Exploring SO2 Photooxidation to Sulfates on Organic Carbon from PM2.5: Kinetics, Mechanism, and Dependence on Physicochemical Properties

Abstract

The oxidation process of SO2 on organic carbon (OC) may act as a significant source of sulfates, whereas the photochemical conversion of SO2 to sulfates on OC in atmospheric PM2.5 is not well understood. The SO2 uptake and sulfate formation during the reaction of SO2 with OC under irradiation were systematically explored in a flow tube reactor coupled with spectroscopy and chromatography methods. Light can markedly enhance SO2 uptake and sulfate formation on OC. The photochemical reactivity of OC exhibited seasonal characteristics, as evidenced by distinct SO2 uptake coefficients and sulfate formation mass. Photogenerated electrons, •O2– and •OH, were measured, and •OH originating from OC was confirmed as the primary active species that oxidized SO2 to sulfates. The photochemical reactivity of OC greatly depended on the physicochemical properties, including light absorption properties, hydrophilicity, and •OH generation ability, which were related to the contents of chromophores (aromatic C–H and C═O) and hydrophilic groups (O–H, C–O, and C═O). According to experimental results, SO2 lifetime and sulfate formation rate during the reaction with OC were evaluated as 4.24–56.01 days and 0.04–0.34 μg·m–3·h–1, respectively, highlighting the significant role of OC in atmospheric sulfate sources.