FTIR product study of the OH initiated oxidation of dimethyl sulphide: Observation of carbonyl sulphide and dimethyl sulphoxide

The OH initiated oxidation of dimethyl sulphide (DMS: CH3SCH3) has been performed in a large photoreactor under NOx-free conditions using the 254 run photolysis of OH radicals and long path in situ FTIR to monitor reactants and products. Sulphur dioxide (SO2) and dimethyl sulphoxide (DMSO: CH3SOCH3) were the major sulphur containing products. The formation yield of SO2 was 80 ± 10% S and was sensitive to the prevailing reactor temperature. The yield of DMSO could not be accurately determined due to its rapid further oxidation with OH, however, yields of 20% S were observed in synthetic air in the initial stages of the reaction. The results support that the O2 dependent pathway in the oxidation of DMS will result primarily in the formation of DMSO. Carbonyl sulphide was also observed with a yield of 0.7 ± 0.2% S. From the experimental evidence it is concluded that OCS is being formed via the oxidation of thioformaldehyde (H2C=S) formed in a minor channel involving the reaction of CH3S radicals with O2. Although the yield is small a consideration of the source strengths of DMS reported in the literature shows that the oxidation of DMS could contribute between 0.10 and 0.28 Tg (OCS) yr−1 to the global OCS budget.