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 (DM S) have been investigated as a function of temperature (284, 295, and 306 K ) and different initial NOx (NO + NO2) concentrations: initial NO was varie d 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 phot olysis of H2O2 as the OH-radical source and FT-IR spectroscopy to monitor r eactants and products. The major sulphur-containing products identified wer e SO2, dimethyl sulphoxide (DMSO), dim-ethyl sulphone (DMSO2), methane sulp honic acid (MSA), methane sulphonyl peroxynitrate (MSPN) and OCS. The varia tion of the product yields with temperature and NOx concentration are consi stent with the occurrence of both addition and abstraction channels in OH r adical-initiated oxidation of DMS. Distinct trends in the yields of the var ious products have been observed as a function of temperature, initial NOx conditions and also reaction time as NO is consumed in the system. Increasi ng 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 b ehaviour of DMSO2 is supportive of a formation mechanism involving addition Of O-2 to a (CH3)(2)S-OH adduct, formed via the addition channel, followed by sequential reactions with NO and O-2. The mechanisms controlling the co ncentration-time profiles of the individual products under the present expe rimental conditions are discussed in detail and consideration is given to p ossible implications for the photo-oxidation of DMS under ambient condition s. (C) 2001 Elsevier Science Ltd. All rights reserved.