pH-Dependent Photosensitized Generation of Multiphase •OH Radicals: Unravelling the Catalytic Mechanism

Photosensitized reactions at aerosol interfaces are pivotal to regional atmospheric oxidizing capacity, but the complex physicochemical architecture of atmospheric particles severely impedes mechanistic elucidation of photochemical pollution. We report that polycyclic aromatic hydrocarbons (PAHs) in soot photosensitization processes can function as critical catalysts driving hydroxyl radicals (•OH) formation. Photochemical experiments and quantum-chemical calculations reveal that photoexcited PAHs activate two distinct •OH generation channels. On the one hand, the conversion of excited-state PAHs to •PAH⁺ triggers the generation of •OH radicals through a proton-coupled electron-transfer (PCET) sequence. Meanwhile, the direct electron transfer between hydroxide ions (OH^-) and •PAH⁺ can also produce additional •OH radicals and reduce •PAH⁺ to reform PAHs. Further pH-dependent experiments support that the relative dominance of two different channels in aqueous microenvironments depends on the interfacial acidity. These findings deepen our understanding of aerosol photosensitization and reveal that particulate acidity, beyond modulating the surface uptake of pollutants, can also control their secondary conversion by regulating interfacial oxidation capacity.