Spontaneous sulfur redox chemistry in atmospheric aqueous microdroplets

The high electric field at the microdroplet interface plays a crucial role in regulating aqueous aerosol chemistry. However, the impact of charge at the air-water interface on sulfur-containing aerosols remains unclear. Here we show, using various microdroplet systems, how interfacial reactions of sulfur-containing aerosols proceed under different charge conditions. Field observations were also conducted across multiple regions to characterize the molecular composition of aqueous sulfur-containing aerosols. In small negatively charged droplets, sulfite is spontaneously oxidized to sulfate, with an estimated rate comparable to reported rates of sulfite oxidation by common atmospheric oxidants. Meanwhile, sulfur radicals like sulfate radicals are generated, leading to the rapid formation of organosulfates. In large positively charged droplets, sulfate is spontaneously reduced to sulfite radicals and sulfite, promoting the formation of organosulfonates. These results provide key insights into how charge at the air–water interface affects sulfur aerosols and contribute to our understanding of air pollution.