Distinguishing NOx Source Contributions to Wet and Dry Nitrate Deposition in the United States using Stable Isotopes

Nitrate isotopes in wet deposition are useful indicators of NOx source contributions to nitrate formation and NOx oxidation pathways at local and regional scales. Here, we examine whether nitrogen and oxygen isotopes (d15N and d18O, respectively) provide similarly useful information in: 1) wet deposition at the continental scale; and 2) dry deposition at the regional scale. For wet deposition analyses, weekly archived samples (2000) from 156 NADP sites across the United States were pooled into bimonthly, volume-weighted composites and analyzed for d15N and d18O of nitrate. For dry deposition analyses, weekly nitric acid and particulate nitrate samples from eight CASTNET sites in New York, Pennsylvania, and Ohio were pooled into monthly composites from April 2004- April 2005 and analyzed for d15N and d18O. We present spatial and temporal variations in both N and O isotopes, and investigate the critical question of whether these variations are a function of atmospheric processes and/or NOx source contributions. Building on our results from the Northeastern US (Elliott et al., in press), we examine relationships between d15N values in wet and dry deposition and surrounding NOx emissions. At the national scale, we determined that d15N in wet deposition is strongly correlated with the distribution of major NOx sources, including stationary, mobile, and biogenic emissions. Correlations with biogenic sources are strongest during the warmer months and areas of intensive agriculture, particularly the mid-west. Although d15N at high elevation site is correlated with major NOx sources, the correlations are distinct from other regions. d18O values are strongly correlated with temperature and solar radiation, furthering the notion that seasonal variations in photochemistry influence d18O values. In nitric acid and particulate nitrate dry deposition, d15N and d18O exhibit seasonal trends similar to those of wet nitrate deposition, although d15N values are significantly higher in the dry fractions. Similar to the case with wet deposition, d15N values in dry deposition are strongly correlated with surrounding NOx emissions from stationary sources. Finally, we discuss the use of isotopes in wet and dry deposition as a potential tool for monitoring tool long-term reductions in source contributions.