Fast Peroxy Radical Isomerization and OH Recycling in the Reaction of OH Radicals with Dimethyl Sulfide

Abstract

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 SCH O radicals from the dominant H-abstraction channel. It is experimentally shown that these peroxy radicals undergo a two-step isomerization process finally forming a product consistent with the formula HOOCH 2 SCHO. The isomerization process is accompanied by OH recycling. The rate-limiting first isomerization step, CH 3 SCH 2 O 2  CH 2 SCH 2 OOH, followed by O 2 addition, proceeds with k = (0.23  0.12) s-1 at 295 ± 2 K. Competing bimolecular CH 3 SCH 2 O 2 reactions with NO, HO 2 or RO 2 radicals are less important for trace-gas conditions over the oceans. Results of atmospheric chemistry simulations demonstrate the predominance (95%) of CH 3 SCH 2 O 2 isomerization. The rapid peroxy radical isomerization, not yet considered in models, substantially changes the understanding of DMS´s degradation processes in the atmosphere.

Global airborne sampling reveals a previously unobserved dimethyl sulfide oxidation mechanism in the marine atmosphere

“…The relationships observed between HPMTF, DMS, SO 2 , and SO 4 2− strongly suggest that HPMTF is formed by DMS oxidation, a conclusion that is supported by recent theoretical and laboratory oxidation studies that identify HOOCH 2 SCHO as a product of DMS autoxidation (16,17). Production of HPMTF in the atmosphere is initiated by the H abstraction reaction of OH with DMS and subsequent molecular oxygen (O 2 ) addition to produce the methylthioxymethyl-peroxy radical (CH 3 SCH 2 OO•).…”

“…Our model results show that once formed, the fate of CH 3 SCH 2 OO• depends on the competition between isomerization to •CH 2 SCH 2 OOH and reaction with hydroperoxyl radical (HO 2 ) and nitric oxide (NO). The estimated first-order rate of 2.1 s −1 at 293 K for the first H shift in CH 3 SCH 2 OO• (16) and the rate recently determined from laboratory kinetic studies, 0.23 s −1 at 295 K (17), are considerably greater than experimental and calculated autoxidation rates for analogous peroxy radicals (24,27,28). Using the multiconformer transition state theory (MC-TST) approach of Møller et al (27) (see SI Appendix, section 5, for details), which has shown good agreement with experimentally determined H shift rate coefficients (29), we calculate the first (and rate-limiting) H shift rate to be significantly slower (0.041 s −1 at 293 K) than previously determined (16).…”

“…Here we present atmospheric observations of a newly discovered stable intermediate in the DMS oxidation process, hydroperoxymethyl thioformate (HPMTF; HOOCH 2 SCHO). HPMTF is formed from the methylthiomethylperoxy radical (CH 3 SCH 2 OO•), the primary product of the hydrogen abstraction (H abstraction) reaction of OH with DMS, through subsequent unimolecular hydrogen shifts (H shifts), a process that was initially theoretically proposed (16) and more recently validated in a laboratory study (17). HPMTF has not been previously reported in the atmosphere and therefore has not been widely incorporated into atmospheric models describing sulfur oxidation in the remote marine atmosphere.…”

Coral Reef Emissions of Atmospheric Dimethylsulfide and the Influence on Marine Aerosols in the Southern Great Barrier Reef, Australia

“…DMS a has an atmospheric residence time of ~1 day (Khan et al, 2016) before removal mechanisms such as oxidation to SO 2 occur (Andreae & Crutzen, 1997; Barnes et al, 2006). However, Berndt et al (2019) suggest that DMS oxidation can occur far more rapidly via an isomerization pathway, as does Swan et al (2016) who suggests that DMS oxidation can occur in a matter of hours over warm, well‐lit tropical coral reef waters (Swan et al, 2016), where hydroxyl concentrations are high (Andreae & Crutzen, 1997). If DMS‐derived sulfates significantly influence AOD over the southern GBR, a positive correlation is expected between daily mean DMS a and AOD during calm conditions (≤ 2 m s −1 ).…”

CAPRAM reduction towards an operational multiphase halogen and DMS chemistry treatment in the chemistry transport model COSMO-MUSCAT(5.04e)

“…According to the current knowledge, in the gas phase, the oxidation of DMS or its oxidation products through H-abstraction leads to a corresponding peroxyl radical, which can be further oxidised into an alkoxy radical. Recently, it was suggested that the methylthiomethylperoxyl radical (CH3SCH2O2) undergoes a rapid unimolecular H-shift (Wu et al, 2015;Berndt et al, 2019). The final stable product will be an oxidised organic sulfur compound that is characterised by an aldehyde and an organic hydrogen peroxide functionality.…”