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

Ammonia (NH<inf>3</inf>) 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 NH<inf>3</inf> emission; however, the major sources and atmospheric processes affecting its abundance are poorly understood. The present study aims to understand the wintertime sources of NH<inf>3</inf> 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<inf>2.5</inf>. A distinct diurnal variation in the stable isotopic signature of total nitrogen (δ¹⁵N-TN) and ammonium (δ¹⁵N–NH<inf>4</inf>⁺) were observed; although, average day and night time concentrations of TN and NH<inf>4</inf>⁺ were similar. Mixing model results using δ¹⁵N–NH<inf>3</inf> reveal the dominance of non-agricultural emissions (NH<inf>3</inf> slip: 47 ± 24%) over agricultural emissions (24 ± 11%), combustion sources (19 ± 14 %), and biomass burning (10 ± 8%) for atmospheric NH<inf>3</inf>. Diurnal variability in source contributions to NH<inf>3</inf> was insignificant. Further, significantly negative correlations of δ¹⁵N–NH<inf>4</inf>⁺ with ambient relative humidity (RH) and daytime NO<inf>3</inf>⁻-N concentration were observed, and attributed to the possibility of NH<inf>4</inf>NO<inf>3</inf> volatilization during day-time owing to lower RH and higher temperature, resulting in isotopic enrichment of the remaining NH<inf>4</inf>⁺ in aerosol phase. This study, a first of its type from India, highlights the importance of non-agricultural NH<inf>3</inf> emissions over the agriculture dominated IGP region, and the role of local meteorology on the isotopic fractionation of δ¹⁵N in aerosol NH<inf>4</inf>⁺.