Photoinduced Uptake and Oxidation of SO2 on Beijing Urban PM2.5.

Sulfate, as a major component of aerosol particles, greatly contributes to haze formation and affects global climate change. Although formation pathways of sulfate aerosols from the conversion of SO2 have been extensively studied, the discrepancy between field observations and model simulations suggests that there are still unknown sulfate sources. Herein, we report for the first time a photoinduced SO2 uptake and oxidation pathway in Beijing urban PM2.5 aerosols. In comparison with the NO2- and O3-induced SO2 oxidation pathways, this SO2 photo-oxidation in Beijing urban PM2.5 could make an important contribution to the daytime sulfate formation. Reactive species, such as •OH radicals and H2O2, are the major oxidants leading to sulfate formation in PM2.5. The water-soluble matter (WSM) and water-insoluble organic matter (WISOM) in PM2.5 were identified as the main photo-oxidant producers. Our work highlights an important daytime sulfate source in the atmosphere and provides new insight into the photochemical aging of ambient aerosols.