Oxidative potential, environmentally persistent free radicals and reactive oxygen species of size-resolved ambient particles near highways

Particulate matter (PM) is a group of atmospheric pollutants with an uncertain toxicity, particularly when collected near highways. This study examined the oxidative potential (OP) of, as well as the environmentally persistent free radicals (EPFRs) and reactive oxygen species (ROS) present in PM samples collected near highways in Xiamen, China. Our findings revealed that PM had a relatively high OP, ranging from 3.8 to 18.5 nmol/min/μg, surpassing values reported in previous research. The oxidative potential of the water-insoluble fraction (OPWIS), which accounted for 68% of the total oxidative potential (OPTotal), demonstrated rapid toxicity, whereas the oxidative potential of the water-soluble fraction (OPWS) displayed a steadier toxicity release pattern. The primary free radicals detected in PM were oxygen-centered. The measured concentration of EPFRs was 6.073 × 1014 spins/m3, which is lower than that reported in previous studies, possibly because of the high relative humidity of the road environment in Xiamen. We also investigated the interaction between PM and water near highways and observed the generation of R and OH radicals. Additionally, we analysed the sample composition and evaluated the contributions of the different components to OPTotal. Transition metals (Fe, Cu, and Zn) were identified as the major contributors, accounting for 33.2% of the OPTotal. The positive correlation observed between EPFRs and ROS suggests that EPFRs may be involved in ROS generation. The correlation analysis indicated that the oxidative potential measured using the DTT method (OPDTT) could serve as an indicator of ROS generation. Finally, based on the relationship between OPDTT, EPFRs, and ROS, we propose that reducing the emission of transition metals, particularly Fe, represents an effective control measure for mitigating PM toxicity near highways.