Integrated Evaluation of Oxidative Potential and Antioxidant Activity of Ambient PM2.5 in Lahore: Decoupling between Mass Concentration and Redox Reactivity

Abstract

Ambient fine particulate matter (PM_2.5) exerts adverse effects largely through redox-mediated mechanisms; however, conventional mass-based metrics do not adequately capture its chemical reactivity. Here, we present a nine-month field investigation (August 2024–April 2025) of the redox behavior of ambient PM_2.5 in Lahore, Pakistan, using an integrated multiassay approach. Oxidative potential was quantified using the dithiothreitol (DTT) assay, while antioxidant capacity was evaluated using DPPH and ABTS assays to characterize both pro-oxidant and electron-donating components of atmospheric aerosols. PM_2.5 concentrations exhibited pronounced seasonal variability, increasing from 166.14 ± 5.24 μg m^–3 in August to a maximum of 400.23 ± 63.82 μg m^–3 in November. Volume-normalized oxidative potential (DTT_v) ranged from 6.71 to 20.22 nmol min^–1 m^–3 and increased significantly with PM_2.5 concentration (r = 0.518, p < 0.001), indicating greater oxidative burden during high-pollution episodes. In contrast, mass-normalized oxidative potential (DTT_m) ranged from 0.02 to 0.13 nmol min^–1 μg^–1 and exhibited a significant inverse relationship with PM_2.5 concentration (r = −0.714, p < 0.001), suggesting enhanced intrinsic particle reactivity during lower-pollution periods. Antioxidant activity showed similar seasonal trends, with DPPH antioxidant capacity ranging from 57.50–133.57 μg AAE mg^–1 (aqueous extract) and 58.57–144.64 μg AAE mg^–1 (ethanolic extract), while ABTS antioxidant capacity ranged from 83.35–205.49 μg TE mg^–1; all were positively associated with PM_2.5 concentration. The contrasting behavior of DTT_v and DTT_m, together with the observed antioxidant responses, demonstrates that aerosol redox properties are governed not only by particle abundance but also by chemical composition and atmospheric transformation processes. These findings highlight the importance of integrating oxidative-potential and antioxidant-capacity measurements to provide a more chemically informed assessment of urban aerosol reactivity and potential health relevance in highly polluted environments.