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 - )  Cl OH - additional OH - keeps sea salt pH high high pH favors aqueous S(IV) + O 3 and therefore increases nss-SO 4 2- in sea salt but decreases SO 2 in the gas phase 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 sulfidebiogenic S from ocean S(IV) sum of: SO 2, HSO 3 -, SO 3 2- intermediate product S(VI) sum of: H 2 SO 4, HSO 4 -, SO 4 2- final products; important aerosol constituent MSA methyl sulfonic acidsemi-final product; important aerosol constituent nss-SO 4 2- non-sea-salt sulfate“S(VI)”

Sulfur cycle volcanoes industry, traffic SO 2 H 2 SO 4 CCN radiation aqueous phase oxidation S(IV)  S(VI): H 2 O 2, O 3, HOBr, HOCl cloud albedo nss-SO 4 2- DMS DMSO, SO 2, H 2 SO 4 sea salt

pH dependence of S(IV) oxidation production of nss-SO 4 2-: O 3 + S(IV): only above pH ~ 6, but then very fast H 2 O 2 + S(IV) HOCl + S(IV) HOBr + S(IV) Seinfeld and Pandis, 1998

pH of sea salt definition: pH = - log 10 [H + ] surface ocean water: pH ~ 8.1 sea salt pH buffer: –HCO H +  CO 2 + H 2 O –this consumes all acidity (H + ) until HCO 3 - is depleted, only then the aerosol pH starts changing uptake of acids like HNO 3, H 2 SO 4, 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”: –H 2 SO 4 + Cl -  HSO HCl –HNO 3 + Cl -  NO 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 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, lab study: “molecular chlorine is generated from the photolysis of ozone in the presence of sea salt” O 3 + hv + sea salt  …  Cl 2 Cl potentially important in atmosphere for oxidation of CH 4 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, lab study, molecular dynamics modeling, and kinetic modeling only detection of gas phase products new mechanism proposed: 2 (OH + Cl - )  Cl OH - (on surface) “daytime Cl conc are in good agreement with estimates based on NMHC destruction…” Knipping et al.

+ + Formation of Hydroxyl Radicals Ozone: O 3 Molecular Oxygen: O 2 Excited Oxygen Atom: O( 1 D) Water Vapor: H 2 O Hydroxyl Radical: OH Add NaCl particles to chamber Add humid air to a relative humidity above NaCl deliquescence point CE M MC 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 Q1Q3 Xe lam p photolysis lamps Spectrometer Aerosol Chamber (Top View) Add ozone Photolyze at 254 nm (generate OH radicals) Measure gaseous reactants and products using FTIR, DOAS, and API- MS. The Experiments Eladio Knipping

Molecular Dynamics Simulations of NaCl / H 2 O Possibility for Surface Chemistry? 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 + Picture Courtesy of Pavel Jungwirth and Douglas Tobias Eladio Knipping

O 3, H 2 O 2 OH O 3, H 2 O 2 OH + Cl – Known Aqueous Phase Chemistry Cl 2 Potential Surface Reactions OH Cl – + OH Cl – Cl – OHO3O3 2 OH – Cl 2 Proposed Mechanism for Cl 2 Production OH Cl – + Cl – → Cl 2 – + OH – Eladio Knipping

Jungwirth and Tobias, J. Phys. Chem. B, 2000, 104, , 105, , 106, more detailed molecular dynamics modeling polarizability of halides is reason for surface segregation Jungwirth and Tobias

J&T, 2001 J&T, 2002

Knipping and Dabdub, J. Geophys. Res., 2002, 107, paper no very detailed modeling of lab experiment: current knowledge not enough to explain lab results, proposed reaction: 2 (OH + Cl - )  Cl 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, lab studies of deliquesced NaCl that was deposited on a filter, 3800 ppm O 3, 81% rh, several hours reaction time only detection of particulate products proposed reaction: 2 (OH + Cl - )  Cl OH - surface mechanism as source of alkalinity “back of the envelope” calculations and speculations about atmospheric implications 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-SO 4 2- formation in sea salt  rapid deposition of sea salt  smaller climate effect of SO 2 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 Cl 2..” 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.” HNO 3 is more important than H 2 SO 4 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 - )  Cl OH - –base with 2 (OH + Cl - )  Cl OH - and without kinetic limitations for uptake (--)

Sander, et al.: comment base run including surface reaction without 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 NO 3 - 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 - )  Cl 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 - )  Cl OH - surface mechanism as source of alkalinity 2 comments: i) pH of aerosol ii) kinetics, model results Oum et al., 1998: lab, O 3 + hv + sea salt  …  Cl 2

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 SO 2 and formation of new CCN via: –S(IV) + H 2 O 2 –S(IV) + O 3 –S(IV) + HOBr –S(IV) + HOCl and the deposition of sea salt particles