Laboratory Studies of the Kinetics of Aqueous Phase Reactions of SO4- and OH Radicals with Organo-Sulfur Compounds of Atmospheric Interest

As dimethyl sulfide (DMS) undergoes gas-phase oxidization in the atmosphere, the products become progressively less volatile and can be partially partitioned into the condensed phase. Therefore condensed phase transformations may play an important role in the atmospheric chemistry of these species. As part of a research program aimed at developing a detailed understanding of DMS oxidation and its role in the global climate system, we are investigating the aqueous phase reactions between radicals commonly found in the atmospheric condensed phase and organo-sulfur compounds of atmospheric interest. The present study employs a Laser Flash Photolysis-Long Path UV-Visible Absorption technique to investigate the aqueous phase reactions of SO₄^- and OH radicals with the organo-sulfur species dimethyl sulfoxide (DMSO), dimethyl sulfone (DMSO2), methane sulfinate (MSI), and methane sulfonate (MS). Laser flash photolysis of either S₂O₈^2- or H₂O₂ at 248 nm is used to produce the free radicals. Reactions of SO₄^- radicals are studied by monitoring the time dependence of the SO₄^- absorption at 450 nm under pseudo-first order conditions with the organo-sulfur compounds in large excess over SO₄^-. A competitive kinetics technique is employed in the studies of OH reactions, where the reaction rates of the organo-sulfur species with OH are measured relative to the rate of the reaction of SCN^- with OH. The product (SCN)₂^- from reactions of SCN^- with OH (SCN^- + OH -> SCN + OH^-, SCN + SCN^- -> (SCN)₂^- ), whose concentration decreases with increasing concentration of the organo-sulfur species, is monitored as the kinetic probe (at 475 nm). Room temperature (293+/-1 K) rate coefficients for SO₄^- reactions with DMSO, DMSO2, MSI, and MS in units of M^-1s^-1 are found to be (2.6 +/- 0.3) x 10⁹, (3.4 +/- 0.3) x 10⁶, > 5 x 10⁸, and (1.2 +/- 0.1) x 10⁴, respectively, while OH reaction rates in the same units are found to be (5.9 +/- 1.0)x 10⁹, (1.4 +/- 0.5) x 10⁷, (1.1 +/- 0.5) x 10⁹, and (1.0 +/- 0.2) x 10⁷, respectively. Rate coefficients for the SO₄^- + DMSO2 and MS reactions have not previously been reported. Both previous studies and our studies demonstrate that DMSO and MSI are very reactive species, while DMSO2 and MS are much less reactive. The rate coefficient we report for the OH + MS reaction is much smaller than previously reported values. Temperature-dependent kinetics studies are now in progress and updated results from these studies will be reported.