Surfactants Are Influencing the Photosensitized Multiphase Chemistry of SO ₂

Abstract The air‐water interface acts as a gateway to absorption and aqueous‐phase reaction of gases in the atmosphere. Numerous aqueous organic aerosols are emitted into the troposphere by biomass burning. The surface of such aqueous organic aerosols is then potentially covered by organic surfactants, modifying its properties and hence also the photosensitized oxidation of gaseous sulfur dioxide (SO 2 ). It is therefore desirable to explore how such reactions are influenced by the presence of surfactants. First, we investigated the effect of surfactants on the uptake rate of SO 2 on vanillin, 4‐benzoylbenzoic acid, and humic acid films. The uptake coefficient of SO 2 is increased by approximately a factor of 2–3 under acidic conditions (a pH of 2), compared to that on the humic acid alone film, especially. Second, ultra‐high‐performance liquid chromatography coupled with high‐resolution mass spectrometry was employed to analyze organosulfur compounds, demonstrating the effect of the surfactant‐covered surface on the products. It was found that surfactants, by promoting the interfacial abundance of photosensitizers, facilitate the formation of organosulfur compounds, indicating that the concentration effect is a critical factor in photosensitized interfacial chemistry of aerosols between the bulk liquid water and the gas phase, as it induces abundant product formation at the air‐aqueous interface.