Dark Brown Carbon over the Indo-Gangetic Plain: An Overlooked Yet Major Driver of Regional Radiative Forcing

Brown carbon (BrC) is a major organic carbonaceous aerosol fraction distinguished by its light absorption attribute, which potentially alters Earth’s radiative budget. Dark brown carbon, commonly referred to as Tar Balls (TBs), exhibits markedly stronger light absorption compared to other BrC fractions. TBs are well recognized in wildfire emissions, but their occurrence remains inadequately explored and characterized from other emission sources. This work examines the presence of strongly light-attenuating TBs in the Indo-Gangetic Plain (IGP), a globally identified air-pollution hotspot heavily influenced by biomass burning, particularly crop-residue fires post harvest.

The study encompasses three strategically selected sites across the Western, Central, and Eastern IGP, where simultaneous sampling was conducted during the early post-monsoon period. This was followed by further examination of  the prevalence and occurrence patterns of TB particles, and the associated morphological and physicochemical traits using Scanning electron microscopy (SEM) and Transmission Electron Microscopy (TEM), alongside assessing the sources of PM_2.5 pollution at each site using PMF model.

The TB-to-soot aggregate ratio, representing TB number concentration, increased from 0.85 in the Western IGP to 1.35 in the Central IGP. The results underscore that distinct regional particle profiles are consistent with prevailing primary and secondary pollution sources at the two sites, respectively. TB particles were scarcely detected at the Eastern IGP site, dominated by urban emissions during the study period, suggesting that their origin is primarily linked to biomass-burning. Overall, the TB number fraction in this study at Western and Central IGP, which is potentially driven by crop residue burning, was 15 times lower than previously reported for wildfire-derived TBs. TB chemistry varied spatially, with fresh biomass-derived TB particles at the Western IGP showing a higher C/O ratio of 3.60, while aged ones at the Central IGP exhibited a lower C/O ratio of 2.59. This study reported a notably lower C/O ratio and higher Nitrogen concentrations for the TBs as compared to extensively studied wildfire-derived TBs documented in past, with the ratio reaching values as high as 20–25.

The findings indicate pronounced variability in TB traits based on emission source, emphasizing the necessity of comprehensive, source-specific TB assessments across all potential origins accompanied by a thorough characterization of optical parameters to reduce uncertainties in radiative forcing effect estimates.