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 (OS_C), 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.