Sources and processes affecting the abundances of atmospheric NH_x using δ¹⁵N over northwestern Indo-Gangetic plain

Ammonia (NH₃) is the major constituent among all the reactive nitrogen species present in the atmosphere, and the most essential species for secondary inorganic aerosol formation. Recent satellite-based observations have identified the Indo-Gangetic Plain (IGP) as a major hotspot of global NH₃ emission; however, the major sources and atmospheric processes affecting its abundance are poorly understood. The present study aims to understand the wintertime sources of NH₃ over a semi-urban site (Patiala, 30.3°N, 76.4°E, 249 m amsl) located in the IGP using species specific δ¹⁵N in PM_2.5. A distinct diurnal variation in the stable isotopic signature of total nitrogen (δ¹⁵N-TN) and ammonium (δ¹⁵N-NH₄⁺) were observed; although, average day and night time concentrations of TN and NH₄⁺ were similar. Mixing model results using δ¹⁵N-NH₃ reveal the dominance of non-agricultural emissions (NH₃ slip: 47 ± 24%) over agricultural emissions (24 ± 11%), combustion sources (19 ± 14 %), and biomass burning (10 ± 8%) for atmospheric NH₃. Diurnal variability in source contributions to NH₃ was insignificant. Further, significantly negative correlations of δ¹⁵N-NH₄⁺ with ambient relative humidity (RH) and daytime NO₃^--N concentration were observed, and attributed to the possibility of NH₄NO₃ volatilization during day-time owing to lower RH and higher temperature, resulting in isotopic enrichment of the remaining NH₄⁺ in aerosol phase. This study, a first of its type from India, highlights the importance of non-agricultural NH₃ emissions over the agriculture dominated IGP region, and the role of local meteorology on the isotopic fractionation of δ¹⁵N in aerosol NH₄⁺.