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

Global inputs of NO_x are dominated by fossil fuel combustion from both stationary and vehicular sources and far exceed natural NO_x sources. However, elucidating NO_x 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 (δ¹⁵N) in wet nitrate deposition across 33 sites in the midwestern and northeastern U.S. We demonstrate that spatial variations in δ¹⁵N are strongly correlated with NO_x emissions from surrounding stationary sources and additionally that δ¹⁵N is more strongly correlated with surrounding stationary source NO_x emissions than pH, SO₄^2-, or NO₃^- concentrations. Although emission inventories indicate that vehicle emissions are the dominant NO_x source in the eastern U.S., our results suggest that wet NO₃^- deposition at sites in this study is strongly associated with NO_x emissions from stationary sources. This suggests that large areas of the landscape potentially receive atmospheric NO_y deposition inputs in excess of what one would infer from existing monitoring data alone. Moreover, we determined that spatial patterns in δ¹⁵N values are a robust indicator of stationary NO_x contributions to wet NO₃^- deposition and hence a valuable complement to existing tools for assessing relationships between NO₃^- deposition, regional emission inventories, and for evaluating progress toward NO_x reduction goals.