Sources of gaseous NH_3 in urban Beijing from parallel sampling of NH_3 and NH~+_4 , their nitrogen isotope measurement and modeling

摘要

Atmospheric gaseous ammonia (NH_3) is the most abundant alkaline gas in the atmosphere while aerosol ammonium (NH~+_4 ) constitutes a majority of the inorganic cation concentration in total PM_(2.5) mass and plays a vital role in severe haze formation. This study tried to shed some light on sources of gaseous NH_3 through integrating the parallel measurements of δ~(15)N values in NH~+_4 and ambient NH_3, NH_3 source signature measurement, IsoSource model, and chemistry and transport model (CTM). As a result, predicted initial δ~(15)N (NH_3) values ranging from -42.0%o to -4.9%. were derived from daily δ~(15)(NH~+_4) values of fine particulate NH~+_4, and δ~(15)N(NH_3) values ranging from -26.8‰ to -17.2‰ were obtained from weekday/weekend δ~(15)N(NH_3) values, respectively. During summer, non-agricultural sources (e.g. fossil fuel sources, urban waste) contributed 63% to ambient NH_3 in urban Beijing, derived from δ~(15)N(NH_3) values whereas 64% to ambient NH_3, derived from δ~(15)N(NH~+_4) values. These results suggested that non-agricultural sources were main contributors to gaseous NH_3 even during summer and agricultural sources were not likely the main source of gaseous NH_3 in urban Beijing. To further reduce the uncertainty of isotope-based source apportionment results, more laboratory and field studies are necessary to refine the δ~(15)N(NH_3) source profile of NH_3 using validated collection technique as overlapping exist between δ~(15)N(NH_3) values in agricultural sources such as livestock breeding waste (-42.5‰ to -29.1‰) and fertilizer application (-51.5‰ to -35.0‰).