Unraveling seasonality in origin and processing of carbonaceous aerosols over western India using dual-carbon isotopes

This study investigates the sources and characteristics of PM2.5 in Ahmedabad, a rapidly growing city in western India situated between the highly polluted Indo-Gangetic Plain and the pristine Arabian Sea. Chemical, optical, and dual-carbon isotope (13C and 14C) analyses were conducted to study carbonaceous aerosols (CAs). PM2.5 samples were collected from November 2020 to June 2021, covering postmonsoon, winter, spring, and summer seasons. CAs contributed 42 ± 10%, 53 ± 11%, 55 ± 12%, and 29 ± 6% of the PM2.5 mass during these seasons, respectively. Of this, 64 ± 3%, 64 ± 3%, 56 ± 3%, and 49 ± 4% originated from biomass burning (BB) or biogenic sources (fbio), as estimated from radiocarbon analysis. Stable carbon isotope ratios (δ13C) showed higher values in postmonsoon (−25.4 ± 0.8‰), followed by lower values in winter (−25.9 ± 0.4‰) and the lowest and similar values during spring and summer (−26.5 ± 0.3‰). These changes in δ13C values aligned with the CA oxidation proxy from aerosol mass spectrometry (f44), attributable to atmospheric processing of organic carbon (OC) rather than source shifts. A lower value of fbio_BC (0.20 ± 0.08) compared to that of fbio_OC (0.62 ± 0.07) suggests the contribution of biogenic sources to OC. The ratio of fbio_OC/fbio_BC is proposed as a proxy to differentiate biogenic contributions to CAs. This ratio suggests that the highest biogenic influence is in summer, followed by spring, postmonsoon, and winter. Even though the higher biogenic contribution was in the summer, winter aerosols were highly oxidized as they were transported from the polluted Indo-Gangetic Plain, as evidenced by elevated δ13C and f44. The findings highlight that even in a large urban center, regional transport of aged aerosols and seasonal biomass burning play a significant role in local air quality, alongside local fossil fuel emissions.