H₂O₂-Driven Sulfate Formation at Air-Water Interfaces: Stepwise Mechanism and Accelerated Kinetics

Sulfate is a key component of fine particulate matter (PM_2.5) with a profound impact on climate and air quality. From a global perspective, H₂O₂ acts as the dominant oxidant driving sulfate production, yet its acceleration mechanism at the air-water interface has remained poorly understood. Using a series of theoretical methods, we reveal that the prereaction complex exhibits a preference at the air-water interface. This interfacial reaction predominantly proceeds through a stepwise pathway, involving the formation of a HOOSO₂^- intermediate first from the nucleophilic attack of HSO₃^- on H₂O₂ and its subsequent isomerization to sulfate, with a low rate-determining step barrier (4.1 kcal/mol). Interestingly, compared to its bulk phase, the interfacial reaction not only proceeds with a lower reaction barrier but also exhibits a shift in the rate-determining step. This distinction is attributed to the enhanced interfacial reactivity from the effects of the interfacial electric field and partial solvation environment, which account for 89% of the total barrier reduction. Our findings elucidate that the air-water interface serves as a key region for H₂O₂-driven sulfate production, especially under rising atmospheric H₂O₂ levels from global wildfires, thereby providing the molecular-level mechanistic understanding necessary for refining atmospheric aerosol models.