Nitrogen Isotopes as Indicators of NO_x Source Contributions to Atmospheric Nitrate Deposition Across the Midwestern and Northeastern United States

摘要

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 N0_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 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.