Source variability and atmospheric processing of NH_x using δ¹⁵N: Linkages with WSOC fluorescence indices

NH_x i.e., ammonia gas (NH₃) and particulate ammonium (NH₄⁺), are key reactive nitrogen species in the atmosphere, playing important roles in nitrogen cycling, atmospheric chemistry, air quality, and ecosystem productivity. The Indo-Gangetic Plain (IGP) is among the global hotspots of NH₃ emission, yet studies investigating the sources of NH₃ and its partitioning to NH₄⁺ over this region remain limited. Using stable isotope (δ¹⁵N) and mixing model (MixSIAR), the present work investigates the major sources of NH₃ and factors governing its partitioning to NH₄⁺ over a semi-urban site (Patiala, 30.3° N, 76.4° E, 250 m above sea level) in the northwestern IGP during post monsoon, a notorious time-period for the poor air quality due to large-scale crop-residue burning. Concurrent ageing of inorganic and organic aerosols was also examined by coupling δ¹⁵N-NH₄⁺ with fluorescence indices of water-soluble organic carbon (WSOC). Source apportionment by MixSIAR revealed that overall biomass burning (37 ± 18%) and agricultural emissions (33 ± 10%) were the major NH₃ sources, followed by coal-fired power plants (21 ± 10%) and vehicular emissions (10 ± 5%). Source contributions were modulated by wind speed with calmer conditions dominated by local combustion-related emissions, and windier periods enhancing the contribution from regional agricultural sources. The effect of meteorology, particularly RH, on NH₃-NH₄⁺ partitioning was evident mainly under calmer conditions, where higher RH coincided with an elevated δ¹⁵N-NH₄⁺, indicating favoured partitioning. Windier conditions enhanced the atmospheric mixing and masked this effect. Additionally, significant relationship between δ¹⁵N-NH₄⁺ and WSOC fluorescence indices indicated the combined influence of common sources (biomass burning) and simultaneous ageing of NH₄⁺ and organic aerosols under same atmospheric conditions. Such findings are important for developing effective mitigation strategies over NH₃ hotspots and for improving understanding of NH_x dynamics, their effects on secondary organic aerosols formation, and controlling factors.