquity of water as an atmospheric aerosol constituent, there is a need to investigate

SOA formation under high RH conditions such that the particle water content is representative of

that in the atmosphere.

6.2 Summary and Future Directions of SOA Aging Studies

One of the objectives of this thesis was to determine the importance of photolysis as an SOA

aging process in the atmosphere. In Chapter 3, we demonstrated that photolysis can be an

important sink of some SOA components, and that the atmospheric lifetime of SOA with respect

to photolysis is dependent on the RH cycles it experiences. The RH dependence of other aging

processes, such as heterogeneous oxidation have recently been explored. Chan et al., observed

that the aerosol phase, which is controlled by the liquid water content in the particle, affected the

kinetics of the heterogeneous OH oxidation of succinic acid particles.13

Similarly, the reactive

uptake coefficient of OH by levoglucosan particles was shown to increase by a factor of 4 with a

40 % increase in RH. Conversely, in the same study by Slade et al., a decrease in OH uptake by

methyl-nitrocatechol particles was observed with increasing RH, which can be attributed to the

accumulation of water on the surface of the particle due to low water solubility of methyl-

nitrocatechol.14

These contrasting results clearly demonstrate that more systematic studies on the

RH dependence of SOA aging processes are warranted.

From our study, we determined the average effective photolysis quantum yield for organic mass

loss to be 1.2 ± 0.2. The large quantum yield suggests that indirect photolysis likely played a role

112

but its contribution was not constrained in our work. The effects of such secondary reactions of

photolysis may be important to the fate of organic particles in the atmosphere. For example, the

photolysis of peroxides, which are known constituents of SOA,15,16

can be a potential

photochemical OH source. Additionally, recent work has shown that photosensitized reactions, a

secondary photolysis process, can be important to the SOA formation and processing.17

Further

mechanistic studies of SOA photolysis are necessary to better understand the importance of

photolysis as this area of photochemical aging remains relatively unexplored compared to other

aging processes.

It is important to note that most SOA photolysis studies were conducted using laboratory

generated SOA. Since it is well known that the chemical composition of laboratory generated

SOA is different than ambient SOA,9 it is important to photolyze ambient SOA as this is the

ultimate demonstration of the importance of photolysis as a SOA aging process in the

atmosphere.

6.3 Summary and Future Directions of SOA-CCN Studies

In Chapter 4, we have demonstrated that oxidative aging by both gas-phase and aqueous-phase

OH can increase the hygroscopicity of ambient SOA at Whistler, BC and that there is a general

linear relationship of organic hygroscopicity and the degree of oxygenation. Subsequent to our

work, Rickards et al., have compiled all relevant literature results, as illustrated in Figure 8 of

their paper, where they demonstrated that while there is indeed a general trend of increasing

organic hygroscopicity and its degree of oxidation, considerable scatter can be observed.18

This

suggests that the hygroscopic response of SOA cannot be completely captured by a single

particle bulk property. For example, different SOA precursors will different molecular weights,

where their oxidation will have different number of oxidation products (i.e., different number of

moles of solutes). Additionally, as discussed in Section 1.2.2.4, the evolution of SOA via

functionalization, fragmentation and oligomerization can result in different changes in O:C

ratios. Nevertheless, the predictive power of this linear relationship has yet to be tested using

ambient SOA (e.g., a CCN closure study using this linear relationship).

Many current studies of CCN activity of ambient aerosol, including our work described in

Chapter 4, were conducted at ground level. However, the ultimate test of our understanding of

CCN activation would be to conduct studies in situ (i.e., at cloud level) as the composition of

113

particles will be different aloft compared to ground level. Additionally, most CCN counters are

operated around room temperature (i.e., 297 K), while the temperatures at cloud level are much

lower. In this regard, the effects of temperature on CCN activation may not be well represented

in previous studies. For example, recent work has shown that the adsorption of gas-phase

surfactant onto the gas-particle interface can promote CCN activation.19

As partitioning to the

particle is favoured with decreasing temperature, this process can contribute significantly to

CCN activation at cloud level.

While the thermodynamics of the effect of organics on particle activation are generally well

understood, as discussed in Section 1.3, the impacts of organics on droplet growth kinetics

remain unclear. The contrasting results from both laboratory and ambient studies pose a

challenge to assess the importance of droplet growth kinetics in the atmosphere. One of the goals

of this thesis is to determine the impacts of organics on droplet growth kinetics by comparing

growth kinetics of water on sulfate particles containing simple mixtures of model organic

compounds (carbonyls) and complex organic mixtures (α-pinene SOA and diesel engine

exhaust). In Chapter 5, we demonstrated that in most cases, smaller droplets arise when particles

contained organics, where increasing amounts of particulate organics resulted in greater

suppression in growth kinetics. However, the modeled results indicate that the observed

suppression in droplet growth is not large enough such that this process will be rate limiting in

the atmosphere. Nevertheless, the process was only studied for a limited range of conditions

(e.g., temperature, RH, and types of organics) and there are still likely to be other conditions

such that kinetic limitations can prevail.20

Given that only a limited number of studies have

investigated the effects of organics on droplet growth kinetics, additional laboratory studies

investigating the effects of droplet growth by other types of organics and ambient measurements

are warranted. In particular, more ambient observations of such kinetic limitations are important

as they will enable an assessment of the extent to which slow droplet growth actually occurs in

the atmosphere.

6.4 References

(1) Aljawhary, D.; Lee, A. K. Y.; Abbatt, J. P. D. High-Resolution Chemical Ionization Mass

Spectrometry (ToF-CIMS): Application to Study SOA Composition and Processing.

Atmos. Meas. Tech. 2013, 6, 3211–3224.

114

(2) Gaston, C. J.; Riedel, T. P.; Zhang, Z.; Gold, A.; Surratt, J. D.; Thornton, J. A. Reactive

Uptake of an Isoprene-Derived Epoxydiol to Submicron Aerosol Particles. Environ. Sci.

Technol. 2014, 48, 11178–11186.

(3) Riedel, T. P.; Lin, Y.-H.; Budisulistiorini, S. H.; Gaston, C. J.; Thornton, J. A.; Zhang, Z.;

Vizuete, W.; Gold, A.; Surratt, J. D. Heterogeneous Reactions of Isoprene-Derived

Epoxides: Reaction Probabilities and Molar Secondary Organic Aerosol Yield Estimates.

Environ. Sci. Technol. Lett. 2015, 2, 38–42.




2009, 107, 6640-6645.

(5) Yao, X.; Yan Ling, T.; Fang, M.; Chan, C. K. Comparison of Thermodynamic Predictions

for in Situ pH in PM2.5. Atmos. Environ. 2006, 40, 2835–2844.

(6) Markovic, M. Z.; VandenBoer, T. C.; Baker, K. R.; Kelly, J. T.; Murphy, J. G.

Measurements and Modeling of the Inorganic Chemical Composition of Fine Particulate

Matter and Associated Precursor Gases in California’s San Joaquin Valley during CalNex

2010. J. Geophys. Res. Atmos. 2014, 119, 2 6853-6866.

(7) Hennigan, C. J.; Izumi, J.; Sullivan, A. P.; Weber, R. J.; Nenes, A. A Critical Evaluation

of Proxy Methods Used to Estimate the Acidity of Atmospheric Particles. Atmos. Chem.

Phys. Discuss 2014, 14, 27579–27618.

(8) Wania, F.; Lei, Y. D.; Wang, C.; Abbatt, J. P. D.; Goss, K.-U. Using the Chemical

Equilibrium Partitioning Space to Explore Factors Influencing the Phase Distribution of

Compounds Involved in Secondary Organic Aerosol Formation. Atmos. Chem. Phys.

Discuss 2014, 14, 26545–26590.




Atmos. Chem. Phys. 2009, 9, 5155–5236.




(11) Chowdhury, Z.; Zheng, M.; Schauer, J. J.; Sheesley, R. J.; Salmon, L. G.; Cass, G. R.;

Russell, A. G. Speciation of Ambient Fine Organic Carbon Particles and Source

Apportionment of PM2.5 in Indian Cities. J. Geophys. Res. Atmos. 2007, 112, D15303.

(12) Huang, R.-J.; Zhang, Y.; Bozzetti, C.; Ho, K.-F.; Cao, J.-J.; Han, Y.; Daellenbach, K. R.;

Slowik, J. G.; Platt, S. M.; Canonaco, F.; et al. High Secondary Aerosol Contribution to

Particulate Pollution during Haze Events in China. Nature 2014, 514, 218–222.

115

(13) Chan, M. N.; Zhang, H.; Goldstein, A. H.; Wilson, K. R. Role of Water and Phase in the

Heterogeneous Oxidation of Solid and Aqueous Succinic Acid Aerosol by Hydroxyl

Radicals. J. Phys. Chem. C 2014, 118, 28978-28992.

(14) Slade, J. H.; Knopf, D. A. Multiphase OH Oxidation Kinetics of Organic Aerosol: The

Role of Particle-phase State and Relative Humidity. Geophys. Res. Lett. 2014, 41, 5297–

5306.

(15) Docherty, K. S.; Wu, W.; Lim, Y. B.; Ziemann, P. J. Contributions of Organic Peroxides

to Secondary Aerosol Formed from Reactions of Monoterpenes with O3. Environ. Sci.

Technol. 2005, 39, 4049–4059.



2011, 13, 12199–12212.

(17) Sumner, A. J.; Woo, J. L.; McNeill, V. F. Model Analysis of Secondary Organic Aerosol

Formation by Glyoxal in Laboratory Studies: The Case for Photoenhanced Chemistry.

Environ. Sci. Technol. 2014, 48, 11919–11925.

(18) Rickards, A. M. J.; Miles, R. E. H.; Davies, J. F.; Marshall, F. H.; Reid, J. P.

Measurements of the Sensitivity of Aerosol Hygroscopicity and the Κ Parameter to the

O/C Ratio. J. Phys. Chem. A 2013, 117, 14120–14131.

(19) Sareen, N.; Schwier, A. N.; Lathem, T. L.; Nenes, A.; McNeill, V. F. Surfactants from the

Gas-phase May Promote Cloud Droplet Formation. Proc. Natl. Acad. Sci. 2013, 110,

2723–2728.




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