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

Abstract

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 NO3 with 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–3 for the OH-initiated experiment and 2.4 μg m–3 for the NO3-initiated experiments, both far lower than group contribution method estimates, which ranged from 1.8 × 102 to 8.5 × 108 μg 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.