Aqueous Phase Reaction Kinetics of Organic Sulfur Compounds of Atmospheric Interest

Abstract

Dimethyl Sulfide (CH3SCH3, DMS) is the most important natural sulfur compound emitted from the ocean and its oxidation in the atmosphere has been proposed to play an important role in climate modification because some products from DMS oxidation become non-volatile and could participate in particle formation and growth processes. Although it has been demonstrated that aqueous phase reactions are potentially important for understanding DMS oxidation, the kinetics database for aqueous phase transformations is rather limited. In this work, a laser flash photolysis (LFP) ??ng path UV-visible absorption (LPA) technique was employed to investigate the kinetics of the aqueous phase reactions of four organic sulfur compounds produced from DMS oxidation, i.e., dimethylsulfoxide (DMSO), dimethyl-sulfone (DMSO2), methanesulfinate (MSI) and methanesulfonate (MS), with four important aqueous phase radicals, OH, SO4 and #8722;, Cl and Cl2 and #8722;. The temperature-dependent kinetics of the OH and SO4 and #8722; reactions with DMSO, DMSO2 and MS were studied for the first time. OH is found to be the most reactive, while Cl2 and #8722; is the least reactive toward all the sulfur species studied. The less oxidized DMSO and MSI are found to be more reactive than the more oxidized DMSO2 and MS for each radical. The kinetic data have been employed in a Trajectory Ensemble Model to simulate DMS oxidation in the marine atmosphere as a means of assessing the contribution of aqueous phase reactions to the growth of particulate matter. For the first time, oxidation of organic sulfur compounds by SO4 and #8722;, Cl and Cl2 and #8722; are included in the model to simulate DMS chemistry. Our simulations suggest that aqueous phase reactions contribute >97% of MS and ~90% of NSS (Non-Seasalt Sulfate) production, and aqueous phase reactions of the organic sulfur compounds contribute 30% of total particle mass growth. When our kinetic data for the MS + OH reaction were used in the model, it was found that MS + OH could consume ~20% of MS and produce ~8% of NSS, within 3 days under typical marine atmospheric conditions.