FT-IR product study of the photo-oxidation of dimethyl sulfide: Temperature and O2 partial pressure dependence

Abstract

The products of the OH-radical initiated oxidation of dimethyl sulfur (DMS) have been investigated under NO_x free conditions using the photolysis of H₂O₂ as the OH radical source and FT-IR spectroscopy to monitor reactants and products at 1000 mbar total pressure (N₂+O₂). The products were investigated as a function of the temperature (284, 295 and 306±2 K) and the O₂ partial pressure (20, 200 and 500 mbar). Dimethyl sulfoxide (DMSO), SO₂ and methane sulfonic acid (MSA: CH₃SO₃H) were the major sulfur-containing products and evidence has been found for the formation of methane sulfinic acid (MSIA: CH₃S(O)OH) in considerable yield. The variation of the formation of DMSO and SO₂ with temperature and O₂ partial pressure is consistent with a mechanism involving both addition and abstraction channels. The molar formation yields of DMSO support that a major fraction of the OH-DMS adduct formed in the addition channel reacts with O₂ to form DMSO. Due to the uncertainty in the literature rate coefficients used for the corrections in the formation yield of DMSO the possibility of other channels cannot be completely excluded. High overall molar formation yields of SO₂ have been measured, suggesting that further oxidation of the products in both the addition and abstraction channels result mainly in SO₂ production under the NO_x free conditions employed in the experiments. Consideration is made of the implications of the results for the atmospheric oxidation of DMS under conditions of low NO_x such as occur in the remote marine boundary layer.