Source quantification of PM2.5 using δ13C values along with corresponding organic carbon, elemental carbon, and select inorganic ions over two COALESCE network locations

Ratios of stable carbon isotopes reported as values of δ¹³C ‰, are often used to provide information about the origin of aerosol particles because these stable carbon isotopes are conserved through time and change predictably during atmospheric processes. As part of the COALSECE network ambient aerosol measurement campaign, PM_2.5 samples were collected at two regionally representative sites during 2019 (Bhopal and Mysuru) in India with the objectives of identifying and estimating their potential sources at regional level and quantitatively estimating the anthropogenic impact on their carbon content by coupling the δ¹³C values with their corresponding organic carbon (OC) and elemental carbon (EC) concentrations along with inorganic water soluble ion concentrations. The EC, OC, water soluble inorganic ions and δ¹³C_TC values were determined using a variety of analyses.

At Bhopal, the average OC and EC concentrations were 9.5 and 2.4 µg/m³, respectively, with an average δ¹³C value of -26.6 ± 0.6‰. At Mysuru, the average OC and EC concentrations were 4.5 and 1.0 µg/m³, respectively, with an average δ¹³C of -26.2 ± 0.6‰. Notable differences were observed in the seasonality of the δ¹³C valueswith slight increase (-25.8±0.5‰) during the winter (Jan, Feb) and a decrease (-27.0±0.3‰) during the monsoon (Jun, Jul, Aug, Sep) in Bhopal. Further, based on the MODIS derived fire spots and back trajectories, we infer that δ¹³C values (-27.5 to -26.0‰) in Bhopal during post-monsoon season (Oct, Nov, Dec) were predominately associated with biomass burning. Further, the enrichment in both non-sea salt potassium and sulphate/nitrate was significantly higher than the other inorganic species, suggesting that biomass burning in Bhopal during post-monsoon was aged and less fresh and may have transported from the Indo-genetic plains during post harvesting periods. In contrast, δ¹³C values at Mysuru did not exhibit pronounced seasonality and ranged between -25.3 to -26.7‰ during all of 2019, suggesting the influence of proximal sources.

Finally, we use the δ¹³C values with priors in a Bayesian mixing model MixSIAR to resolve the TC at both sampling locations into fossil fuel combustion and non-fossil fuel combustion carbon. We find that in Bhopal fossil fuel combustion accounted for 53.6±12.2% of the TC, whereas, in Mysuru, it accounted for 60.4±6.3% of the TC.