Intermodel comparison of atmospheric nitrogen deposition estimates for the conterminous United States and critical load exceedance for selected national parks

McDonnell, Todd; Sullivan, Timothy; Knees, Brian; Bell, Michael; Felker-Quinn, Emmi

Atmospheric deposition of reactive nitrogen (Nr), including both reduced and oxidized forms, can have adverse impacts on aquatic and terrestrial ecosystem elements of National Parks, including eutrophication and acidification processes. Critical loads (CLs) of atmospheric Nr deposition are used to quantify ecosystem impacts associated with a given, tipping point, level of deposition. Critical load exceedance is determined as the difference between ambient deposition and the CL. Exceedances can be calculated using a variety of methods for estimating deposition. This report quantifies differences in atmospheric deposition estimates generated by two modeling approaches (Total Deposition [TDep] and Community Multiscale Air Quality [CMAQ]) at two points in time (2003 and 2011) throughout the conterminous US. The extent to which calculations of CL exceedance varied based on these two approaches for modeling deposition and the timeframes of exceedance were evaluated throughout 10 National Parks. At the national scale, estimates of total N deposition from TDep were generally higher than those from CMAQ, and agreement between TDep and CMAQ was better in 2011 than 2003. TDep estimates of deposition tended to be higher in mountainous areas near the west coast, whereas eastern mountain regions generally showed higher values from CMAQ. Locations where total N deposition estimated by CMAQ was higher than TDep by the largest magnitudes were mostly near densely populated urban areas. Exceedance of CLs protective of lichen species richness, herbaceous species richness, tree growth, and/or probability of tree survival occurred within all of the National Park lands included in this study. The magnitude and spatial extent of exceedance was dependent at many locations on the source of deposition data. Grand Teton and Yosemite National Parks showed substantially more areas in exceedance of lichen CLs based on TDep as compared with CMAQ. Relatively minor differences in the spatial patterns of exceedance of CLs protective of herbaceous species richness were observed between the two deposition models. Few substantial differences in the magnitude or extent of exceedance between TDep and CMAQ occurred for the CLs protective of tree growth. TDep indicated greater effects on probability of tree survival for Grand Teton, Great Smoky Mountains, North Cascades, and Yosemite National Parks and CMAQ indicated greater effects on probability of tree survival for Acadia, Glacier, and Rocky Mountain National Parks and Sleeping Bear Dunes National Lakeshore. These differences highlight the importance of accurately modeling deposition to provide the foundation for informed management decisions.

Intermodel comparison of atmospheric nitrogen deposition estimates for the conterminous United States and critical load exceedance for selected national parks

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