Efficient Formation of Atmospheric Triplets and Organosulfur from Photochemistry of Non/weakly Light-absorbing Biomass Burning Tracers and S(IV)

Abstract

Biomass burning (BB) events emit S(IV) and biomass-burning organic aerosol (BBOA). While some BBOA constituents generate atmospheric oxidants through photosensitization, a major fraction of BBOA consists of phenolics with negligible or weak absorption of solar radiation. Therefore, these phenolics, together with S(IV), have generally been viewed as atmospheric substrates of other oxidants. Here, we show that their photochemical interactions constitute an efficient source of brown carbon (BrC) and organosulfur products. Under simulated solar irradiation, aqueous S(IV) photochemistry initiates the decay of non/weakly light-absorbing phenolic BB tracers, including phenols, methoxyphenols, and catechols, with methoxyphenols (guaiacol, GUA; and syringol, SYR) exhibiting the most substantial decay. S(IV) photooxidation generates sulfur-centered radicals, which trigger the formation of organosulfur and BrC products, including quinone-like photosensitizers. Scavenging experiments and kinetic modeling demonstrate that triplets (3PS*)-mediated pathways arising from BrC photosensitization dominate the methoxyphenol decay, in turn leading to new BrC formation. Model results reveal that, across varying atmospheric GUA and S(IV) precursor levels, these 3PS* reach 10–15–10–14 M, comparable to those originating from primary photosensitizers in aqueous-phase BBOA. This work identifies a previously overlooked source of photosensitizers and extends the role of non/weakly light-absorbing BB tracers from passive substrates to active contributors to atmospheric oxidative capacity.