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 (δ 15 N) in wet nitrate deposition across 33 sites in the midwestern and northeastern U.S. We demonstrate that spatial variations in δ 15 N are strongly correlated with NO x emissions from surrounding stationary sources and additionally that δ 15 N is more strongly correlated with surrounding stationary source NO x emissions than pH, SO 4 2- , or NO 3 - concentrations. Although emission inventories indicate that vehicle emissions are the dominant NO x source in the eastern U.S., our results suggest that wet NO 3 - 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 δ 15 N values are a robust indicator of stationary NO x contributions to wet NO 3 - deposition and hence a valuable complement to existing tools for assessing relationships between NO 3 - deposition, regional emission inventories, and for evaluating progress toward NO x reduction goals.