Spatial and temporal variation in sources of atmospheric nitrogen deposition in the Rocky Mountains using nitrogen isotopes

Publication date: March 2018
Source:Atmospheric Environment, Volume 176
Author(s): Leora Nanus, Donald H. Campbell, Christopher M.B. Lehmann, M. Alisa Mast
Variation in source areas and source types of atmospheric nitrogen (N) deposition to high-elevation ecosystems in the Rocky Mountains were evaluated using spatially and temporally distributed N isotope data from atmospheric deposition networks for 1995-2016. This unique dataset links N in wet deposition and snowpack to mobile and stationary emissions sources, and enhances understanding of the impacts of anthropogenic activities and environmental policies that mitigate effects of accelerated N cycling across the Rocky Mountain region. δ¹⁵N−NO3⁻ at 50 U.S. Geological Survey Rocky Mountain Snowpack (Snowpack) sites ranged from −3.3‰ to +6.5‰, with a mean value of +1.4‰. At 15 National Atmospheric Deposition Program (NADP)/National Trends Network wet deposition (NADP Wetfall) sites, summer δ¹⁵N−NO3⁻ is significantly lower ranging from −7.6‰ to −1.3‰ while winter δ¹⁵N−NO3⁻ ranges from −2.6‰ to +5.5‰, with a mean value of +0.7‰ during the cool season. The strong seasonal difference in NADP Wetfall δ¹⁵N−NO3⁻ is due in part to variation in the proportion of N originating from source regions at different times of the year due to seasonal changes in weather patterns. Snowpack NO3⁻ and δ¹⁵N−NO3⁻ are significantly related to NADP Wetfall (fall and winter) suggesting that bulk snowpack samples provide a reliable estimate at high elevations. Spatial trends show higher NO3⁻ concentrations and δ¹⁵N−NO3⁻ in the Southern Rocky Mountains located near larger anthropogenic N emission sources compared to the Northern Rocky Mountains. NADP Wetfall δ¹⁵N−NH4⁺ ranged from −10‰ to 0‰, with no observed spatial pattern. However, the lowest δ¹⁵N−NH4⁺(−9‰), and the highest NH4⁺ concentration (35 μeq/L) were observed at a Utah site dominated by local agricultural activities, whereas the higher δ¹⁵N−NH4⁺ observed in Colorado and Wyoming are likely due to mixed sources, including fossil fuel combustion and agricultural sources. These findings show spatial and seasonal variation in N isotope data that reflect differences in sources of anthropogenic N deposition to high-elevation ecosystems and have important implications for environmental policy across the Rocky Mountain region.



http://ift.tt/2CDfANz