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

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 CH₃SCH₂O₂ radicals from the dominant H-abstraction channel. A fast CH₃SCH₂O₂ 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 CH₃SCH₂O₂ 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 CH₃SCH₂O₂ radicals undergo a two-step isomerization process finally forming a product consistent with the formula HOOCH₂SCHO. The isomerization process is accompanied by OH recycling. The rate-limiting first isomerization step, CH₃SCH₂O₂ → CH₂SCH₂OOH proceeds with k = (0.23 ± 0.12) s^-1 at 295 ± 2 K. Competing bimolecular CH₃SCH₂O₂ reactions with NO, HO₂ or RO₂ radicals are less important for trace-gas conditions over the oceans.  Results of atmospheric chemistry simulations demonstrate the predominance (≥95%) of CH₃SCH₂O₂ isomerization. The rapid peroxy radical isomerization, not yet considered in models, substantially changes the understanding of DMS´s degradation processes in the atmosphere.