An emerging approach for quantifying NH3 emission sources is to investigate the nitrogen isotopic composition of atmospheric NH3 (δ15N-NH3). δ15N-NH3 values associated with fossil fuel combustion sources differ from those of agricultural sources thus providing a tool for determining the contribution of different NH3 sources to ambient atmospheric concentrations. This work presents application of this tool to infer NH3 sources on a regional scale by measuring monthly δ15N-NH3 values at nine National Atmospheric Deposition Program Ammonia Monitoring Network (AMoN) sites over a year period. These AMoN sites are located away from urban areas and point source pollution in effort to represent regional atmosphere unaffected by direct sources (e.g. vehicles, livestock). US agricultural regions had low and seasonally variable δ15N-NH3 values associated with conventional agricultural activity regimes, while rural nonagricultural areas had higher and seasonally consistent δ15N-NH3 values associated with a constant “natural” (e.g. soil, vegetation, bi-directional flux, ocean) emission source. The majority of sites in the US experienced a decrease in δ15N-NH3 values related to the country’s spring agricultural activity peak. While these results are valuable when characterizing spatial and temporal trends of regional NH3 sources, they do not represent NH3 dynamics in urban areas where a majority of the population can be affected by poor air quality.
Atmospheric NH3 concentrations in urban areas may result from transport of NH3 emissions from rural agricultural sources but urban sources (e.g. vehicles, sewage, and industry) may also be primary contributors. Ammonia concentrations in urban areas vary widely making it difficult to qualify sources contributing to ambient NH3. This work presents a pilot study in an urban air shed, Pittsburgh, PA, USA, that measured the isotopic composition of NH3 at ten sites over a one month period and concluded that a majority of the atmospheric NH3 can be attributed to vehicle and industrial sources. To build on this pilot study we collected monthly NH3 samples at eight sites over the period of one year in a coastal urban air shed, Corpus Christi, TX, USA and measured the nitrogen isotopic composition. This urban air shed experiences the expected vehicle and industrial emissions of a large city but it is also regularly subjected to marine air masses and is surrounded by agricultural land. We will report spatial and temporal trends in NH3 concentration and δ15N-NH3 values in an effort to delineate sources affecting NH3 concentrations in this air shed. In addition, a pilot study is currently underway to use this stable isotope technique to investigate NH3 emissions collected at ten sites over one month in Morelia, MI, MX. Preliminary results will be presented to compare varying urban air shed types. Knowledge of NH3 sources in disparate urban areas is important for determining location specific air quality attainment strategies and possible future NH3 emission reduction goals.