Reactive Oxygen Species-Related Inside-to-Outside Oxidation of Soot Particles Triggered by Visible-Light Irradiation: Physicochemical Property Changes and Oxidative Potential Enhancement.

Modifications of the physicochemical properties and oxidative potential (OP) of soot due to visible-light irradiation and its underlying mechanisms during atmospheric aging have not been elucidated. In this study, two types of soot obtained using different air/fuel ratios (A/F) were aged under visible light with or without ozone (O3) at an atmospherically relevant level in an environmental chamber. Physicochemical characteristics and OP of aged soot were systematically measured using the dithiothreitol (DTT) assay (OPDTT). Regardless of the presence of O3, visible light markedly promoted oxidation of soot, which led to consumption of polycyclic aromatic hydrocarbons, formation of oxygen-containing functional groups, and enhancement of OPDTT values. Compared to low-A/F soot, high-A/F soot contained more elemental carbon but less organic carbon and was more sensitive to visible light by exhibiting greater changes. It was proposed that elemental carbon in soot under visible-light irradiation initiated an inside-to-outside oxidation pathway, where reactive oxygen species played an important role. This study clarified the solar irradiation-triggered self-oxidation process in soot, which is important to its atmospheric and health effects.