This work presents investigations on the gas-phase chemistry of dimethyl sulfide (DMS: CH₃-S-CH₃) 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 kDMS x 1011 (cm³ molecule-1 s-1) (K) ~0 mbar O₂ 205 mbar O₂ 500 mbar O₂ 299 0.50±0.10 0.78±0.18 0.95±0.19 <290 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: O₂ partial pressure (mbar) kDMS (cm³ 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 NOₓ concentrations (NO + NO₂: 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: CH₃S(O)CH₃) and sulfur dioxide (SO₂), in the absence of NOₓ, and dimethyl sulfoxide, dimethyl sulfone (DMSO₂: CH₃S(O)₂CH3) and sulfur dioxide, in the presence of NOₓ. Formation of dimethyl sulfone, methyl thiol formate (MTF: CH₃SCHO) and carbonyl sulfide (OCS), in the absence of NOₓ, and of methyl sulfonyl peroxynitrate (MSPN: CH₃S(O)₂OONO₂) and methane sulfonic acid (MSA: CH₃S(O)₂OH), in the presence of NOₓ, has also been observed. Using Ion Chromatography (IC) evidence has been found for the formation of methane sulfinic acid (MSIA: CH₃S(O)OH) and methane sulfonic acid, both in the absence and presence of NOₓ.
The variation of the product yields with temperature, NOₓ 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/DMSO₂ 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 DMSO₂ concentration. A reaction sequence involving a DMS-OH adduct and O₂ 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 O₂-dependent pathway will be DMSO with a branching ratio of ≥0.95 and DMSO₂, if formed at all, with a ratio of ≤0.05.
The work has confirmed that the reaction of CH₃S radicals with O₂ to produce CH₂S and its further oxidation is the most probable pathway for the production of carbonyl sulfide (OCS) under NOₓ-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 SO₂ was always lower than the fraction of the reaction proceeding via the O₂-independent pathway. In the NOₓ-free systems this has been attributed to the formation of products such as CH₃SCHO (methyl thiol formate), CH₃SCH₂OH (methyl thiomethanol) and possibly CH₃SCH₂OOH. Of these products only CH₃SCHO 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 NOₓ the low SO₂ yields can be explained by the formation of sulfur reservoir species such as CH₃S(O)₂OONO₂, CH₃S(O)OONO₂, CH₃S(O)ONO₂, CH₃S(O)NO₂ and CH₃S(O)NO. The low levels of SO₂ observed also support that the further oxidation of DMSO formed in the O₂-dependent pathway is not leading to formation of high yields of SO₂ 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.