Utilizing stable isotopes to characterize reactive nitrogen emissions and deposition

Background/Question/Methods

Reactive nitrogen, including nitrogen oxides (NO_x = NO + NO₂) and ammonia (NH₃), are significant contributors to global water and air quality degradation – resulting in a myriad of human health and ecosystem impacts.  Natural and anthropogenic sources can be significant contributors to total reactive nitrogen; however source partitioning remains a challenge for researchers and policy makers. This research investigates the use of stable isotopes of NH₃ and NO_x as monitoring tools for understanding sources, transport, and fate of reactive nitrogen pollutants. Particular emphasis is placed on characterizing the isotopic composition of NO_x (δ¹⁵N-NO_x) and NH₃ (δ¹⁵N-NH₃) from livestock operations, fossil fuel combustion, cropland, pasture, and prairie ecosystems.

Results/Conclusions

For initial measurements of livestock NH₃ emissions, a transect radiating outward from a 150 head dairy barn and downwind to the edge of a forest was sampled for NH₃ using Ogawa passive samplers.  The δ¹⁵N-NH₃ values inside the barn and adjacent to the ventilating fans range from -23‰ to -29‰ indicating a largely negative δ¹⁵N value for livestock waste emissions.  d¹⁵N values progressively increased away from the barn, indicating decreasing contributions of livestock emissions to ambient NH₃.  Additionally, to characterize a major source of fossil fuel NO_x, two coal-fired power plants, one using selective catalytic reduction (SCR) and one not using SCR, were sampled.  The δ¹⁵N-NO_x values of the plant using the SCR and the non-SCR plant were ~+20‰ and ~+10‰ respectively. 
In addition to these initial measurements of NO_x and NH₃, field deployments in the spring and summer of 2010 will characterize d¹⁵N values from a conventionally managed corn field, a 30,000 head cattle concentrated animal feeding operation, vehicle emissions in a highly trafficked tunnel, and biogenic emissions from a tallgrass prairie.  This research will result in the most comprehensive survey to date of the isotopic composition of emission sources of reactive nitrogen and will serve as a critical foundation for subsequent studies aimed at the sources, transport, and fate of reactive nitrogen pollutants.