Reactive Oxygen Species-Related Inside-to-Outside Oxidation of Soot Particles Triggered by Visible-Light Irradiation:...

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 (OP^DTT). 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 OP^DTT 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.