Perhemiacetal formation and Cl/NO3-initiated chemistry of hydroperoxymethylthioformate (HPMTF) in atmospheric DMS oxidation†

Abstract

The emission of dimethylsulfide (DMS) is an important source of sulfur in the atmosphere. Its oxidation leads to enhanced particle formation, where OCS is a critical reaction intermediate as it can reach the stratosphere and oxidize to low-volatility H₂SO₄ acting as a condensation nucleus. The mechanism for OCS formation from DMS is currently understood to proceed through the hydroperoxymethylthioformate intermediate (HOOCH₂SCHO, HPMTF), and experimental data indicate that the OH-initiated HPMTF oxidation generates high yields of OCS. The total atmospheric OCS formation is assumed to remain limited due to competition by phase transfer of the soluble HPMTF to water droplets, but the fate of HPMTF, once it transitions to the aqueous phase, remains unclear. In this work, we theoretically study the formation of cyclic thioperhemiacetal isomers of HPMTF both in the gas phase and in acidic aqueous phase, finding that formation of thioperhemiacetal can be rapid when catalyzed by acids. The subsequent oxidation of thioperhemiacetal is shown not to form OCS, but rather lead to formic and thioformic acid, HCOOH + HCOSH. Based on these theoretical predictions we propose that thioperhemiacetal formation is the main loss process blocking OCS formation from HPMTF in the aqueous phase. To complement the models incorporating the OH-initiated HPMTF oxidation, we also theoretically predict the rate coefficients of HPMTF with Cl atoms and NO₃ radicals. The reaction with Cl is found to be fast and leads primarily to OCS, while the reaction with NO₃ is slow and does not contribute appreciably to HPMTF loss.