Long-life type — The dominant fraction of EPFRs in combustion sources and ambient fine particles in Xi'an

Abstract

Environmentally persistent free radicals (EPFRs) are new environmental health risk substances that have received much attention in recent years. The characterization of EPFRs is essential for understanding their sources and formation mechanisms in atmospheric particulate matter (PM), yet their physicochemical characteristics have not yet been fully and extensively studied. To clarify the nature of “persistent” free radicals in atmospheric PM, we systematically evaluated the decay characteristics of EPFRs in both combustion sources and ambient PM2.5 based on the planned measurement of EPFRs in a large number of samples (more than 130). The content of EPFRs changed over time for up to 2 years. This study quantifies EPFRs with different lifetimes in different sources of PM2.5. The results indicate that the EPFRs were dominated by the long-life type (>70%), with a 1/e lifetime of several years, in both combustion sources and ambient PM2.5. The decay characteristics of the EPFRs, including the temporal variations in the concentrations and g-factors of EPFRs in ambient PM2.5, were similar to those of combustion source samples but different from those of EPFRs produced by the interaction of metal oxides and aromatics. The interaction of metal oxides and aromatics may produce short-life EPFRs, and the formation mechanism of long-life EPFRs may be more complicated. This study provides important basic knowledge for understanding the sources and formation mechanisms of EPFRs in atmospheric PM.