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 metal-free, UVA-driven mechanism for sulfate radicals (SO4·-) generation at 370 nm, supported by laboratory experiments and quantum chemical calculations. Photolysis of the [SO32- +O2] complex yields a [SO3·-+O2·-] pair; the SO3·- radical subsequently reacts with O2 to form peroxomonosulfate (SO5·-), which then oxidizes S(IV) to produce SO4·-. These sulfate radicals rapidly oxidize guaiacol, a representative biomass burning phenol, in bulk solution, producing SOA enriched in organosulfates. Microdroplet experiments show ∼100-fold rate enhancement due to interfacial effects. Box modeling indicates that this aqueous UVA pathway represents a potentially important and previously underappreciated source of sulfate. This work establishes a photochemical link between S(IV) oxidation and SOA formation, with implications for aerosol composition, oxidative capacity, and climate-relevant processes.