Outdoor Health Risk of Atmospheric Particulate Matter at Night in Xi'an, Northwestern China

The deterioration of air quality via anthropogenic activitiesduringthe night period has been deemed a serious concern among the scientificcommunity. Thereby, we explored the outdoor particulate matter (PM)concentration and the contributions from various sources during theday and night in winter and spring 2021 in a megacity, northwesternChina. The results revealed that the changes in chemical compositionsof PM and sources (motor vehicles, industrial emissions, coal combustion)at night lead to substantial PM toxicity, oxidative potential (OP),and OP/PM per unit mass, indicating high oxidative toxicity and exposurerisk at nighttime. Furthermore, higher environmentally persistentfree radical (EPFR) concentration and its significant correlationwith OP were observed, suggesting that EPFRs cause reactive oxygenspecies (ROS) formation. Moreover, the noncarcinogenic and carcinogenicrisks were systematically explained and spatialized to children andadults, highlighting intensified hotspots to epidemiological researchers.This better understanding of day-night-based PM formation pathwaysand their hazardous impact will assist to guide measures to diminishthe toxicity of PM and reduce the disease led by air pollution. The significant increase in outdoor healthrisks of atmosphericparticulate matter at night is associated with a change in its chemicalcomposition and source.