Pollution characteristics and light-driven evolution of environmentally persistent free radicals in PM2.5 in two typical northern cities of China

Environmentally persistent free radicals (EPFRs) in PM2.5 can pose significant health risks by generating reactive oxygen species (ROS). In this study, Beijing and Yuncheng were chosen as two representative northern cities of China that mainly relied on natural gas and coal respectively as the energy source for domestic heating in winter. The pollution characteristics and exposure risks of EPFRs in PM2.5 around the heating season of 2020 were investigated and compared between the two cities. Through laboratory simulation experiments, the decay kinetics and secondary formation of EPFRs in PM2.5 collected in both cities were also studied. EPFRs in PM2.5 collected in Yuncheng in the heating period showed longer lifetime and lower reactivity, suggesting that EPFRs originated from coal combustion were more stable in the atmosphere. However, the generation rate of hydroxyl radical (·OH) by the newly formed EPFRs in PM2.5 in Beijing under ambient conditions was 4.4 times of that in Yuncheng, suggesting higher oxidative potential of EPFRs from the atmospheric secondary processes. Accordingly, the control strategies of EPFRs and their health risks were raised for the two cities, which would also have direct implication for the control of EPFRs in other areas of similar atmospheric emission and reaction patterns.