Sulfate Radical-Initiated Aqueous Oxidation of Biomass Burning Tracers: A Potential Source of HULIS and aqSOA

Given the limited studies on the aqueous-phase oxidation of methoxyphenols by sulfate radicals (SO4•–), this work provided the first insight into the formation of aqueous-phase secondary organic aerosols (aqSOA) and the optical properties of SO4•–-initiated aqueous oxidation of two representative methoxyphenols (guaiacol (GUA) and syringol (SYR)). Variations in the UV–vis and fluorescence spectra of GUA and SYR solutions indicated the effective formation of humic-like substances (HULIS). The optical properties were essentially unaffected by pH (3–7), whereas they were significantly influenced by nitrate ions. The aqSOA mass yields of GUA and SYR reached up to 0.79 and 0.87 at 2 h, respectively, and both decreased slightly as the reactions progressed. The oxidation degree of aqSOA was characterized by the carbon oxidation state (OSC), which increased rapidly to 0.07 for GUA and 0.21 for SYR in the initial stage and then remained almost unchanged. Additionally, the O/C ratios of aqSOA from GUA and SYR reached 0.80 and 0.87, respectively, comparable to those of ambient low-volatility oxygenated organic aerosols (LV-OOA). These results suggest that the SO4•–-initiated aqueous oxidation of methoxyphenols could serve not only as a potential source of HULIS in atmospheric aqueous environments but also as a nonignorable pathway for aqSOA formation.