Ambient black carbon and its dependence on in-situ stubble burning

Crop residue burning (CRB) in Northern India is a serious concern that poses substantial health risks and deteriorates air quality in the Indo-Gangetic Plain (IGP) region via regional transport. Despite governmental regulations and various interventions, the practice remains prevalent. In this study, we investigate the influence of CRB on ambient black carbon (BC) level in rural areas of Punjab during the post monsoon period. BC source contributors were quantified using aethalometer model (AM) and positive matrix factorization (PMF) approaches. Interestingly, despite the distinct mathematical framework of these two models, their results showed good agreement. Although, the measurements were conducted in the agricultural field, CRB contributed on average 48% to the total BC, periodically exceeding 90% during the active burning hours (1:00 PM – 6:00 PM), resulting in strong diurnal variation. In addition to the CRB several other BC sources e.g., traffic emissions (20.2%), power plant emissions (24.0%), waste incineration (2.0%), and a Pb-rich industrial factor (6.1%) were also quantified. The BC concentration measured during the study ranged from 1.45 to 85.0 µg/m³ with an average of 12.6 µg/m³. Absorption Angstrom Exponent (AAE), an important aerosol parameter, is also calculated and observed to be 1.7 ± 0.3, with hourly variations ranging from 1.1 to 2.4, suggesting a significant influence of CRB. Furthermore, spectral variation in absorption characteristics of BC and brown carbon (BrC) was also calculated, providing insight into the optical properties of fresh biomass-burning carbonaceous aerosols. These findings highlight the persistent dominance of CRB emissions in shaping rural aerosol composition. Moreover, when transported by prevailing winds, these aerosols substantially influence air quality across the Indo-Gangetic Plain. In this study, forward wind trajectory analysis further demonstrates that air masses originating in Punjab frequently move toward the IGP region, supporting the observed regional impact of CRB emissions.