Examining the transport of ammonia emissions across landscapes using nitrogen isotope ratios

The proportion of reactive nitrogen in wet deposition attributable to ammonium (NH4+) has increased over the last three decades in the U.S. due to steadily increasing NH3 emissions and concomitant reductions in NOx emissions. The importance of examining NH3 emission sources and transport is amplified as the fraction of NH3 contributing to reactive nitrogen budget increases. Presented here is a study illustrating how the nitrogen stable isotopic composition of NH3 (δ15N-NH3) can be used to characterize the transport of NH3 emissions at the landscape-scale. To accomplish this, ambient NH3 was sampled across varying land use types (e.g. conventionally managed cornfield, tallgrass prairie, concentrated animal feeding operation (CAFO), dairy operation, urban setting) and analyzed for δ15N-NH3 values. Ambient δ15N-NH3 values at a conventionally managed cornfield were used in an isotope mixing model to make first order approximations of NH3 source contributions to ambient air over the cornfield. Results suggest that while volatilized fertilizer is a primary contributor to ambient NH3 after fertilizer application, during periods of low or no fertilization, vehicle NH3 emissions can be a substantial contributor to ambient NH3 over cornfields that are adjacent to roadways. These source approximations can aid in evaluating NH3 emission abatement techniques to local landscapes. Modeled NH3 deposition flux at a CAFO was found to contribute a considerable amount of nitrogen to the landscape and δ15N-NH3 values were used to trace the livestock source contributing to this flux. Ambient NH3 concentrations and δ15N-NH3 values were measured across an urban region and δ15N-NH3 values indicated primarily fossil fuel-based emissions and large spatial variations in NH3 concentrations.