Characterization of Oxidative Potential and Environmentally Persistent Free Radicals in a Tropical Coastal Megacity of India

Abstract The burden of human health in urban clusters across India is associated with increasing fine particulate matter, PM2.5. However, associating PM2.5 health impacts with its mass burden alone without quantifiable relation to toxicity indicators over polluted regions may cause inaccurate estimations of disease burden resulting from varying chemical composition. Here, we present 17-day measurement results of oxidative potential quantified using hydroxyl radical assay (OP–OH) and environmentally persistent free radicals (EPFRs) in the coastal city of Chennai. We quantify EPFR concentration and OP–OH activity to be 34 ± 21 pmol m–3 and 11 ± 5.4 pmol min–1 m–3, respectively, and observed a weak association between OP–OH and EPFRs. Westerly and mixed wind separation helped characterize the distinct physical and chemical properties of PM2.5. During westerly winds, EPFRs were moderately correlated with particle number concentration <30 nm (R2 = 0.5) and strongly correlated with traffic markers (R2 > 0.68; hydrocarbon-like organic aerosols (HOA), NO2, Fe, Sn, and Ti). During mixed winds, OP–OH exhibited moderate correlations with transition metals (Cr and Zn; R2 > 0.34) and particle number concentration >100 nm (R2 = 0.5) and a clear correlation with EPFRs (R2 = 0.59). Our result underscores the complexity of EPFRs and OP–OH in PM2.5 over polluted urban clusters, suggesting the importance of particle size dependence along with chemistry.