Oxidative potential and hydroxyl radical generation capacity of ambient PM2.5 over a high-altitude site in northeastern Himalaya: Role of long-range transport

Abstract

This study investigates the dithiothreitol (DTT)-based oxidative potential (OP) and corresponding hydroxyl radical (•OH) generations capacity of PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) over a high-altitude site, Shillong (25.7 °N, 91.9 °E; 1064 m above mean sea level), located in the northeastern Himalaya. Measured OP and •OH are reported in two units: per m3 of filtered air (OPV and OHV) or per unit mass of PM2.5 in μg (OPM and OHM). Based on the characteristic ratios of chemical species in different sources, PM2.5 were classified into three source categories: Biomass Burning (BB) (N = 10), Secondary Organic Aerosols (SOA) (N = 12), and Mixed (N = 12). Consistently higher OP and •OH generation per m3 of air (OPV: 6.2 ± 1.0 (average ± SD, 1σ) nmol DTT min−1 m−3, and OHV: 1.2 ± 0.31 nmol •OH m−3) in all BB-dominated PM2.5 samples indicate relatively hazardous exposure dose from BB emissions among all sources. Characteristic OC/EC, WSOC/OC, and nss-K+/EC ratios evoked that SOA from biogenic precursors was a significant source for the observed OP and •OH in SOA-categorized samples. Distinctly different chemical composition of PM2.5 over Shillong (about 55% composed of organic matter) makes it relatively more (2–4 times) intrinsically redox-active compare to a reported study over another high-altitude site in a semi-arid region of India. Despite ∼1.7 times lower PM2.5 mass over Shillong (characterized by more aged PM2.5) compare to Patiala (30.3°N, 76.4°E; 250m amsl, a semi-urban city, characterized by fresher PM2.5 from mixed sources), the observed OPV over Shillong was about 1.2 times higher, and the OPM was about 2 times higher. It is attributed to the effect of atmospheric processing on particle's redox-activity. While examining the relationship of OPV and OHV with chemical species, it was observed that •OH generation capacity was more influenced by WSOC species' origin (primary or secondary) compare to DTT consumption. Such an observation restricts the practicality of particle-induced OP to be considered as the only metric for aerosol particle toxicity, and suggests also to consider simultaneous measurements of •OH generations. For the BB-dominated category, OC-normalized OP (OPOC) was correlated strongly with the fractions of N-containing water-soluble organic aerosols (i.e., a fraction of WSON and a fraction of familyCHO1N). Corresponding OC/EC, nss-K+/EC and nss-SO42−/EC ratios hint that coal combustion, in addition to BB, could be a significant source of redox-active WSON species in these samples. Such studies are important for identifying redox-active aerosol species and developing appropriate mitigation policies for healthy air.