Remarkably High Oxidative Potential of Atmospheric PM<sub>2.5</sub>Coming from a Large-Scale Paddy-Residue Burning over the Northwestern Indo-Gangetic Plain
Anil Patel, Rangu Satish, Neeraj Rastogi
Abstract
High PM2.5 (particulate matter with an aerodynamic diameter 2.5 μm or less) concentrations are known to cause severe chronic cardiovascular and respiratory diseases. The present study assesses the dithiothreitol (DTT)-based oxidative potential (OP) measured on PM2.5 filters extracted with a mixture of water and methanol. PM2.5 samples were collected during paddy-residue burning that occurs every year in the northwest Indo-Gangetic Plain (IGP) during October–November. The entire study period was classified into four categories: pre-intense burning, Diwali, intense burning, and post-intense burning. On average, the PM2.5 mass concentrations were 154 ± 59, 340 ± 74, 271 ± 122, and 156 ± 19 μg m–3; volume-normalized OP (OPV) values were 7.6 ± 2.8, 9.4 ± 1.1, 14 ± 5.1, and 12 ± 1.9 nmol DTT min–1 m–3; and mass-normalized OP (OPM) values were 51 ± 18, 29 ± 7, 55 ± 12, and 78 ± 9 pmol DTT min–1 μg–1 during pre-intense burning, Diwali, intense burning, and post-intense burning periods, respectively. The intense burning period was associated with a relatively high mass fraction of organic aerosols compared to Diwali that was associated with a relatively higher fraction of inorganic aerosols. It likely caused higher OPV during the intense burning period compared to Diwali, which highlights the role of chemical composition in PM2.5 OP. The observed OPV value during the intense burning period is the globally highest reported value (average 14 nmol DTT min–1 m–3). Biomass burning markers, such as organic carbon/elemental carbon (OC/EC) and K+/EC, correlated significantly with OPV during the intense burning period, attesting the effect of burning emissions on the observed OP. The relationship of OPOC (ratio of OPV and OC mass concentration) with specific brown carbon chromophores during the post-intense burning period indicates the plausible link between atmospheric aging of redox-active organic aerosols and their optical properties. Development of a mitigation strategy for such high DTT-active PM2.5 is important to avoid a wide array of possible health effects on the inhabitants of the study and downwind regions.