Bulk Phase Dominates Sulfur Dioxide Hydrolysis over Interfacial Processes

Sulfur dioxide (SO₂) hydrolysis is a critical step in secondary sulfate formation, which significantly affects air quality and climate change. Since the 1980s, debate has persisted over whether this reaction occurs mainly at the air-water interface or in the bulk phase. In this study, we investigate SO₂ hydrolysis in heterogeneous systems using molecular dynamics simulations that are driven by a deep neural network potential with ab initio accuracy. In previous studies, rapid interfacial reactions have been proposed to account for the unexpectedly high SO₂ uptake coefficients. In contrast, our results reproduce the observed uptake coefficients but show that interfacial hydrolysis contributes only 1%. We find that hydrolysis is accelerated in the bulk phase, where the denser hydrogen-bond network enhances SO₂ electrophilicity and lowers the reaction barrier. The theoretical simulations in this work help to improve the understanding of aqueous sulfate aerosol formation and microdroplet chemistry.