HR: 16:15h
AN: B54A-02    [Abstracts]
TI: What can Nitrate Isotopes in Precipitation tell us about NOx Sources, Atmospheric Cycling, and Source Areas? Results from the First National Survey in the United States.
AU: * Elliott, E M
EM: eelliott@usgs.gov
AF: USGS, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Kendall, C
EM: ckendall@usgs.gov
AF: USGS, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Harlin, K
EM: kharlin@uiuc.edu
AF: NADP Central Lab, Illinois State Water Survey 2204 Griffith Drive, Champaign, IL 61820-7495 United States
AU: Butler, T
EM: tjb2@cornell.edu
AF: Cornell University, Center for the Environment Rice Hall, Ithaca, NY 14853 United States
AU: Carlton, R
EM: RCarlton@epri.com
AF: EPRI, 3420 Hillview Ave , Palo Alto, CA 94304 United States
AU: Wankel, S
EM: sdwankel@usgs.gov
AF: USGS, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Boyer, E
EM: boyer@nature.berkeley.edu
AF: UC- Berkeley, 137 Mulford Hall # 3114, Berkeley, CA 94720 United States
AU: Burns, D
EM: daburns@usgs.gov
AF: USGS, New York Water Science Center 425 Jordan Road , Troy, NY 12180-8349 United States
AB: Atmospheric deposition is a major source of nitrate exported to coastal waters and a key contributor to eutrophication of surface waters worldwide. In order to reduce N loads to surface waters, it is important to understand the relative contributions of major NOx sources to wet and dry deposition to watersheds. In the United States, the two largest NOx sources are vehicular emissions (54 percent) and stationary fuel combustion (40 percent). Reducing emissions from these sources is critical to improving air and surface water quality. However, using nitrate concentration data alone, it is difficult to establish relationships between individual NOx sources and wet deposition of nitrate. Previous research has shown that different NOx sources can have different isotopic compositions and can be used to identify NOx sources to wet deposition. To address this research need, we have completed the first national survey of nitrate isotopes in wet deposition using samples collected by the National Atmospheric Deposition Program (NADP). Archived samples (2000) from 156 NADP sites across the United States were pooled into bimonthly, volume-weighted composites and analyzed for δ¹⁵N, δ¹⁸O, and mass-independent δ¹⁷O of nitrate using the microbial denitrifier method. Nitrate concentrations in the archived samples were stable over several years, indicating that the probability of isotopic fractionation associated with sample storage is very low. 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 or NOx source contributions. In our analyses (n=883), we determined that δ¹⁵N values ranged from -11 to +3 permil, whereas δ¹⁸O values ranged from +63 to +94 permil. On average, both δ¹⁵N and δ¹⁸O values are higher in the winter than in the summer (approximately 2 and 10 permil higher, respectively). In the Northeastern and Mid-Atlantic US, we observed strong correlations between δ¹⁵N and NOx emissions from nearby electric generating units. Along a regional N deposition gradient from Ohio to Maine, δ¹⁵N values are strongly correlated with nitrate deposition, sulfate deposition, and mean annual pH. These results suggest that in this region, δ¹⁵N may be a powerful tracer of stationary source NOx emissions. Despite the fact that vehicle emissions are the largest NOx source in the eastern US, we found no correlations between δ¹⁵N and vehicle NOx emissions in this region. This raises some interesting questions about the fate of ground-level vehicle NOx emissions. Similar metrics will be applied to data across the US and the applicability of using these techniques to trace NOx sources at this scale will be discussed.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0469 Nitrogen cycling
DE: 0478 Pollution: urban, regional and global (0345, 4251)
DE: 1808 Dams
DE: 1854 Precipitation (3354)
SC: Biogeosciences [B]
MN: Fall Meeting 2005