Aqueous Formation and Evolution of Phenolic Secondary Organic Aerosol: Excited by NO2- and Organic Triplet Excited State.

Liquid-phase photochemical aging critically affects the composition of secondary organic aerosols (SOAs), their optical properties, and health-related reactivity. This study examines the aqueous photochemical oxidation of guaiacol in two atmosphere-relevant systems: nitrite (NO2-) and triplet-excited organic (3C*). Using integrated analysis, we systematically elucidate oxidant-specific reaction mechanisms. Both NO2- and 3C* markedly accelerate guaiacol degradation (rate constants 6.3 × 10-3 and 5.8 × 10-3 min-1) compared to direct photolysis, but via distinct mechanisms: 3C* promotes triplet-state hydrogen abstraction, whereas NO2- degradation involves ∼46% hydroxyl radicals and the remainder mainly involves reactive nitrogen species. Product analysis indicates that NO2- photochemistry preferentially forms nitrophenols, whereas 3C*-mediated oxidation favors hydroxylated products and oligomerization. These distinct reaction pathways lead to divergent functional evolution: NO2--driven reactions rapidly enhance oxidative potential before stabilizing due to the reduced reactivity of nitroaromatic compounds. In contrast, the 3C* system exhibits sustained hydroxylation and molecular coupling processes, resulting in a continuous increase in oxidative potential. By establishing oxidant-specific terminal constraints, this study shows that aqueous oxidant type controls BrC evolution and oxidation potential, providing a framework to understand SOA aging and its effects on aerosol radiative and chemical reactivity.