SMPS

INTRODUCTION

INVESTIGATIONS OF THE HETEROGENEOUS REACTION BETWEEN AMMONIA AND

SULFURIC/SULFUROUS ACID AEROSOLSThomas Townsend, Colette Noonan and John R. Sodeau

Centre for Research into Atmospheric Chemistry, Department of Chemistry,

University College Cork, and Environmental Research Institute, Cork, Ireland.

JSodeau@chemistry.ucc.ie

Sulfuric acid aerosol is formed by the oxidation of SO2 in

gas/aqueous/aerosol phase. The preferred form of sulfuric acid

in the aerosol phase is ammonium sulfate (NH4)2SO4. If there

is not enough ammonia present, sulfuric acid exists either as

H2SO4(aq) or NH4HSO4.

10

Soluble trace gases such as Ammonia,

NH3, are produced from agricultural

sources and represents a significant

atmospheric pollutant in Ireland.

FTIR

Wet/dry H2SO4 yields the same dissociation products, bisulfate ion at 1180, 1035 and 890cm-1.

At low [H2SO4] the major “dry” particle is (NH4)2SO4/H2SO4.Varying the humidity yields the (NH4)2SO4/H2SO4/H2O particle.

At higher [H2SO4], ammonium bisulfate is formed as seen from the FTIR both wet and dry. NH3(g) is visible at 966cm-1

Sulfurous acid produces SO2 in three different phases in the 1350cm-1 region; aqueous, liquid and gas.

Bisulfite isomers (1200cm-1 and 1030cm-1) and sulfite (950cm-1) are observed on introduction of NH3.

NOx Monitor

Low [H2SO4], increase of NH3 does not affect the [NH4+]. High [H2SO4], [NH4

+] increases proportionally with increase in NH3..

The „acidic‟ bisulfate hydrogen is responsible for increased formation of [NH4+].

An increase of about 0.1ppm [NH4]+ occurs to H2SO3 at 1%wt on addition of NH3. This may be due to the formation of

ammonium sulfite which favours alkaline conditions as opposed to the HSO3-/OHSO2

- isomers (bisulfite and hydrogen sulfonate).

SMPS

NH4HSO4 particle diameter: 70nm; (NH4)2SO4 particle diameter: 200nm. Bimodal distributions are seen for 400ccm [H2SO4].

Humidity and ammonia introduction results in larger sized but smaller numbers of particles for the sulfuric acid system.

Humidity appears to increase the amount of particles for H2SO3, increased [NH3] results in smaller particle number with larger

diameters suggesting more extensive incorporation of ammonia into these acidic aerosols.

Our investigations into the reaction between

NH3 and H2SO4 aerosols, have shown that the

main products formed were, NH4HSO4 and

(NH4)2SO4. Their relative importance

depended on both flow concentration and

humidity.

Studies into sulfurous acid have shown

typical sulfur dioxide absorptions in

different phases. On addition of ammonia,

bisulfite isomers and SO32- were observed at

high humidities.

Vary the interaction times, probe mechanistic

and kinetic details.

Investigate the interactions between NH3,

H2SO4, H2SO3 and other organics such as

malonic and succinic acid.

Aerosol Flow

NH3/N2

Comp. Air

Unit

Humidifier

MCT

Exhaust

RH

To

FTIR

NH3

Analyser

SMPS

TSI 3081

P

Dilution Unit

Carrier Flow

Aerosol sampling NH3/NH4+ sampling

CatalyticConverter

Denuder

Aerosol

Generator

Aerosol Flow

NH3/N2

Comp. Air

Unit

Comp. Air

Unit

Humidifier

MCT

Exhaust

RH

To

FTIRFTIR

NH3

Analyser

SMPS

TSI 3081

SMPS

TSI 3081

P

Dilution UnitDilution Unit

Carrier Flow

Aerosol sampling NH3/NH4+ sampling

CatalyticConverter

DENUDER Coated with

oxalic acid, removes NH3(g)

Aerosol Generator

CHEMILUMINESCENCE

NOx Monitor. Catalytic

oxidation of NH3/NH4+ to

NO.

PARTICLE SIZER (SMPS)

Monitors aerosol fraction,

particle size, mass, surface

area, volume.

FTIR Digilab FTS 3000

using an MCT detector

and BaF2 windows:

monitors both gas and

aerosol condensed phase.

AEROSOL GENERATION

H2SO4 /C2H2O4 aerosol

generated by passing a flow

(200-500 ccm) of air over a

heated solution or via a

nebuliser.

RELATIVE HUMIDITY (RH)

tuned between 1% and 70%.

AMMONIA (100ppm

standard, NH3/N2) diluted to

ppb range. Admitted to flow

tube via a 6mm diameter

movable, glass injector.

AEROSOL FLOW-REACTOR

Made of glass, ID: 10cm, maximal

reactive length Z: 80cm.

Operated at room temperature and

atmospheric pressure.

EXPERIMENTAL SET-UP

NOx Monitor

FTIR

Aerosols

….are tiny particles suspended in the air. Those larger than

about 1μm in size are mainly produced by windblown dust

and sea-spray. Aerosols smaller than 1μm are mostly formed

by condensation processes e.g. conversion of SO2 gas released

from volcanic eruptions to sulfate-type particles.

RESULTS: H2SO4 System

200cc H2SO4

500cc H2SO4

400cc H2SO4

300cc H2SO4

Increasing flows of H2SO4 with 200ccm of NH3 at 30%RH

0.00E+00

1.00E+04

2.00E+04

3.00E+04

4.00E+04

5.00E+04

6.00E+04

10 100 1000Diameter (nm)

Co

nc.

(d

N#

/cm

3)

0.00E+00

5.00E+04

1.00E+05

1.50E+05

2.00E+05

2.50E+05

50

0cc

m

H2

SO

4

on

ly

200ccm 300ccm 400ccm 500ccm

Various stages of a typical experiment (500ccm H2SO4 at 30%RH)

0.00E+00

2.00E+05

4.00E+05

6.00E+05

8.00E+05

1.00E+06

1.20E+06

1.40E+06

1.60E+06

10 100 1000Diameter (nm)

Conc. (dN

#/c

m3)

Dry H2SO4 Wet H2SO4 100ccm ammonia 200ccm ammonia 300ccm ammonia

NH3 vs. [NH4+] @ various [H2SO4] (2%RH)

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350[NH3] ccm

[N

H4

+] p

pm

WET: 35%rh

DRY: 1.5%rh

H2SO4

Effects of particles:

Health implications when inhaled

Reduce and distort visibility

Climate change

Provide surfaces for “new” catalytic chemical reactions

DISCUSSION

FUTURE WORK

Sulfurous acid (H2SO3) has never been characterized or isolated on

Earth. This is caused by the unfavourable conditions for the hydration

product of SO2 (the first step of its oxidation) within the atmosphere.

Such conditions (high temperature, water content and oxidation),

however, can be found on the Jupiter moons Io and Europa.

S OO

Sulfur Dioxide

+ H2O

S

O

O

O

Bisulfite

H

OHSO2

S

Hydrogen Sulfonate

O

O O

H

HSO3

-O S

O

O-

NH4+

NH4+

Ammonium Sulfite

NH3

H

H

pH

2 - 7

pH

7 - 12

HO S

O

OH

Sulfurous Acid (H2SO3)

Size Distributions of 4%wt H2SO3 under various conditions

0.00E+00

5.00E+05

1.00E+06

1.50E+06

2.00E+06

2.50E+06

3.00E+06

3.50E+06

10 100 1000Particle Diameter (nm)

Co

nc.

(dN

#cm

3)

H2SO3 (14.6% RH) H2SO3 (32.3% RH)

H2SO3 + 100ccm NH3 (40.3% RH) H2SO3 + 300ccm NH3(42.3% RH)

SMPS data for 1%wt H2SO3 at different humidities

0.00E+00

2.00E+05

4.00E+05

6.00E+05

8.00E+05

1.00E+06

1.20E+06

1.40E+06

1.60E+06

1 10 100Particle diameter (nm)

co

nc.

(dn

#/c

m3)

11.3% RH 22.4% RH 38.1% RH 48.2% RH 54.4% RH

The aim of this work was to investigate

the interaction of ammonia with sulfuric and sulfurous acid aerosols. The

H2SO4 system was initially studied followed by the H2SO3 counterpart. A

relationship was attempted to be established by comparing the size of these

particles, FTIR spectra and ammonium production.

AIM OF THIS WORK

CONCLUSION

H2SO3 15%RH

H2SO3 + NH3 55%RH

DRY: 1.5%rh

WET: 35%rhLow H2SO4; NH3

High H2SO4;

NH3

WET: 35%rh

DRY: 1.5%rh

RESULTS: H2SO3 System

ACKNOWLEDGEMENTS!: We would like to thank the following funding

institutions for supporting this research, as well as the CRAC lab crew!