Enhanced oxidative potential and SO₂ heterogeneous oxidation on candle soot after photochemical aging: Influencing mechanisms of different irradiation wavelengths

Photochemistry plays a significant role in the atmospheric aging processes of soot. However, the physicochemical properties and changes in environmental and health effects of soot particles from sacrificial sources after photochemical aging remain unclear. The reaction mechanisms of soot under different irradiation wavelengths require further investigation. In this study, candle soot from sacrificial sources was subjected to photochemical aging using ultraviolet (UV) and visible light. The experimental results of oxidation potential (OP) and heterogeneous oxidation of sulfur dioxide (SO₂) indicated that both UV and visible light promoted the photooxidation of candle soot, leading to significant increases in oxygen-containing functional groups, environmentally persistent free radicals, and negative charges on soot surfaces. After photochemical aging, candle soot exhibited higher OP values and enhanced SO₂ oxidation and sulfate formation. UV light had a stronger photooxidation ability on candle soot than visible light. Mechanistic analysis revealed that the photochemical aging mechanisms driven by reactive oxygen species were different under these two wavelengths. Photosensitive aging induced by organic carbon under UV light was stronger than the photocatalytic oxidation induced by element carbon under visible light. Our research findings provided new insights into the photochemical aging mechanisms and health impacts of soot.