Atmospheric degradation of dimethyl sulfone mediated by [rad]OH, [rad]Cl and NO3[rad], and the C-centered dimethyl sulfone radical + 3O2 reaction: A kinetics and mechanistic study

Abstract

Mechanistic and kinetics studies of the gas phase reactions of dimethyl sulfone (DMS(O)2; CH3S(O)2CH3) mediated by hydroxyl radical (OH), chlorine atom (Cl) and nitrate radical (NO3) have been extensively investigated using quantum chemistry calculations along with kinetic modeling. The reaction of DMS(O)2 + OH/Cl/NO3 can in principle undergo abstraction and substitution pathways. The results revealed the dominant path to be abstraction of an H-atom from the methyl group of DMS(O)2 by OH, Cl and NO3, with barriers of 2.1, 4.2, and 9.8 kcal mol−1 respectively relative to their starting reactants, to produce CH2S(O)2CH3. The barrier heights for the substitution paths involved in the DMS(O)2 + OH, DMS(O)2 + Cl and DMS(O)2 + NO3 reactions were found to be very high (>30 kcal mol−1) and therefore inaccessible under tropospheric conditions. The rate coefficients for all possible H-atom abstractions associated with the DMS(O)2 + OH, DMS(O)2 + Cl and DMS(O)2 + NO3 reactions were calculated using Master equation solver for multi-energy well reactions (MESMER) code in the temperature range of 200–320 K and 1 atm. The overall rate coefficients for the reaction of DMS(O)2 initiated by OH radical, Cl atom and NO3 radical at 298 K and 1 atm were estimated to be 4.6 × 10−13, 9.1 × 10−14 and 3.7 × 10−15 cm3 molecule−1 s−1, respectively. The atmospheric lifetime of DMS(O)2 with respect to its reactions with OH, Cl and NO3 was also estimated. The major product CH2S(O)2CH3 further reacts with ground state molecular oxygen (3O2) to form the CH3S(O)2CH2OO adduct. Computational methods also showed that the rate of unimolecular isomerization of the CH3S(O)2CH2OO adduct is slow compared to its reactions with NO and hydroperoxyl (HO2) radical. Ultimately, the CH3S(O)2CH2OO adduct leads to formation of formic acid, sulfur dioxide, formaldehyde, methanol, carbon dioxide, sulfene, and OH radical as final products under high NO and HO2 radical atmospheric conditions.