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

Abstract. In ambient air, aqueous-phase oxidation may turn precursors into more light-absorbing and toxic products, leading to air quality deterioration and adverse health effects. In this study, we investigated eugenol degradation in aqueous phase under direct photolysis, and triplet excited organic (3C*) and hydroxyl radical (OH) as oxidants. Results showed degradation rates of eugenol followed the order of 3C* > OH > direct photolysis. Relative contributions of reactive oxygen species (ROS) and 3C* were evaluated via quenching and O2-free experiments. 3C* played a dominant role in eugenol degradation for 3C*-initiated oxidation, while both O2 and O2•-generated were important for eugenol degradation for OH-initiated oxidation. Rate constants under O2, air and N2 followed the order of ko2＞kAir＞kN2 under both direct photolysis and OH oxidation, and it changed to kAir＞kN2＞ko2 in 3C*-initiated oxidation. Light absorption spectra showed absorbance at 300–400 nm increased as photolysis progressed, and there were new broad fluorescent spectra at excitation/emission (Ex/Em) = 250/(400–500) nm, suggesting the formation of new chromophores and fluorophores, such as humic-like substances (HULIS). Additionally, distinct fluorescence peaks appeared at Ex/Em=(300–350)/300 nm at different stages. Concentration of generated HULIS increased gradually over time, then leveled off. Dithiothreitol (DTT) assay was applied to assess the oxidation potential of products, which was greater than pure eugenol, suggesting more harmful species were produced during oxidation. Detailed reaction pathways were elucidated via analyses of chemical characteristics of the products.