Association between oxidative potential and optical properties of PM2.5 over the northwestern Himalayan region

Particulate matter (PM_2.5) has significant impacts on air quality, climate, and human health. Exposure to PM_2.5 can induce oxidative stress either by transferring oxidants to the lungs or via catalytic production of reactive oxygen species (ROS), leading to various respiratory and cardiovascular diseases. In this study, we quantified the oxidative potential (OP) using the DTT assay in surrogate lung fluid (SLF) for aerosol collected in the northwestern Himalayas, examined its sources and chemical drives, and its potential links with aerosol optical properties_. The annual mean volume-normalized DTT (OP_v) was 0.30 ± 0.15 nmol min^-1m^-3 while the absorption coefficient (b_{abs_aq_365}) was 7.9 ± 7.8 Mm^-1. OP_v showed a strong seasonality, with the highest value in the winter season (0.35±0.17 nmol min^-1m^-3), followed by the post-monsoon and summer. BrC absorption showed a similar trend with the highest levels in winter (b_{abs_aq_365}: 16.1 ± 7.6 Mm^-1), which was 6 and 4.4 times higher than the summer and post-monsoon seasons, respectively (p<0.05). The mass absorption efficiency (MAE) for BrC also peaked in the winter at 1.85 ± 0.44 m² g^-1 with lower E₂/E₃ values (5.8 ± 0.6) as compared to the summer (0.8 ± 0.3 m² g^-1, 9.0 ± 2.6) and post-monsoon (1.0 ± 0.4 m² g^-1, 7.1 ± 1.2). This indicated the presence of more aromatic, higher molecular weight chromophores in the winter, which were more resistant to photobleaching. Moderate but significant positive correlations were observed for b_{abs_aq_365} and MAE with OP_v (r=0.40, p<0.01, and r=0.25, p<0.05, respectively), signifying the potential role of light absorbing chromophores in inducing oxidative stress. Also, a significant negative correlation of OP_v with E₂/E₃ (r=-0.28, p<0.05) was observed, indicating that aromatic, high-molecular-weight BrC chromophores possibly enhanced redox activity. Atmospheric aging/SOA formation from fossil fuel and biomass burning emissions played a dominant role in driving both OPᵥ and b_{abs_aq_365}. Random forest (RF) coupled with regression analysis identified Cu, K⁺, and NO₃^- as dominant drivers of OPᵥ, followed by OC, EC, and Mn. Positive matrix factorization (PMF) resolved seven sources, with industrial emissions and secondary formation contributing ~83% of OP_v. These findings emphasize the potential linkages between the light absorbing and stress-inducing roles of aged aerosol in the Himalayan region.

 

Keywords: DTT assay, Mass absorption efficiency (MAE), Surrogate lung fluid (SLF), Positive matrix factorization (PMF), Random forest (RF).