Utility of Isotopic Techniques for Partitioning NOSx Sources Contributing to Landscape Nitrogen Deposition

E.M. Elliott1,2*, Kendall, C.2, Boyer, E.W.3, Burns, D.A.4, Lear G.5, Felix, J.D.1, Golden, H.E.6, Harlin, K.7, Bytnerowicz, A.8, Butler, T.J.9 and Glatz, R.

Dry deposition is a major component of total atmospheric nitrogen deposition and thus an important source of bioavailable nitrogen to ecosystems. However, relative to wet deposition, less is known regarding the sources and spatial variability of dry deposition. This is in part due to difficulty in measuring dry deposition and associated deposition velocities. Passive sampling techniques offer potential for improving our understanding of the spatial distribution and sources of gaseous and aerosol N species, referred to here as dry deposition. We present dual nitrate isotopes (δ15N and δ18O) in actively collected dry and wet deposition across the high deposition region of Ohio, New York, and Pennsylvania. We also present results from initial tests to examine the efficacy of using passive nitric acid collectors as a collection medium for isotopic analysis at a site in New York. Further, we present new measurements of δ15N in stack gases from a coal-fired power plant. Isotopes in actively collected dry deposition, including particulate nitrate and gaseous nitric acid, are compared with those in wet nitrate deposition and surrounding NOx emission sources.

δ15N values in dry and wet fractions are highest at the westernmost sites and lowest at the easternmost sites, and stationary source NOx emissions (e.g., power plants and incinerators) appear to be the primary control on δ15N spatial variability. In contrast, δ18O values show a less consistent spatial pattern in dry deposition. Both δ15N and δ18O show strong seasonality, with higher values in winter than summer. Seasonal variations in stationary source NOx emissions appear to be the most likely explanation for seasonal variations in δ15N, whereas seasonal variations in air temperature and solar radiation indicate variable chemical oxidation pathways control δ18O patterns. Additionally, we demonstrate the utility of passive samplers for collecting the nitric acid (HNO3) component of dry deposition suitable for isotopic analysis. We observe slight differences in δ15N-HNO3 values between simultaneous samples collected actively and passively (0.6‰). However, we observe a larger offset in δ18O values between actively and passively collected samples; the causes for this offset warrant further investigation. Nonetheless, passive sample collection represents a significant cost savings over active sampling techniques and will allow a more extensive understanding of patterns of dry deposition and associated insights to nitrogen sources across landscapes.

1 University of Pittsburgh, Pennsylvania, USA
2 U.S. Geological Survey, Menlo Park, California, USA
3 Pennsylvania State University, University Park, PA, USA
4 U.S. Geological Survey, Troy, New York, USA
5 U.S. Environmental Protection Agency, Washington, D.C.
6 U.S. Environmental Protection Agency, Athens, GA, USA
7 NADP Central Analytical Lab, Champaign-Urbana, IL
8 U.S. Forest Service, Riverside, CA 9 Cornell University and the Institute for Ecosystem Studies
*Corresponding author: Department of Geology & Planetary Science, University of Pittsburgh, 200 SRCC, 4107 O’Hara Street, Pittsburgh, PA 15260, p: 412/624-8882 ,f: 412/624-3914, email: