Rapid Spontaneous Generation of Organosulfur from Inorganic Sulfur in Atmospheric Microdroplets

Organosulfur compounds are important constituents of atmospheric aerosols and have been extensively studied in previous field, laboratory, and modeling investigations. However, current mechanisms cannot fully account for their atmospheric abundance. Ubiquitous in the atmosphere, micrometer-sized droplets serve as distinctive microreactors and may provide an important medium for organosulfur formation. Here, we demonstrate that reaction of inorganic sulfur with oxygenated volatile organic compounds in microdroplets can spontaneously and rapidly produce sulfonates (C-SO₃) and organosulfates (C-OSO₃) within hundreds of microseconds (~220 μs), without any catalyst, external potential, or radiation. Furthermore, some organosulfur species identified in laboratory work were detected in ambient aerosols at an urban site and a high-altitude mountain station, confirming the environmental relevance of this pathway. This transformation is driven by the strong interfacial electric field of microdroplets, which promotes the loss of one electron from SO₃^2- to form SO₃^-•. SO₃^-• subsequently undergoes nucleophilic addition and radical coupling with unsaturated oxygenated volatile organic compounds to generate organosulfur. Our findings offer a new perspective on atmospheric organosulfur formation and highlight the critical, yet previously overlooked, role of microdroplet interfaces in the formation of secondary organic aerosols.