Near-source emission profiling of post-monsoon crop residue fires in N-W India

Post-monsoon crop residue fires of North-West (N-W) India continue to have a significant contribution to global cropland burned area estimates and poor air quality in downwind urban cities, yet detailed understanding of the fire smoke evolution and in situ emission characteristics is lacking and remains poorly constrained. To address this research gap, targeted STUB-BURN (Stubble Burning Emissions Study) field measurements of speciated fine particulate matter (PM 2.5 ) and gaseous species were obtained during the Oct–Nov period in 2023. We investigated sources of organic aerosols (OA) and other major species in PM 2.5 by applying positive matrix factorization and found that while ongoing crop residue burning activity drives major species in PM 2.5 , oxygenated organic aerosols (OOA) dominate the overall organic composition (~68%) even in near-source measurements. Though freshly emitted biomass burning organic compounds are observed to undergo rapid oxidation, such a high contribution of OOAs in this case might be partly caused by capture vaporizer (CV) induced oxygenation in CV-ACSM mass spectrometers. No net mass enhancement or reduction in dilution-corrected ΔOA/ΔCO was observed w.r.t. aging indicator (O:C ratio) for nine plume events. Inferred in situ mean emission factor values via the carbon mass balance method for seventeen species based on two plume events under open field scale combustion conditions are lower than average estimates from widely used fire emission inventories. Given the large variability in fire-to-fire emissions driven by combustion conditions among other factors, the study reinforces the need to account for localized fire characteristics to represent regional contributions more accurately within global emission estimates.