Abstract
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.
Highlights
Hydrology and Earth SystemNitrogen (N) deposition has greSatclyieinnccreeassed over the last century due to fossil fuel combustion and production of industrial fertilizer (Aber et al, 2003; Fenn et al, 2003b; Galloway et al, 2004)
For present-day conditions, we find 24 out of 45 parks to be in critical load (CL) exceedance and 14 more to be marginally so
Excess deposition of N to natural ecycoclsiynsgt,emansdcacnaudseecarceOiadsieficcbeaiotaidoninveSarnsicdtyie,eudntrisocrpuehpitcastoioilnnouftriwena-t ters (Driscoll et al, 2001; Fenn et al, 2003a; Galloway et al, 2003). This excess deposition is of particular concern in US national parks, where legislation dictates that natural resources be preserved unimpaired (NPS, 2001)
Summary
Nitrogen (N) deposition has greSatclyieinnccreeassed over the last century due to fossil fuel combustion and production of industrial fertilizer (Aber et al, 2003; Fenn et al, 2003b; Galloway et al, 2004). Excess deposition of N to natural ecycoclsiynsgt,emansdcacnaudseecarceOiadsieficcbeaiotaidoninveSarnsicdtyie,eudntrisocrpuehpitcastoioilnnouftriwena-t ters (Driscoll et al, 2001; Fenn et al, 2003a; Galloway et al, 2003) This excess deposition is of particular concern in US national parks, where legislation dictates that natural resources be preserved unimpaired (NPS, 2001). NOx is produced in combustion by oxidation of atmospheric N2 and fuel nitrogen It is oxidized in the atmosphere on a timescale of a day to nitric acid (HNO3), which is removed rapidly by wet and dry deposition. Soil cycling of fertilizer N is a source of NOx. NH3 is removed rapidly from the atmosphere by wet and dry deposition, to HNO3. A continental-scale model analysis evaluated with relevant observations, as presented here, can provide a general perspective on N deposition to US national parks as well as projections for the future
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