Kinetics and mechanism of the oxidation of dimethyl sulfide by hydroperoxides in aqueous medium Study on the potential contribution of liquid-phase oxidation of dimethyl sulfide in the atmosphere

Conventional and multi-wavelength stopped-flow spectrophotometry has been used to study the kinetics of the oxidation of dimethyl sulfide (DMS) by hydroperoxides, ROOH=hydrogen peroxide (H 2 O 2 ), peroxo formic acid (HCO 3 H), peroxo acetic acid (CH 3 CO 3 H), peroxo nitrous acid (ONOOH), peroxo monosulfuric acid anion (PMS=HSO 5 - ), and the H 2 O 2 analogue hypochlorous acid (HOCl), in aqueous solution in the pH range 0–14 at 293 K and I=1.0 M. The reaction between DMS and ROOH and between dimethyl sulfoxide (DMSO) and ROOH is a second-order process, leading to DMSO and dimethyl sulfone (DMSO 2 ), respectively. It was shown by gas chromatography that, except for the oxidant ONOOH, DMSO and DMSO 2 are the only oxidation products. It follows from the pH dependence of the second-order rate constant k 2 that both the hydroperoxide ROOH, rate constant k ROOH , and its anion ROO - , rate constant k ROO , oxidize DMS and DMSO, respectively. The data for k ROOH (dm3 mol -1 s -1 ) and for k ROO (dm3 mol -1 s -1 ) at 293 K for the formation of DMSO and DMSO 2 are presented. For the oxidation of DMS k ROOH >k ROO ; for the step DMS→DMSO, k ROOH ranges from 4780 (PMS) to 0.018 (H 2 O 2 ), whereas k ROO lies in the range 88 (PMS) to 0.0018 (H 2 O 2 ); for the step DMSO→DMSO 2 , k ROOH ranges from 349 (HOCl) to 2.7×10 -6 (H 2 O 2 ), whereas k ROO lies in the range 18 (PMS) to 8.4×10 -5 (H 2 O 2 ). A mechanistic interpretation of the oxidation reactions, based on the ambifunctional character of both ROOH and DMSO, is presented. The relevance of in-cloud oxidation of DMS by atmospheric hydroperoxides such as CH 3 CO 3 H and HCO 3 H is discussed and substantiated.