Evolution of environmentally persistent free radicals on tire wear particles under UV irradiation: Contributions of relative humidity and ozone concentration

Tire wear particles (TWPs) were one of the major pollutants of atmospheric particulate matter, which were carriers of environmentally persistent free radicals (EPFRs), thus emerging as an incipient ecological threat. The levels of atmospheric humidity and ozone varied in different regions, which led to differences in the generation and accumulation of EPFRs. EPFRs on TWPs altered during atmospheric photooxidation, yet the effects of key environmental parameters remained poorly understood. This study systematically investigated the evolution of EPFRs on TWPs under ultraviolet irradiation with controlled relative humidity and ozone exposure. EPFRs were detected on original TWPs, which originated from polymers, and carbon black prolonged the lifespan of EPFRs. Electron Paramagnetic Resonance (EPR) analysis demonstrated humidity-dependent EPFRs accumulation dynamics. Under low relative humidity, sustained photon absorption promoted polymer chain excitation, driving EPFRs accumulation ( p  < 0.01) through the induction of photooxidation chain reactions. Under high humidity, light scattering effect of water molecules reduced the accumulation of EPFRs ( p  < 0.05) and retarded the photooxidation process of TWPs. During photooxidation in ozone, ozone concentration significantly affected the accumulation and evolution of EPFRs. Photoaging in an ozone atmosphere led to a rapid accumulation of EPFRs ( p  < 0.01), which was positively correlated with ozone concentration. The formation and accumulation of EPFRs in the ozone atmosphere resulted not only from the photooxidation chain reaction of polymers but also from the Norrish reactions of the ozonation products under photon excitation. These findings emphasized the key roles of atmospheric relative humidity and ozone in regulating the evolution of EPFRs during the photooxidation of TWPs. This provided fundamental knowledge for predicting the EPFRs carried by TWPs in areas with different meteorological conditions and ozone pollution levels. Future air quality assessments should consider photooxidation processes of TWPs under actual atmospheric parameters and atmospheric pollutant concentrations to better assess ecological risks of atmospheric particulate matter.