Heterogeneous photochemical uptake of SO2 on typical brown carbon species: A significant sulfate source

The heterogeneous reactions of SO 2 on organic aerosols (OA) have been proposed as a significant source of sulfates. The photochemical conversion of SO 2 to sulfates on the surface of light-absorbing OA, i.e., brown carbon (BrC), remained slight understanding. Herein, the photochemical uptake of SO 2 on typical BrC species, including aromatic carbonyl compounds (ACs), polycyclic aromatic hydrocarbons (PAHs) and phenols (PHs), was systematically investigated by a flow tube reactor. Light can significantly promote the uptake of SO 2 on all BrC species used here, especially for ACs. The initial and steady-state uptake coefficients (γ i and γ ss ) of SO 2 exhibited positive relationships with the light intensity, whereas they negatively correlated with the initial SO 2 concentration. Significant sulfate formation was clearly observed with in situ attenuated total internal reflection infrared (ATR-IR) spectrometer, and H 2 O and O 2 had obvious enhancement effects on the sulfate production. Photoinduced •OH, which primarily originated from •O 2 − that was generated through an electron transfer process on BrC under irradiation, was determined to be a key driver for the oxidation of SO 2 to sulfates. Finally, SO 2 lifetime and sulfate formation rate were evaluated on the basis of experimental results, highlighting potential contributions of BrC photochemistry to atmospheric SO 2 loss and sulfate sources.