Nitrate isotopes in wet deposition are useful indicators of NOx source contributions to nitrate formation and NOx oxidation pathways1. Here we examine whether nitrogen and oxygen isotopes (d15N and d18O, respectively) in dry deposition provide similarly useful information using dry deposition fractions collected from: 1)CASTNET samplers (nitric acid (HNO3) and particulate nitrate (NO3 -)); and 2) passive diffusion samplers (nitrogen dioxide (NO2), ammonia (NH3), and HNO3).

We present the first regional scale analyses of the isotopic composition of dry deposition using CASTNET samples from 8 sites in New York, Pennsylvania, and Ohio. Weekly samples were pooled into monthly composites from April 2004-April 2005 and both HNO3 and particulate NO3 - fractions were analyzed for d15N and d18O. We observed distinct spatial and temporal patterns in both d15N and d18O of HNO3 and particulate NO3 -. Potential causes for these patterns will be explored by comparing the CASTNET isotope data with the distribution of major NOx emission sources, as well as with volume-weighted monthly precipitation samples collected simultaneously at co-located NADP sites.

In addition, we examine the efficacy of using passive diffusion samples to assess the isotopic composition of various atmospheric N sources, and their utility for assessing multiple source contributions to individual sites. Passive diffusion samplers were deployed for up to one month to collect NH3, NO2, and HNO3 in various settings across New York. The deployment environments included parking garages, urban areas, vehicle tailpipes, agricultural areas, and confined animal feedlots. Preliminary results of d15N in these deposition fractions will be presented, with a focus on the potential for separating source contributions to dry N deposition. Further, knowledge gaps necessitating additional work will be identified and presented. Finally, using a synthesis of these data sets, we will present isotopes of wet and dry deposition as a potential method for distinguishing source contributions to individual sites, and for monitoring long-term reductions in
source contributions associated with the Clean Air Interstate Rule (CAIR).