Modeling and measurements of oxygen isotope tracers of sulfate formation: Implications for the sulfur budget in the marine boundary layer Becky Alexander, Rokjin Park, Daniel J. Jacob, Robert M. Yantosca Harvard University, Department of Earth and Planetary Sciences Joël Savarino, Charles. C.W. Lee, Mark H. Thiemens University of California, San Diego, Department of Chemistry and Biochemsitry Mian Chin NASA Goddard Space Flight Center AGU, Fall 2003

Mass-Dependent Fractionation SMOW Rain and Cloud Water Basaltic and Sedimentary Rocks Starting O 2 Experimental: O + O 2  O 3 * Thiemens and Heidenreich 1983 Residual O 2 Product O 3 Measured: Tropospheric O 3 Mass-Independent Fractionation:  17 O/  18 O  1 Mass-Independent Fractionation Mass-Dependent Fractionation:  17 O/  18 O  0.5  17 O  30‰ Mass-Independent Fractionation:  17 O=  17 O-0.5*  18 O

Source of  17 O Sulfate SO 2 in isotopic equilibrium with H 2 O :  17 O of SO 2 = 0 ‰ 1) SO 2 + O 3 (  17 O=30-35‰)   1 7 O ~ 8-9 ‰  17 O of SO 4 2- a function relative amounts of OH, H 2 O 2, and O 3 oxidation Savarino et al., ) SO 2 + OH (  17 O=0‰)   17 O = 0 ‰ 2) SO 2 + H 2 O 2 (  17 O=1-2‰)   17 O ~ ‰ Aqueous Gas

Aqueous versus Gas Phase Oxidation Biological regulation of the climate? (Charlson et al., 1987) DMS OH NO 3 SO 2 H 2 SO 4 OH New particle formation CCN H2O2H2O2 Light scattering Gas-phase Aqueous-phase O3O3

pH dependency of O 3 oxidation and its effect on  17 O of SO 4 2- H2O2H2O2 O3O3 H2O2H2O2 O3O3 Lee et al., 2001

Pre-INDOEX Jan. 1997INDOEX March 1998 INDOEX cruises –  17 O sulfate Sulfate collected on front deck with a High Volume Air Sampler Collection time of ~48 hours per sample Laboratory conversion: SO 4 2-  O 2 Triple isotope mass-spectrometer (  17 O,  18 O)

GEOS-CHEM Global 3-D model of atmospheric chemistry and transport 4ºx5º horizontal resolution, layers in vertical Driven by assimilated meteorology Off-line sulfur chemistry (uses monthly mean OH and O 3 fields from a coupled chemistry/aerosol simulation) Includes aqueous and gas phase chemistry: S(IV) + OH (gas-phase) S(IV) + O 3 /H 2 O 2 (in-cloud, pH=4.5) S(IV) + O 3 (sea-salt aerosols, function of sea-salt alkalinity flux to the atmosphere) Modeled after results from Chamedies and Stelson (1992)

GEOS-CHEM  17 O Sulfate Simulation SO 2 + OH (gas phase)  17 O=0‰ S(IV) + H 2 O 2 (in cloud)  17 O=0.85‰ S(IV) + O 3 (in cloud, sea-salt)  17 O=8.75‰ Assume constant, global  17 O value for oxidants  17 O ‰ methodreference O3O3 35 Photochemical model Lyons 2001 H2O2H2O (1.7) Rainwater measurements Savarino and Thiemens 1999 OH0 ExperimentalDubey et al., 1997

Pre-INDOEX Sagar Kanya Cruise #120 January 1997 ITCZ

INDOEX Sagar Kanya Cruise #133 March 1998 ITCZ

0%50%100% Percent (%) change in concentrations (yearly average) Case A: SO 2 /SO 4 2- concentration without sea-salt chemistry Case B: With sea-salt chemistry SO 2 (decrease)SO 4 2- (small increase) Effect of sea-salt chemistry on SO 2 and SO 4 2- concentrations

50% 0%100% Effect of sea-salt chemistry on gas-phase sulfate production rates Mar/Apr/MayJun/Jul/Aug Sep/Oct/NovDec/Jan/Feb Percent (%) decrease (seasonal average):

Aqueous versus Gas Phase Oxidation Biological regulation of the climate? (Charlson et al., 1987) DMS OH NO 3 SO 2 H 2 SO 4 OH New particle formation CCN H2O2H2O2 Light scattering Gas-phase Aqueous-phase O3O3

Conclusions Sulfate  17 O provides information on relative oxidation pathways (gas OH versus aqueous O 3,H 2 O 2 ) in the atmosphere Measurements from INDOEX show that O 3 oxidation is an important mechanism for sulfate formation over the ocean The magnitude and trend of  17 O sulfate can be represented in the GEOS-CHEM model only with the addition of chemical formation in sea-salt aerosols Sulfate  17 O measurements provide an additional constraint for chemical transport models  improve our understanding of sulfur chemistry and the sulfur budget

Acknowledgements Funding: Measurements: NSF Modeling: NASA NOAA CGC Postdoctoral Fellowship (B. Alexander)