Spatiotemporal Variations in Fossil vs Non-Fossil Source Contributions to Carbonaceous Aerosols across Diverse Indian Environments: An Assessment via Radiocarbon

Carbonaceous aerosols (CAs) are a dominant component of particulate matter over South Asia, yet the relative contributions of fossil-fuel combustion and nonfossil sources remain poorly constrained across India’s diverse emission environments. Here, we present a comprehensive assessment of CA sources using dual-carbon isotopes (¹⁴C and ¹³C), complemented by chemical composition (OC, EC, and WSOC), and aerosol mass spectrometer (AMS)-derived oxidation indicators (f₄₃ and f₄₄). Aerosol samples were collected across eight geographically distinct locations spanning the Indo-Gangetic Plain (IGP), semiarid western India, southern India, the Himalayan foothills, and the Bay of Bengal during the postmonsoon, winter, spring, and summer seasons. Radiocarbon-derived nonfossil fractions of total carbon (f_{bio_TC}) ranged from 0.39 to 0.86 (mean = 0.71 ± 0.13), with the highest values observed during the postmonsoon paddy-residue-burning period over the IGP (up to 0.82 at Patiala), reflecting the overwhelming dominance of biomass-burning-derived carbon, and the lowest values during summer at Delhi (0.45) and Ahmedabad (0.49), indicating a clear seasonal shift toward fossil-fuel-dominated CAs. Stable carbon isotope signatures (δ¹³C), together with oxidation indicators (f₄₃ and f₄₄), reveal pronounced regional contrasts in aerosol aging and secondary processing, with aged, highly oxidized biomass-burning aerosols prevailing over northern and northeastern India during the postmonsoon and winter, and fresher primary fossil-fuel emissions dominating during summer and at southern and western sites. Comparison with MERRA-2 reanalysis reveals a systematic underestimation of TC concentrations across all sites and seasons (mean bias ∼−42%), most severe during the PRB period over the IGP and in winter over northeastern India, pointing to deficiencies in fire emission inventories and aerosol transport representation in current reanalysis products. These findings provide robust, observation-based constraints on the seasonal and regional evolution of CA sources across India, offering critical inputs for refining emission inventories, improving chemical transport model performance, and designing targeted mitigation strategies for reducing the air-quality and climate impacts of carbonaceous aerosols over South Asia.