Aqueous Formation and Evolution of Phenolic Secondary Organic Aerosol: Excited by NO ₂ ^– 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 (NO 2 – ) and triplet-excited organic ( 3 C*). Using integrated analysis, we systematically elucidate oxidant-specific reaction mechanisms. Both NO 2 – and 3 C* 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: 3 C* promotes triplet-state hydrogen abstraction, whereas NO 2 – degradation involves ∼46% hydroxyl radicals and the remainder mainly involves reactive nitrogen species. Product analysis indicates that NO 2 – photochemistry preferentially forms nitrophenols, whereas 3 C*-mediated oxidation favors hydroxylated products and oligomerization. These distinct reaction pathways lead to divergent functional evolution: NO 2 – -driven reactions rapidly enhance oxidative potential before stabilizing due to the reduced reactivity of nitroaromatic compounds. In contrast, the 3 C* 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.