Laskin et al.“Reactions at interfaces as a source of sulfate formation in

sea-salt particles”

Science, 301, 340 – 344, 2003

Roland von Glasow

• Idea of paper

• Sulfur cycle and sea salt aerosol

• History of the paper

• The paper and the comments to it

Outline

• OH reacts with Cl- at surface of sea salt aerosol:

2(OH + Cl-) Cl2 + 2 OH-

• additional OH- keeps sea salt pH high

• high pH favors aqueous S(IV) + O3 and therefore increases nss-SO4

2- in sea salt but decreases SO2 in the gas phase

Idea of paper

• Idea of paper

• Sulfur cycle and sea salt aerosol

• History of the paper

• The paper and the comments to it

Outline

Some terms

DMS dimethyl sulfide biogenic S from ocean

S(IV) sum of: SO2, HSO3-, SO3

2- intermediate product

S(VI) sum of: H2SO4, HSO4-, SO4

2- final products; important aerosol constituent

MSA methyl sulfonic acid semi-final product; important aerosol constituent

nss-SO42- non-sea-salt sulfate “S(VI)”

Sulfur cycle

volcanoes industry, traffic

SO2

H2SO4

CCN

radiation

aqueous phase oxidation S(IV) S(VI):

H2O2, O3, HOBr, HOCl

cloud albedo

nss-SO42-

DMS

DMSO, SO2, H2SO4 sea salt

pH dependence of S(IV) oxidation

production of nss-SO4

2-:

• O3 + S(IV): only above pH ~ 6, but then very fast

• H2O2 + S(IV)

• HOCl + S(IV)• HOBr + S(IV)

Seinfeld and Pandis, 1998

pH of sea salt• definition: pH = - log10[H+]• surface ocean water: pH ~ 8.1• sea salt pH buffer:

– HCO3- + H+ CO2 + H2O

– this consumes all acidity (H+) until HCO3- is depleted, only then

the aerosol pH starts changing

• uptake of acids like HNO3, H2SO4, HCl decrease pH rapidly• sea salt pH function of particle age and size• “auto-acidification” of young sea salt by old sea salt via HCl• additionally “acid displacement”:

– H2SO4 + Cl- HSO4- + HCl

– HNO3 + Cl- NO3- + HCl

pH determinations• indirect (acid balance):

– Bermuda, “moderately polluted”, pH of super-micron aerosol: 3.5 – 4.5– Hawaii, “clean”, pH of sub-micron aerosol: 2.6 – 5.3, super-micron aerosol:

4.5 – 5.4– East Coast of US, “moderately polluted – polluted”, sub-micron aerosol: 1.5

– 2, super-micron aerosol 2 – 3.5

• direct (on minimally diluted filter extracts):– East Coast of US, “moderately polluted – polluted”, sub-micron aerosol:

(2.5), super-micron aerosol 3 - 4

• however: analytics require sampling times of >12h

Keene and Savoie (1998,1999), Pszenny et al. (2004), Keene et al. (2004)

• Idea of paper

• Sulfur cycle and sea salt aerosol

• History of the paper

• The paper and the comments to it

Outline

• Oum, Lakin, DeHaan, Brauers, Finlayson-Pitts, Science, 1998, 279, 74-77

• lab study: “molecular chlorine is generated from the photolysis of ozone in the presence of sea salt”

O3 + hv + sea salt … Cl2• Cl potentially important in atmosphere for oxidation of CH4

and many NMHCs• however: the proposed mechanism cannot work under

atmospheric conditions• see e.g. the rejected comment by Rolf Sander

Oum et al.

• Knipping, Lakin, Foster, Jungwirth, Tobias, Gerber, Dabdub, Finlayson-Pitts, Science, 2000, 288, 301-306

• lab study, molecular dynamics modeling, and kinetic modeling

• only detection of gas phase products• new mechanism proposed:

2 (OH + Cl-) Cl2 + 2 OH- (on surface)

• “daytime Cl conc are in good agreement with estimates based on NMHC destruction…”

Knipping et al.

++

++

Formation of Hydroxyl Radicals

Formation of Hydroxyl Radicals

Ozone: O3

Ozone: O3

Molecular

Oxygen: O2

Molecular

Oxygen: O2

Excited Oxygen Atom: O(1D)

Excited Oxygen Atom: O(1D)

Water Vapor:

H2O

Water Vapor:

H2OHydroxy

l Radical:

OH

Hydroxyl

Radical: OH

Hydroxyl

Radical: OH

Hydroxyl

Radical: OH

•Add NaCl particles to chamber

•Add humid air to a relative humidity above NaCl deliquescence point

•Add NaCl particles to chamber

•Add humid air to a relative humidity above NaCl deliquescence point

CEMMC

T

water regulated temperature

control

CPC

DMA

gas inlet

P, T, %RH

560L Stainless Steel and Aluminum Chamber

FTIR

Differential Optical Absorption Spectroscopy (DOAS)

Aerosol Generation and Measurement

Atmospheric Pressure Ionization Mass Spec (API-MS)

Spectrometer

Q1 Q3

Xelamp

photolysis lamps

Spectrometer

Aerosol Chamber

(Top View)

Aerosol Chamber

(Top View)

•Add ozone•Photolyze at 254 nm (generate OH radicals)•Measure gaseous reactants and products using FTIR, DOAS, and API-MS.

•Add ozone•Photolyze at 254 nm (generate OH radicals)•Measure gaseous reactants and products using FTIR, DOAS, and API-MS.

The ExperimentsThe Experiments Eladio Knipping

Molecular Dynamics Simulations of NaCl / H2O

Molecular Dynamics Simulations of NaCl / H2O

Possibility for Surface Chemistry?Possibility for Surface Chemistry?

Snapshot of the open surface of an infinite “slab” consisting of 96 NaCl and 864 water molecules per unit cell.

Snapshot of the open surface of an infinite “slab” consisting of 96 NaCl and 864 water molecules per unit cell.

Model predicted surface coverage:

•11.9% Cl-

•<0.2% Na+

Model predicted surface coverage:

•11.9% Cl-

•<0.2% Na+

Picture Courtesy of Pavel Jungwirthand Douglas Tobias

Picture Courtesy of Pavel Jungwirthand Douglas Tobias

Eladio Knipping

O3 , H2O2O3 , H2O2 OHOH

O3 , H2O2O3 , H2O2

OH + Cl– OH + Cl–

Known Aqueous

Phase Chemistry

Known Aqueous

Phase Chemistry

Cl2Cl2 Cl2Cl2

Potential Surface Reactions

Potential Surface Reactions

OH•Cl–

+ OH•Cl–

OH•Cl–

+ OH•Cl–

Cl– Cl– OHOHO3O3

2 OH– 2 OH– Cl2Cl2

Proposed Mechanism for Cl2 ProductionProposed Mechanism for Cl2 Production

OH•Cl– + Cl–

→ Cl2– + OH–

OH•Cl– + Cl–

→ Cl2– + OH–

Eladio Knipping

• Jungwirth and Tobias, J. Phys. Chem. B, 2000, 104, 7702-7706

2001, 105, 10468-10472 2002, 106, 6361-

6373

• more detailed molecular dynamics modeling• polarizability of halides is reason for surface

segregation

Jungwirth and Tobias

Jungwirth and Tobias

J&T, 2001

J&T, 2002

• Knipping and Dabdub, J. Geophys. Res., 2002, 107, paper no. 4360

• very detailed modeling of lab experiment: current knowledge not enough to explain lab results, proposed reaction:

2 (OH + Cl-) Cl2 + 2 OH- (on surface)

• “contribution of interfacial mechanism to chloride deficits measured in the atmosphere is minimal”

Knipping and Dabdub

• Idea of paper

• Sulfur cycle and sea salt aerosol

• History of the paper

• The paper and the comments to it

Outline

• Laskin, Gaspar, Wang, Hunt, Cowin, Colson, Finlayson-Pitts, Science, 2003, 301, 340-344

• lab studies of deliquesced NaCl that was deposited on a filter, 3800 ppm O3, 81% rh, several hours reaction time

• only detection of particulate products• proposed reaction:

2 (OH + Cl-) Cl2 + 2 OH-

surface mechanism as source of alkalinity

• “back of the envelope” calculations and speculations about atmospheric implications

Laskin et al.

Laskin et al.

unreacted NaCl

reacted NaCl

Laskin et al.

unreacted NaCl

reacted NaCl

Cl : Na

O : Na

esp. small but also supermicron particles lose Cl and gain O:

• “in the MBL the NaOH generated in this reaction will provide a previously unrecognized buffering mechanism”

• buffering more nss-SO42- formation

in sea salt

rapid deposition of sea salt

smaller climate effect of SO2

Laskin et al.: main idea

• regarding unexplained large Cl- depletion in measurements of sea salt aerosol: “ an alternative explanation is the mechanism proposed here, in which chlorine is displaced from the interface as Cl2..”

• however: Knipping and Dabdub, 2002: “contribution of interfacial mechanism to chloride deficits measured in the atmosphere is minimal”

Laskin et al.: other idea

• “However, measurements indicate that acidification rates are greater and pHs lower than those inferred and, consequently, the influence on S(VI) production was substantially overestimated.”

• HNO3 is more important than H2SO4 in acidifying sea salt in clean areas

• none of their samples ever indicated alkalinity production in sea salt aerosol (from polluted to clean environments)

Keene and Pszenny: comment

Sander, et al.: comment• “Their extrapolation to atmospheric conditions,

however, neglected to include gas-phase diffusion limitations. The proposed reaction is not important for regulating sea-salt aerosol pH and sulfate production in the marine troposphere.”

• neglect of gas phase diffusion limitations for OH uptake in “back of the envelope” calculations: 10x too fast

• model runs– base (--)– base with 2 (OH + Cl-) Cl2 + 2 OH- – base with 2 (OH + Cl-) Cl2 + 2 OH- and without kinetic

limitations for uptake (--)

Sander, et al.: comment

base run

including surface reactionwithout gas-phase diffusion

Laskin et al.: reply

• pH: measurements often not in clean air, their idea doesn’t affect final pH only its temporal evolution

• NO3- is also enriched at interface and its

photolysis might be an important OH source without gas phase limitations

Summary of paper history

Knipping et al., 2000: lab (only detection of gas phase), MD model, kinetic model:2 (OH + Cl-) Cl2 + 2 OH- surface mechanism as source of Cl

Knipping and Dabdub, 2002: more kinetic modeling

Jungwirth and Tobias, 200x: more MD modeling

Laskin et al., 2003: lab (only detection of particulate phase)2 (OH + Cl-) Cl2 + 2 OH- surface mechanism as source of alkalinity2 comments: i) pH of aerosol

ii) kinetics, model results

Oum et al., 1998: lab, O3 + hv + sea salt … Cl2

Conclusions

• there is still a lot to do to understand sea salt pH• surface reactions have a great potential• oxidation of S(IV) in sea salt does decrease gas

phase SO2 and formation of new CCN via:– S(IV) + H2O2

– S(IV) + O3

– S(IV) + HOBr– S(IV) + HOCl

and the deposition of sea salt particles