Nitrogen isotopes as indicators of NOx source contributions to atmospheric nitrate deposition across the Midwestern and northeastern United States

Global inputs o NOx are dominated by fossil fuel combustion from both stationary and vehicular sources and far exceed natural NOx sources. However, elucidating NOx sources to any given location remains a difficult challenge, despite the need for this information to develop sound regulatory and mitigation strategies. We present results from a regional-scale study of nitrogen isotopes (delta N-15) in wet nitrate deposition across 33 sites in the midwestern and northeastern U.S. We demonstrate that spatial variations in delta N-15 are strongly correlated with NOx emissions from surrounding stationary sources and additionally that delta N-15 is more strongly correlated with surrounding stationary source NOx emissions than pH, SO42-, or NO3- concentrations. Although emission inventories indicate that vehicle emissions are the dominant NOx source in the eastern U.S., our results suggest that wet NO3- deposition at sites in this study is strongly associated with NOx emissions from stationary sources. This suggests that large areas of the landscape potentially recieve atmospheric NOy deposition inputs in excess of what one would infer from existing monitoring data alone. Moreover, we determined that spatial patterns in delta N-15 values are a robust indicator of stationary NOx contributions to wet NO3- deposition and hence a valuable complment to existing tools for assessing relationships between NO3- depostion, regional emission inventories, and for evaluting progress toward NOx reduction goals.