AQAST research highlights

Present and future nitrogen deposition to
National Parks in the United States: Critical Load Exceedences
by Raluca Ellis et al

Ellis, R.A., et al, submitted to Atmos. Chem. Phys., 2013.

Ellis et al 2013

Figure: Critical load exceedance (total deposition minus critical load) for nitrogen deposition in 2006 (top) and in 2050 (bottom, assuming the optimistic RCP2.scenario). National parks with CL exceedance are shown in red, and those with no CL exceedance are shown in green. Marginal cases where the park itself does not have CL exceedance but an adjacent model gridsquare does (~50 km away) are shown in orange.

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 GEOSā€Chem global chemical transport model with 1/2° x 2/3° horizontal resolution over North America. We estimate CL values in the range 2.55 kg N ha-1 yr-1 for the different parks with the goal of protecting 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 (4085%) of the deposition originates from NOx emissions (fuel combustion). We then project future changes in N deposition using the Representative Concentration Pathway (RCP) emission scenarios for 2050. These feature 5273% declines in US NOx emissions relative to present but 1950% 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 RCP scenarios. Even in total absence of anthropogenic NOx emissions, 1418 parks would still have a CL exceedance. Returning all parks to N deposition below CL by 2050 will require at least a 55% decrease in anthropogenic NH3 emissions relative to RCP-projected 2050 levels.

Mar 2013