Photochemistry of acetylacetone: Droplets-mediated H2O2 generation and SO2 oxidation

H₂O₂-driven oxidation has been considered to be the main cause of sulfate formation in haze events. However, field observations have confirmed that the known H₂O₂ sources cannot explain its ambient levels. In this study, the photochemistry of acetylacetone (AA), a ubiquitous β-diketone in the atmosphere, was investigated under ultraviolet (UV) irradiation using droplets formed in-situ by deliquescence of hygroscopic ammonium sulfate ((NH₄)₂SO₄) and sodium dihydrogen phosphate (NaH₂PO₄). Special emphasis was placed on elucidating its contributions to H₂O₂ formation and SO₂ oxidation under varying conditions. Results show that AA photolysis significantly boosted H₂O₂ production. In (NH₄)₂SO₄ droplets, H₂O₂ peaked early after UV irradiation, while in NaH₂PO₄ droplets, it reached fourfold higher and peaked later because phosphate promotes the AA photolysis. Hydrogen atom donors like citric acid redirected radical termination from H₂O₂ to organic hydroperoxides (ROOHs). Cl⁻ and Br⁻ altered radical processes: Cl⁻ suppressed H₂O₂ production but enhanced ROOHs production, while Br⁻ exhibits stronger, concentration-dependent effects on both species. SO₂ oxidation was achieved by various oxidizing species such as H₂O₂ and ROOHs produced during AA photolysis. Key products such as pyruvic acid, peroxyacetic acid and propanedioic acid were identified and possible UV-driven reaction pathways were revealed. These results uncover new H₂O₂ production and sulfate formation pathways, and show the complexity of aerosol-phase photochemistry. This work improves our understanding of atmospheric H₂O₂ source and redox processes, offering new insights for the oxidation potential of atmospheric organic aerosols and atmospheric oxidation capacity, and enhancing atmospheric chemistry models to better predict air quality and climate.