Patterns in atmospheric circulation affect emission sources contributing to nitrogen deposition in the Columbia River Gorge, Pacific Northwest USA

Abstract:

The Columbia River Gorge separating Oregon and Washington provides an ideal setting to investigate how atmospheric circulation patterns determine types of emission sources contributing to atmospheric deposition. Up-gorge and down-gorge atmospheric circulation patterns each provide a different suite of emission sources. Up-gorge airflow originates in the Portland-Vancouver metro area dominated by urban and industrial sources. Down-gorge patterns originate in the Columbia River basin, which is dominated by agricultural production. We tested the dependence of emission sources contributing to atmospheric deposition on circulation patterns by measuring the isotopic composition of nitrate (NO₃⁻) in 2003–2004 precipitation samples from the WA98-Columbia River Gorge NADP & USNIP site. Circulation patterns were determined using back-trajectory analysis with the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model using the archived EDAS meteorological dataset.

We observed a significant difference (P=0.01) between up-gorge and down-gorge patterns with mean δ¹⁵N-NO₃⁻ of +1.8 and −2.1‰ for up- and down-gorge, respectively. The differences observed between these two patterns is likely tied to the different emission sources of N found in these different geographic areas. The lower δ¹⁵N of down-gorge sources is due to the large amount of agricultural production in the Columbia River basin. Observed values for the up-gorge patterns likely result from industrial and fossil fuel emissions of NO_x, the precursor of deposited NO₃⁻, in the Portland-Vancouver area. The significantly greater amount of NO₃⁻ in precipitation from up-gorge patterns (0.72 mg/L) compared to down-gorge patterns (0.36 mg/L, P=0.01) supports the influence of urban sources rather than relatively clean marine air which characteristically has low amounts of NO₃⁻. No significant differences are found in δ¹⁸O_nitrate or Δ¹⁷O_nitrate between the two patterns, suggesting that atmospheric chemistry between the two patterns is not greatly different. Differences in emission sources between geographic areas seem to drive the differences observed in the isotopic composition of N deposition between the different atmospheric circulation patterns in the Columbia River Gorge.