FT-IR product study on the photo-oxidation of dimethyl sulphide in the presence of NOx—temperature dependence

The products of the OH radical-initiated oxidation of dimethyl sulphide (DMS) have been investigated as a function of temperature (284, 295, and 306 K) and different initial NOx (NO+NO2) concentrations: initial NO was varied between 434 and 2821 ppb and NO2 between 135 and 739 ppb. The experiments were performed at 1000 mbar total pressure in synthetic air using the photolysis of H2O2 as the OH-radical source and FT-IR spectroscopy to monitor reactants and products. The major sulphur-containing products identified were SO2, dimethyl sulphoxide (DMSO), dimethyl sulphone (DMSO2), methane sulphonic acid (MSA), methane sulphonyl peroxynitrate (MSPN) and OCS. The variation of the product yields with temperature and NOx concentration are consistent with the occurrence of both addition and abstraction channels in OH radical-initiated oxidation of DMS. Distinct trends in the yields of the various products have been observed as a function of temperature, initial NOx conditions and also reaction time as NO is consumed in the system. Increasing the initial NO concentration was found to depress the DMSO, SO2 and OCS formation yields and enhance those of DMSO2, MSA and MSPN. The yield–time behaviour of DMSO2 is supportive of a formation mechanism involving addition of O2 to a (CH3)2S---OH adduct, formed via the addition channel, followed by sequential reactions with NO and O2. The mechanisms controlling the concentration–time profiles of the individual products under the present experimental conditions are discussed in detail and consideration is given to possible implications for the photo-oxidation of DMS under ambient conditions.