Summertime oxidative potential of atmospheric PM_2.5 over New Delhi: Effect of aerosol ageing

Exposure to elevated particulate matter (PM) concentrations in ambient air has become a major health concern over urban areas worldwide. Reactive oxygen species (ROS) generation due to ambient PM (termed as their oxidative potential, OP) is shown to play a major role in PM-induced health effects. In the present study, the OP of the ambient PM_2.5 samples, collected during summer 2019 from New Delhi, were measured using the dithiothreitol (DTT) assay. Average volume-normalized OP (OP_V) was 2.9 ± 1.1 nmol DTT min^-1 m^-3, and mass-normalized OP (OP_m) was 61 ± 29 pmol DTT min^-1 μg^-1. The regression statistics of OPv vs chemical species show the maximum slope of OP_V with the elemental carbon (EC, r² = 0.72) followed by water-soluble organic carbon (WSOC, r² = 0.72), and organic carbon (OC, r² = 0.64). A strong positive correlation between OP_m and secondary inorganic aerosols (SIA, such as NH₄⁺ and NO₃^- mass fractions) was also observed, indicating that the sources emitting NO₂ and NH₃, precursors of NO₃^- and NH₄⁺, also emit DTT-active species. Interestingly, the slope value of OPv vs OC for aged aerosols (OM/OC > 1.7, f44 > 0.12 and f43 < 0.04) was 1.7 times higher than relatively fresh organic aerosols (OA, OM/OC < 1.7, f44 < 0.12, f43 > 0.04). An increase in OPv and OPoc with f44 indicates the formation of more DTT active species with the ageing of OA. A linear increase in OP_oc with increasing Nitrogen/Carbon (N/C) ratio suggests that nitrogenous OA have higher OP.