Characteristics, exposure risk, and occurrence of environmental persistent free radicals in PM2.5: a case study in Baoding

Environmental persistent free radicals (EPFRs) in fine atmospheric particulate matter (PM2.5) can help the production of reactive oxygen species, leading to oxidative damage to people. In this study, electron spin resonance spectroscopy was used to detect EPFRs in PM2.5. The exposure risks were evaluated by an equivalent cigarette model. The occurrence of EPFRs in PM2.5 was studied by linking the EPFRs with heavy metals in PM2.5 and traditional gaseous air pollutants. The results indicated that carbon-centered radicals with adjacent oxygen atoms (O-C-•) dominated EPFRs in PM2.5, with the g-factor ranged from 2.0031 to 2.0042. Environmental persistent free radicals in summer samples showed a higher g-factor indicated a greater proportion of oxygen-centered (O-•) EPFRs. The concentration of PM2.5-EPFRs in Baoding City in different seasons was in the order of winter > summer/spring > autumn, with the highest concentration of 1.99 × 1017 spins/g and 2.45 × 1013 spins/m3. Exposure risk assessment indicated that the exposure risk caused by PM2.5-EPFRs inhalation was the highest in December and equivalent to 0.39 cigarettes smoked per day. Heavy metals, such as Cd, Zn, and As, and gaseous pollutants, such as NO2, O3, and CO, were related to the occurrence of PM2.5-EPFRs. Air pollution control in the past few years in China significantly decreased the risk of PM2.5-EPFRs. This study will enhance our understanding of the environmental fate and health risks associated with EPFRs in PM2.5, as well as the current state of atmospheric pollution in the Beijing-Tianjin-Hebei region during the new energy era.