Assessing the Isomerization of a Primary Intermediate (CH₃SCH₂O₂• Radical) in Dimethyl Sulfide Degradation in the Marine Boundary Layer

Abstract

Dimethyl sulfide (DMS) produced by algae is the largest biogenic emission of sulfur, and its OH-initiated Habstraction is considered a key atmospheric oxidation pathway to form CH 3 SCH 2 O 2 • radicals. Recent findings suggested that CH 3 SCH 2 O 2 • radicals undergo unexpectedly rapid isomerization to form the C•H 2 SCH 2 OOH intermediate under low-NO x conditions in the marine boundary layer (MBL). This intermediate and its reaction with O 2 lead to radical chain branching and tropospheric autoxidation, resulting in the formation of oxidized organic aerosol precursor hydroperoxymethylthioformate (HPMTF, HOOCH 2 SCHO) and OH recycling. Here, the detailed mechanism of their isomerization reaction is investigated by using the UCCSD(T)//UM06-2X/Aug-cc-pV(T+d)Z method, with an implicit emphasis on the role of quantum tunneling for fast radical isomerization, which has been studied using small curvature tunneling corrections. We investigated the kinetics using the variational transition state theory, and the results show that the reported rate coefficients agree reasonably well with the experimental values. This study provides important facts for a systematic understanding of the formation of DMS oxidation product HPMTF, a ubiquitous sulfur reservoir in the marine atmosphere, which has not been considered in previous global ocean sulfur models and OH regeneration.