Kinetic and Product Studies of the Hydroxyl Radical Initiated Oxidation of Dimethyl Sulfide in the Temperature Range 250 - 300 K

This work presents investigations on the gas-phase chemistry of dimethyl sulfide (DMS: CH 3 -S-CH 3 ) with hydroxyl (OH) radicals performed in a 336 l quartz glass reactor in the laboratory of the Department of Physical Chemistry of the University of Wuppertal, Germany. In this work kinetic, product and mechanistic data for the reaction of OH radicals with DMS were obtained. The investigations were aimed at achieving a better understanding of the oxidation mechanism for DMS as a function of temperature. Such an insight will help to provide a basis for understanding the atmospheric processes that link DMS photooxidation to particle formation and climate change. Relative rate coefficients were measured for the first time for the gas-phase reactions of hydroxyl radicals with dimethyl sulfide for a wide range of temperature (250 - 299 K) and different partial pressures of oxygen (~0, 205 and 500 mbar) at 1000 mbar total pressure. Using in situ Fourier transform infrared (FTIR) spectroscopy for the analyses the following averaged values of the rate coefficients for the reaction of OH radicals with DMS were obtained: Temperature k DMS x 10 11 (cm 3 molecule -1 s -1 ) (K) ~0 mbar O 2 205 mbar O 2 500 mbar O 2 299 0.50±0.10 0.78±0.18 0.95±0.19 0.57±0.11 0.98±0.23 1.34±0.27 280 0.62±0.13 1.20±0.26 1.54±0.31 270 0.63±0.14 1.51±0.34 1.85±0.37 260 0.64±0.13 1.99±0.47 2.38±0.47 250 0.66±0.14 2.82±0.77 2.83±0.62 From the kinetic data the following Arrhenius expressions for the reaction of OH radicals with DMS, valid in the temperature range 250 to 299 K, have been obtained: O 2 partial pressure (mbar) k DMS (cm 3 molecule -1 s -1 ) ~ 0 (1.56±0.20) x 10 -12 exp[(369±27) / T] 205 (1.31±0.08) x 10 -14 exp[(1910±69) / T] 500 (5.1±0.71) x 10 -14 exp[(1587±24) / T] Detailed product studies were performed on the OH radical initiated oxidation of dimethyl sulfide for different conditions of temperature (260, 270, 280, 290 and 298 K), oxygen partial pressure (~0, 250 and 500 mbar) and initial NO x concentrations (NO + NO 2 : 0 - 3215 ppbv 0 - 106 ppbv). The present work represents the first product study at low temperatures (260 - 280 K). The major sulfur-containing products identified in the OH radical initiated oxidation of DMS using in situ FTIR spectroscopy were: dimethyl sulfoxide (DMSO: CH 3 S(O)CH 3 ) and sulfur dioxide (SO 2 ), in the absence of NO x , and dimethyl sulfoxide, dimethyl sulfone (DMSO 2 : CH 3 S(O) 2 CH3) and sulfur dioxide, in the presence of NO x . Formation of dimethyl sulfone, methyl thiol formate (MTF: CH 3 SCHO) and carbonyl sulfide (OCS), in the absence of NO x , and of methyl sulfonyl peroxynitrate (MSPN: CH 3 S(O) 2 OONO 2 ) and methane sulfonic acid (MSA: CH 3 S(O) 2 OH), in the presence of NO x , has also been observed. Using Ion Chromatography (IC) evidence has been found for the formation of methane sulfinic acid (MSIA: CH 3 S(O)OH) and methane sulfonic acid, both in the absence and presence of NO x . The variation of the product yields with temperature, NO x concentration and partial pressure of oxygen is consistent with the occurrence of both addition and abstraction channels in the OH radical initiated oxidation of DMS. At all temperatures and reaction conditions studied, in the presence of oxygen, the combined yields of dimethyl sulfoxide and dimethyl sulfone are roughly equal to the fraction of the reaction proceeding via the addition channel. The DMSO/DMSO 2 yield ratio observed in the reaction system was sensitive to the initial NO concentration. Increasing NO caused a decrease in the DMSO concentration with an approximately equivalent increase in the DMSO 2 concentration. A reaction sequence involving a DMS-OH adduct and O 2 has been proposed to explain the observation. The observations, however, support that this sequence will be unimportant under most atmospheric conditions and that reaction of the DMS-OH adduct with molecular oxygen will mainly produce DMSO. Based on the work it is suggested that in the OH radical initiated photooxidation of DMS in the remote marine atmosphere, where NO levels are extremely low, the major product of the O 2 -dependent pathway will be DMSO with a branching ratio of ≥0.95 and DMSO 2 , if formed at all, with a ratio of ≤0.05. The work has confirmed that the reaction of CH 3 S radicals with O 2 to produce CH 2 S and its further oxidation is the most probable pathway for the production of carbonyl sulfide (OCS) under NO x -free experimental systems. At around room temperature the measured yield is similar to that reported in other studies; the present study, however, indicates that the process forming OCS may not be efficient at temperatures below 290 K. In all of the experimental systems the yield of SO 2 was always lower than the fraction of the reaction proceeding via the O 2 -independent pathway. In the NO x -free systems this has been attributed to the formation of products such as CH 3 SCHO (methyl thiol formate), CH 3 SCH 2 OH (methyl thiomethanol) and possibly CH 3 SCH 2 OOH. Of these products only CH 3 SCHO has been positively identified. The results support that in the remote marine atmosphere the yield of this compound in the oxidation of DMS will not be more than a few percent even at low temperatures. In the presence of high levels of NO x the low SO 2 yields can be explained by the formation of sulfur reservoir species such as CH 3 S(O) 2 OONO 2 , CH 3 S(O)OONO 2 , CH 3 S(O)ONO 2 , CH 3 S(O)NO 2 and CH 3 S(O)NO. The low levels of SO 2 observed also support that the further oxidation of DMSO formed in the O 2 -dependent pathway is not leading to formation of high yields of SO 2 under the conditions of the experiments. The results from the kinetic and product studies have been used to propose a reaction mechanism for the reaction of OH radicals with DMS which can be implemented over the temperature range 260 - 306 K.