Spatial and temporal patterns of nitrogen isotopic composition of ammonia at U.S. ammonia monitoring network sites.

Ammonia (NH 3 ) emissions and ammonium (NH 4 + ) deposition can have harmful effects on the environment and human health but remain generally unregulated in the U.S. PM 2.5 regulations require that an area not exceed an annual average PM 2.5 value of 12 μg/m 3 (averaged over three years), and since NH 3 is a significant precursor to PM 2.5 formation these are the closest indirect regulations of NH 3 emissions in the U.S. If the U.S. elects to adopt NH 3 emission regulations similar to those applied by the European Union, it will be imperative to first adequately quantify NH 3 emission sources and transport, and also understand the factors causing varying emissions from each source. To further investigate NH 3 emission sources and transport at a regional scale, NH 3 was sampled monthly at a subset of nine Ammonia Monitoring Network (AMoN) sites and analyzed for nitrogen isotopic composition of NH 3 (δ 15 N-NH 3 ). The observed δ 15 N-NH 3 values ranged from −42.4 to +7.1‰ with an average of −15.1 ± 9.7. The observed δ 15 N-NH 3 values reported here provide insight into the spatial and temporal trends of the NH 3 sources that contribute to ambient [NH 3 ] in the U.S. In regions where agriculture is prevalent (i.e., U.S. Midwest), low and seasonally variable δ 15 N-NH 3 values are observed and are associated with varying agricultural sources. In comparison, rural nonagricultural areas have higher and more seasonally consistent δ 15 N-NH 3 values associated with a constant “natural” (e.g. soil, vegetation, bi-directional flux, ocean) NH 3 source. With regards to temporal variation, the peak in U.S. spring agricultural activity (e.g. fertilizer application, livestock waste volatilization) is accompanied by a decrease in δ 15 N-NH 3 values at a majority of the sites, whereas higher δ 15 N-NH 3 values in other seasons could be due to shifting sources (e.g. coal-fired power plants) and/or fractionation scenarios. Fractionation processes that may mask NH 3 source signatures are discussed and require further investigation to optimize the utility of the nitrogen isotopic composition to determine NH 3 sources and dynamics. [ABSTRACT FROM AUTHOR]