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

Open biomass burning across different regions of the world is a major source of gaseous and fine-mode particulate species emitted into the atmosphere. Post-monsoon crop residue fires of North-West (N-W) India continues to have significant contribution to global burned area estimates (GloCAB product – Hall et al., 2024) and air quality impact in downwind urban cities of Indo-Gangetic Plain (IGP) including megacity Delhi. Yet, detailed in-situ observations of the fire smoke’s evolution and emission characteristics near the source are lacking and largely remain uncertain. We conducted STUB-BURN (Stubble Burning emissions study) measurements characterizing speciated fine particulate matter and related gaseous species via a mobile research platform in rural Punjab from 27 Oct to 18 Nov 2023. We observed rapid oxidation of OA with more than half dominated by oxygenated form of OA even in near-source field sampling conditions. By combining organics, metals, and black carbon (BC) in source apportionment technique, roughly ~ 50% is attributed to ongoing crop residue burning. Further, varying contribution of primary and aged OA factors were found in identified nine individual plume events. However, the dilution corrected enhancement ratio of OA w.r.t CO shows no net increase or decrease in mass enhancement with increasing O:C values as an indicator of ageing. Emission factors (EFs) of 17 species are calculated and their variability with global averages used in global fire emission estimates for this region are highlighted. Broadly, obtained EFs under open field scale combustion conditions for major species are up to three-fold lower than average estimates from widely used fire emission inventories. Overall, this study reinforces the need to account for fire characteristics that govern subsequent emissions to represent regional contributions more accurately within global emission estimates.