Summertime oxidative potential of atmospheric PM 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 samples, collected during summer 2019 from New Delhi, were measured using the dithiothreitol (DTT) assay. Average volume-normalized OP (OP) was 2.9 ± 1.1 nmol DTT min m, and mass-normalized OP (OP) was 61 ± 29 pmol DTT min μg. The regression statistics of OPv vs chemical species show the maximum slope of OP with the elemental carbon (EC, r = 0.72) followed by water-soluble organic carbon (WSOC, r = 0.72), and organic carbon (OC, r = 0.64). A strong positive correlation between OP and secondary inorganic aerosols (SIA, such as NH and NO mass fractions) was also observed, indicating that the sources emitting NO and NH, precursors of NO and NH, 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 with increasing Nitrogen/Carbon (N/C) ratio suggests that nitrogenous OA have higher OP.