New pathways of the reaction of OH radicals with dimethyl sulfide based on CH3SCH2O2 isomerization

<p>Dimethyl sulfide (DMS), produced by marine organisms, represents the most abundant, biogenic sulfur emission into the Earth´s atmosphere. The gas-phase degradation of DMS is mainly initiated by the reaction with the OH radical forming first CH<sub>3</sub>SCH<sub>2</sub>O<sub>2</sub> radicals from the dominant H-abstraction channel. A fast CH<sub>3</sub>SCH<sub>2</sub>O<sub>2</sub> isomerization process was proposed as a result of quantum chemical calculations. In the present study, experimental investigations on the product formation from OH + DMS have been conducted in a free-jet flow system at 295 ± 2 K and 1 bar air. Very efficient detection of CH<sub>3</sub>SCH<sub>2</sub>O<sub>2</sub> isomerization products has been achieved by iodide-CI-APi-TOF measurements allowing to run the reaction for close to atmospheric conditions. It is experimentally shown that the CH<sub>3</sub>SCH<sub>2</sub>O<sub>2</sub> radicals undergo a two-step isomerization process finally forming a product consistent with the formula HOOCH<sub>2</sub>SCHO. The isomerization process is accompanied by OH recycling. The rate-limiting first isomerization step, CH<sub>3</sub>SCH<sub>2</sub>O<sub>2</sub> → CH<sub>2</sub>SCH<sub>2</sub>OOH proceeds with k = (0.23 ± 0.12) s<sup>-1</sup> at 295 ± 2 K. Competing bimolecular CH<sub>3</sub>SCH<sub>2</sub>O<sub>2</sub> reactions with NO, HO<sub>2</sub> or RO<sub>2</sub> radicals are less important for trace-gas conditions over the oceans.  Results of atmospheric chemistry simulations demonstrate the predominance (≥95%) of CH<sub>3</sub>SCH<sub>2</sub>O<sub>2</sub> isomerization. The rapid peroxy radical isomerization, not yet considered in models, substantially changes the understanding of DMS´s degradation processes in the atmosphere.</p>