Photochemical impacts on the toxicity of PM2.5

Fine particulate matter (PM2.5) is one of the main pollutants in the atmosphere and poses a huge threat to public health. Secondary aerosol pollutants that are formed by precursor pollutants in the atmosphere represent a large proportion of PM2.5; furthermore, photochemical reactions play an important role in their transformation processes. This review systematically presents the chemical and toxicological transformations of secondary organic aerosol (SOA), secondary inorganic aerosol (SIA), and metal species in PM2.5 under photochemical conditions. Specifically, major SOAs and SIAs are observed to be formed from volatile organic compounds (VOCs) and sulfur/nitrogen oxides through photochemical reactions, while metal species in PM2.5 participate in a series of photochemical reactions. Additionally, we compare the toxicities of precursor pollutants with their secondary pollutants after photochemical reactions. In general, major SOAs are found to be more toxic than their precursors, while major SIAs are less toxic than their precursors. As different metal species have different toxicities, changes in their toxicities after photoreaction depend on the specific metal and the photochemical reaction. By demonstrating that photochemical reactions can affect the toxicities of PM2.5 components, this review provides a promising perspective toward reducing air pollution and its health risks by investigating the related photochemical reactions.