Rapid Secondary Organic Aerosol Formation at the Air–Water Interface from Methoxyphenols in Wildfire Emissions: UVA-Driven S(IV) Photooxidation to Organosulfates

Abstract. Wildfire emissions release large amounts of methoxyphenols, which serve as key precursors of aqueous-phase secondary organic aerosols (SOA). Their transformation is closely coupled with aqueous S(IV) oxidation, jointly driving the formation of sulfate and organosulfates; however, the underlying mechanisms remain poorly understood. Here, we identify a novel, metal-free mechanism for SO₄^•- generation under UVA light (370 nm), supported by experiments and quantum chemical calculations. Photolysis of the [SO₃^2-+O₂] complex yields a [SO₃^•-+O₂^•-] pair that forms peroxomonosulfate (SO₅^•-) and ultimately SO₄^•-. These radicals rapidly oxidize guaiacol, a biomass burning phenol, in bulk solution (k = 3.8 × 10¹⁰ M^-1 s^-1), producing SOA enriched in organosulfates. Microdroplet experiments show 100-fold rate enhancement due to interfacial effects. Box and global modeling indicate that this aqueous UVA pathway is a significant, previously overlooked source of sulfate. This work established a new photochemical link between S(IV) oxidation and SOA formation, with implications for aerosol composition, oxidative capacity, and climate-relevant processes.