Elucidating Formation Mechanisms of Inorganic Sulfates upon OH Radical Oxidation of Organosulfates via Non-Sulfur Radical Pathways: Insights from Methyl Sulfate.

Organosulfates are major reservoirs of sulfur in atmospheric aerosols, yet the molecular steps by which hydroxyl radicals (OH) convert organosulfates into inorganic sulfate in aqueous particles remain poorly constrained. This uncertainty limits the mechanistic interpretation of organosulfate chemical aging and its representation in atmospheric models. Here, we use methyl sulfate, CH3OSO3-, the simplest organosulfate, as a model system to elucidate OH-initiated aqueous oxidation chemistry through integrated quantum-chemical calculations, in situ electron paramagnetic resonance spin trapping, and high-resolution mass spectrometry. Calculations show that OH oxidation is initiated predominantly by methyl H-abstraction rather than direct attack at the sulfate ester, producing a carbon-centered radical that rapidly adds O2 to form a peroxy radical. Subsequent peroxy-peroxy chemistry forms a tetroxide intermediate whose decomposition favors stepwise O-O bond scission to alkoxy sulfate radicals, while classical Russell and Bennett-Summers pathways are kinetically inaccessible. The key sulfate-forming step is a water-assisted, proton-coupled C-O bond cleavage of the alkoxy sulfate radical. Extended hydrogen-bond networks markedly lower the barrier for this fragmentation, enabling direct formation of bisulfate/sulfate and a formyl radical without requiring sulfite or sulfate radical anions as obligatory intermediates. Experimentally, mass spectrometry detects a BMPO-trapped methyl sulfate radical and an adduct consistent with formyl-radical formation, whereas sulfur-centered radical adducts are minor under our conditions. These combined results reveal a non-sulfur radical pathway for inorganic sulfate formation from organosulfates and highlight the central role of explicit solvation and proton transfer in aqueous aerosol sulfur chemistry.