1School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
2Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
3Department of Atmospheric and Oceanic Sciences, Laboratory for Climate and Ocean-Atmosphere Studies, School of Physics, Peking University, Beijing, China
4Department of Earth System Science, University of California, Irvine, CA, USA
5Air Resources Division, National Park Service, Fort Collins, CO, USA
6Air Resources Division, National Park Service, Denver, CO, USA
7USDA Forest Service, University of Vermont Aiken Center, Burlington, VT, USA
8Office of Air and Radiation, United States Environmental Protection Agency, Washington, DC, USA
Received: 27 Feb 2013 – Discussion started: 05 Apr 2013
Abstract. National parks in the United States are protected areas wherein the natural habitat is to be conserved for future generations. Deposition of anthropogenic nitrogen (N) transported from areas of human activity (fuel combustion, agriculture) may affect these natural habitats if it exceeds an ecosystem-dependent critical load (CL). We quantify and interpret the deposition to Class I US national parks for present-day and future (2050) conditions using the GEOS-Chem global chemical transport model with 1/2° × 2/3° horizontal resolution over North America. We estimate CL values in the range 2.5–5 kg N ha−1 yr−1 for the different parks to protect the most sensitive ecosystem receptors. For present-day conditions, we find 24 out of 45 parks to be in CL exceedance and 14 more to be marginally so. Many of these are in remote areas of the West. Most (40–85%) of the deposition originates from NOx emissions (fuel combustion). We project future changes in N deposition using representative concentration pathway (RCP) anthropogenic emission scenarios for 2050. These feature 52–73% declines in US NOx emissions relative to present but 19–50% increases in US ammonia (NH3) emissions. Nitrogen deposition at US national parks then becomes dominated by domestic NH3 emissions. While deposition decreases in the East relative to present, there is little progress in the West and increases in some regions. We find that 17–25 US national parks will have CL exceedances in 2050 based on the RCP8.5 and RCP2.6 scenarios. Even in total absence of anthropogenic NOx emissions, 14–18 parks would still have a CL exceedance. Returning all parks to N deposition below CL by 2050 would require at least a 50% decrease in US anthropogenic NH3 emissions relative to RCP-projected 2050 levels.
Revised: 29 Jul 2013 – Accepted: 05 Aug 2013 – Published: 10 Sep 2013
Ellis, R. A., Jacob, D. J., Sulprizio, M. P., Zhang, L., Holmes, C. D., Schichtel, B. A., Blett, T., Porter, E., Pardo, L. H., and Lynch, J. A.: Present and future nitrogen deposition to national parks in the United States: critical load exceedances, Atmos. Chem. Phys., 13, 9083-9095, doi:10.5194/acp-13-9083-2013, 2013.