Optical and chemical properties and oxidative potential of aqueous-phase products from OH and <sup>3</sup>C<sup>∗</sup>-initiated photooxidation of eugenol

Abstract. Aqueous reactions may turn precursors into light-absorbing and toxic products, leading to air quality deterioration adverse health effects. In this study, we comprehensively investigated eugenol photooxidation (a representative biomass-burning-emitted, highly substituted phenolic compound) in the bulk aqueous phase with direct photolysis, a hydroxyl radical (OH), an organic triplet excited state (3C∗). Results show that degradation rates of followed order 3C∗> OH > photolysis. During 3C∗-initiated oxidation, different reactive oxygen species (ROS), including 3C∗, OH, 1O2, O2⚫-, can participate oxidation eugenol, quenching experiments verified 3C∗ was most important one, while, during OH-initiated O2⚫- more ROS than for degrading eugenol. The rate constants under saturated O2, air, N2 kO2>kAir>kN2 both photolysis but changed kAir>kN2>kO2 3C∗-mediated oxidation. pH dissolved (DO) levels decreased indicating formation acids participation DO Ultraviolet visible (UV-vis) light absorption spectra reaction products showed clear absorbance enhancement 300–400 nm range all three sets experiments, new fluorescence at excitation/emission =250/ (400–500) appeared, suggesting chromophores fluorophores (brown carbon species). These were likely attributed humic-like substances (HULIS), as shown by increases HULIS concentrations Large mass yields (140 %–197 %) after 23 h illumination obtained, high degrees these also observed. Correspondingly, series oxygenated compounds identified, detailed mechanism functionalization dominant pathway proposed. At last, dithiothreitol (DTT) assay applied assess potential end higher DTT consumption those produced upon Overall, our results from using model compound, underscore importance processing biomass burning emissions secondary aerosol (SOA) formation.