Formation and Evolution of Catechol-Derived SOA Mass, Composition, Volatility, and Light Absorption

Phenolic compounds emitted from wildfires contribute to secondary organic aerosol (SOA) and brown carbon (BrC) upon oxidation initiated by hydroxyl (OH) and nitrate radicals (NO₃). We conducted a set of laboratory chamber experiments to study catechol oxidation by OH and NO₃with a focus on the associated SOA formation and evolution under conditions relevant to fresh wildfire plumes. Oxidation products in both gas and particle phases as well as SOA volatility were measured using an iodide-adduct high-resolution time-of-flight chemical ionization mass spectrometer coupled with the filter inlet for gases and aerosols (FIGAERO-CIMS). Nitrocatechol (C₆H₅NO₄) was the dominant particle-phase compound in both OH-initiated and NO₃-initiated oxidation and was strongly associated with particle light absorption at 405 nm, consistent with BrC. Maximum SOA mass yields, ranging from 0.1 to 1.6 for the OH- and NO₃-driven experiments, respectively, varied with the net formation of nitrocatechol. Gas-particle partitioning measurements implied the effective saturation vapor concentration, c*, of nitrocatechol is 12 μg m^-3for the OH-initiated experiment and 2.4 μg m^-3for the NO₃-initiated experiments, both far lower than group contribution method estimates, which ranged from 1.8 × 10²to 8.5 × 10⁸μg m^-3. In extended photochemical aging experiments, wall-loss-corrected photochemical lifetimes of BrC in the chamber were 17.4 ± 0.8 and 12.4 ± 0.1 h, while particulate nitrocatechol had lifetimes of 21 ± 8 and 6.9 ± 0.6 h for OH-initiated and NO₃-initiated conditions, respectively. Implications for phenolic-derived SOA and BrC evolution in wildfire plumes are discussed.