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 atmospherewhile aerosol ammonium(NH4+) constitutes amajority 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 delta N-15 values in NH4+ and ambient NH3, NH3 source signature measurement, IsoSource model, and chemistry and transport model (CTM). As a result, predicted initial delta N-15 (NH3) values ranging from -42.0 parts per thousand to -4.9 parts per thousand were derived from daily delta N-15(NH4+) values of fine particulate NH4+, and delta N-15(NH3) values ranging from -26.8 parts per thousand to -17.2 parts per thousand were obtained from weekday/weekend delta N-15(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 delta N-15(NH3) values whereas 64% to ambient NH3, derived from delta N-15(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 delta N-15(NH3) source profile of NH3 using validated collection technique as overlapping exist between delta N-15(NH3) values in agricultural sources such as livestock breedingwaste (-42.5% to-29.1%) and fertilizer application (-51.5% to -35.0%). (C) 2020 Elsevier B.V. All rights reserved.