EXAMINING THE SOURCES AND TRANSPORT OF REACTIVE NITROGEN EMISSIONS USING STABLE ISOTOPES TECHNIQUES

Reactive nitrogen (Nr) emissions (i.e., NOx, HNO3, and NH3) from the transportation and electricity-generating sectors are declining due to increasingly stringent air quality regulations and development of new emission reduction technologies. In comparison, Nr emissions from the agricultural sector are increasing and are largely unregulated in the US. Identifying Nr emission sources and quantifying Nr source contributions are important initial steps for reducing nitrogen inputs to the environment and have become of particular concern to air quality managers, modelers, and epidemiologists. Stable isotope techniques are an emerging tool used to aid in the quantification of Nr emission sources and transport. This work presents a comprehensive inventory of the isotopic compositions of reactive Nr sources. The inventory reveals distinct differences between the isotopic compositions of fossil fuel and agricultural Nr emission sources. Equipped with these isotopic signatures of Nr sources, the isotopic ratios of ambient Nr were used to trace the transport and deposition of emissions across landscapes including dairy farms, a concentrated animal feeding operation, tallgrass prairies, conventionally managed cornfields, barrier island dunes, and urban systems. The isotopic composition of ambient Nr was used in conjunction with source signatures and isotope mixing models to quantify source contributions to ambient Nr concentrations. After assessing the effectiveness of using isotopic ratios to trace Nr emissions across landscapes, investigations were scaled up to examine Nr regional transport. In collaboration with the National Atmospheric Deposition Program’s Ammonia Monitoring Network, monthly NH3 emission were collected at nine sites across the U.S and analyzed for isotopic composition. Resulting NH3 isotopic composition showed spatial and temporal trends associated with primary regional NH3 sources. To further investigate regional transport of Nr emissions, nitrate isotopes in a Greenland ice core were examined to infer long-term relationships between agricultural activities, soil NOx emissions, and subsequent regional transport and deposition. Results of this investigation demonstrate that nitrate isotopes in ice cores, coupled with newly constrained isotopic compositions of NOx emission sources, provide a novel means for estimating contemporary and historic contributions from individual NOx emission sources to deposition