HOOCH2SCHO formation in the marine boundary layer: Rate coefficient measurement of the intramolecular Hydrogen-shift in the CH3SCH2OO radical

Dimethyl sulfide (CH₃SCH₃, DMS), which is mainly emitted from the oceans, is a major source of sulfur in the atmosphere. The atmospheric oxidation of DMS leads to the formation of sulfur dioxide (SO₂), methane sulfonic acid (CH₃SO₃H, MSA), and sulfate (SO₄^2-) aerosol. In the gas phase, DMS is oxidized by hydroxyl (OH) and reactive halogens (e.g. Cl or BrO) by either H-abstraction or an addition pathway. The addition pathway mainly results in CH₃S(O)CH₃ (DMSO) formation, while the abstraction pathway will lead to peroxy radical (CH₃SCH₂O₂, MSP) formation. Ab initio calculations have predicted that MSP can undergo an intramolecular Hydrogen-shift that would eventually lead to the formation of hydroperoxymethyl thioformate (HOOCH₂SCHO, HPMTF) and regeneration of OH following O₂ addition and a more rapid second H-shift reaction. The rate coefficient for the first H-shift was calculated theoretically to be faster than the bimolecular reactions of MSP under typical NO and HO₂ concentrations in the remote marine atmosphere, i.e., under low NOx conditions. Recent measurements from the ATom field campaign have identified the presence of HPMTF in the marine boundary layer. This chemistry is currently not included in most atmospheric models despite its potential impact on the chemical fate of DMS and OH recycling. In this work, pulsed laser photolysis was used to generate OH or Cl radicals in the presence of excess DMS in a slow flow tube reactor coupled to a chemical-ionization mass-spectrometer (CIMS) to directly measure the formation of HPMTF and determine the first intramolecular H-shift rate coefficient. Measurements were performed at temperatures between 373 and 443 K and at 620 Torr (Syn. Air or O₂). The observed H-shift rate coefficients are slower than previous theoretical calculations, but support the formation of HPMTF under remote marine boundary conditions. In this presentation, the experimental approach and atmospheric implications of the H-shift chemistry will be discussed.