Stable Isotopic Composition of NH3 at Regional and Urban Scales: Source and Transport Implications

Ammonia (NH₃) emissions and subsequent deposition are reported to have harmful effects on the environment and human health but remain generally unregulated in the US. If the country elects to adopt NH₃ emission regulations similar to those applied by the European Union, it will be imperative to first adequately quantify NH₃ emission sources and transport. However, quantification of NH₃ sources and reduction of NH₃ emissions may prove more difficult than previous air quality goals. Unlike other pollutants that are released consistently and predictably through industrial or mechanical processes and are readily quantifiable, NH₃ emissions from area sources (e.g. livestock waste and fertilizer) can be a function of both chemical and biological processes. Therefore, these sources are often highly variable over space and time and can lead to large uncertainties in NH₃ emission source and sink inventories.

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.