Sources of gaseous NH3 in urban Beijing from parallel sampling of NH3 and NH4+, their nitrogen isotope measurement and modeling

Atmospheric gaseous ammonia (NH3) is the most abundant alkaline gas in the atmosphere while aerosol ammonium (NH4+) constitutes a majority of the inorganic cation concentration in total PM2.5 mass and plays a vital role in severe haze formation. This study tried to shed some light on sources of gaseous NH3 through integrating the parallel measurements of δ15N values in NH4+ and ambient NH3, NH3 source signature measurement, IsoSource model, and chemistry and transport model (CTM). As a result, predicted initial δ15N (NH3) values ranging from -42.0‰ to -4.9‰ were derived from daily δ15N(NH4+) values of fine particulate NH4+, and δ15N(NH3) values ranging from -26.8‰ to -17.2‰ were obtained from weekday/weekend δ15N(NH3) values, respectively. During summer, non-agricultural sources (e.g. fossil fuel sources, urban waste) contributed 63% to ambient NH3 in urban Beijing, derived from δ15N(NH3) values whereas 64% to ambient NH3, derived from δ15N(NH4+) values. These results suggested that non-agricultural sources were main contributors to gaseous NH3 even during summer and agricultural sources were not likely the main source of gaseous NH3 in urban Beijing. To further reduce the uncertainty of isotope-based source apportionment results, more laboratory and field studies are necessary to refine the δ15N(NH3) source profile of NH3 using validated collection technique as overlapping exist between δ15N(NH3) values in agricultural sources such as livestock breeding waste (-42.5‰ to -29.1‰) and fertilizer application (-51.5‰ to -35.0‰).