Sources and processes affecting the abundances of atmospheric NH using δN over northwestern Indo-Gangetic plain.

Ammonia (NH) 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 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 over a semi-urban site (Patiala, 30.3°N, 76.4°E, 249 m amsl) located in the IGP using species specific δN in PM. A distinct diurnal variation in the stable isotopic signature of total nitrogen (δN-TN) and ammonium (δN-NH) were observed; although, average day and night time concentrations of TN and NH were similar. Mixing model results using δN-NH reveal the dominance of non-agricultural emissions (NH slip: 47 ± 24%) over agricultural emissions (24 ± 11%), combustion sources (19 ± 14 %), and biomass burning (10 ± 8%) for atmospheric NH. Diurnal variability in source contributions to NH was insignificant. Further, significantly negative correlations of δN-NH with ambient relative humidity (RH) and daytime NO-N concentration were observed, and attributed to the possibility of NHNO volatilization during day-time owing to lower RH and higher temperature, resulting in isotopic enrichment of the remaining NH in aerosol phase. This study, a first of its type from India, highlights the importance of non-agricultural NH emissions over the agriculture dominated IGP region, and the role of local meteorology on the isotopic fractionation of δN in aerosol NH.