Photoenhanced Uptake of SO2 and Organosulfate Formation at the Air–Aqueous Interface

Organosulfate, a common constituent of atmospheric aerosols, contributes significantly to particle mass and the secondary organic aerosol formation. While recent studies have investigated the photosensitized oxidation of SO₂─typically focusing on inorganic sulfate formation─the mechanisms underlying the multiphase formation of organosulfate remain largely unexplored. This study identifies a potential pathway for organosulfate formation via a radical–radical mechanism under highly acidic conditions, involving an interaction between organic radicals and sulfate radicals generated through photosensitization chemistry. Using photosensitizing films composed of vanillin, 4-benzoylbenzoic acid, and humic acid in a vertical wetted-wall flow tube reactor, SO₂ uptake was examined under both irradiated and dark conditions. Product analysis was conducted using ultrahigh-performance liquid chromatography coupled with high-resolution mass spectrometry. Results revealed substantial SO₂ consumption under irradiation, with uptake rates significantly enhanced compared to dark conditions, likely due to oxidation by excited triplet states. The second-order rate constants for SO₂ reacting on the irradiated films follow the order: 4-benzoylbenzoic acid (6.9 × 10⁷ M^–1 s^–1) > vanillin (4.56 × 10⁷ M^–1 s^–1), where the value for 4-BBA is the direct experimentally determined in Wang’s study (refer 21). Our findings suggest photosensitized oxidation may serve as an unrecognized pathway for gas-aqueous organosulfate formation, offering insights on previously unrecognized and overlooked OS sources, particularly those stemming from biomass burning.