Tracing the predominant sources of carbon in PM using δC values together with OC/EC and select inorganic ions over two COALESCE locations.

As part of the COALESCE (Carbonaceous Aerosol Emissions, Source apportionment and Climate Impacts) campaign, ambient PM was collected at two regional sites (Bhopal and Mysuru) in India during 2019. We utilized organic carbon (OC), elemental carbon (EC) and water-soluble inorganic ions together with δC values, to better understand total carbon (TC) sources at these locations. The annual average δC values (-26.2 ± 0.6‰) at Mysuru and Bhopal (-26.6 ± 0.6‰) were comparable. However, at Mysuru, except during winter, day-to-day variability was much lower (narrow range of -26.8 to -26.0‰) than that at Bhopal (range: -28.1 to -24.7‰), suggesting that TC was contributed by few sources, likely dominated by vehicular emissions. Seasonal average δC values at Bhopal increased slightly (-25.8 ± 0.5‰) during the winter (Jan-Feb) and decreased (-27.0 ± 0.3‰) during the monsoon (Jun-Sep) season compared to the annual average. The decrease in δC values during the monsoon season was likely driven by enhanced secondary organic aerosol formation. Further, based on MODIS derived fire spots and back trajectories, we infered that the δC values (-27.5 to -26.0‰) in Bhopal during the post-monsoon season (Oct-Dec) were indicative of dominant biomass burning contributions. The inorganic ions/TC ratio during this season suggested that biomass burning aerosol was aged and may have been transported from crop residue burning in the Indo-Gangetic plains. At Mysuru, like the trend at Bhopal, the δC values during the monsoon season were lower than those during the winter season. Finally, δC values were input to a Bayesian model-MixSIAR to demonstrate the usefulness of such models in apportioning TC. In its simplest implementation, the model separated TC sources into fossil fuel emissions and non-fossil fuel sources . Fossil fuel combustion emissions accounted for 47 ± 19% and 62 ± 22% of the TC at Bhopal and Mysuru, respectively.