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

H 2 O 2 -driven oxidation has been considered to be the main cause of sulfate formation in haze events. However, field observations have confirmed that the known H 2 O 2 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 ((NH 4 ) 2 SO 4 ) and sodium dihydrogen phosphate (NaH 2 PO 4 ). Special emphasis was placed on elucidating its contributions to H 2 O 2 formation and SO 2 oxidation under varying conditions. Results show that AA photolysis significantly boosted H 2 O 2 production. In (NH 4 ) 2 SO 4 droplets, H 2 O 2 peaked early after UV irradiation, while in NaH 2 PO 4 droplets, it reached fourfold higher and peaked later because phosphate promotes the AA photolysis. Hydrogen atom donors like citric acid redirected radical termination from H 2 O 2 to organic hydroperoxides (ROOHs). Cl − and Br − altered radical processes: Cl − suppressed H 2 O 2 production but enhanced ROOHs production, while Br − exhibits stronger, concentration-dependent effects on both species. SO 2 oxidation was achieved by various oxidizing species such as H 2 O 2 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 H 2 O 2 production and sulfate formation pathways, and show the complexity of aerosol-phase photochemistry. This work improves our understanding of atmospheric H 2 O 2 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.