Exceedance of critical loads of nitrogen and sulfur deposition across national parks: Comparing 2015–2017 CMAQ and TDep model outputs

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

doi:10.36967/2305350

Atmospheric deposition can alter biological communities through a variety of pathways. Atmospheric nitrogen (N) deposition causes some plant and animal species to increase growth and/or abundance. Other species then can be outcompeted and eliminated from the biological community. Deposition of reactive N (Nr) to watersheds can impact both aquatic and terrestrial ecosystem elements and contribute to eutrophication and acidification effects. Atmospheric sulfur (S) deposition has been the primary driver of soil and surface water acidification of acid-sensitive ecosystems, resulting in adverse effects on aquatic biota and forest health. Critical loads (CLs) of atmospheric N and S deposition are used to establish thresholds of ecosystem impacts associated with a given level of deposition. The CLs of N for a decline in sensitive lichen species richness (3.1 kg-N ha-1 yr-1) and to protect herbaceous species richness (6-16 kg-N ha-1 yr-1) were chosen to evaluate N impacts at lower and higher levels of deposition. To evaluate S impacts, CLs for a decline in sensitive lichen species richness (2.3 kg-S ha-1 yr-1) and the CL to protect acid neutralizing capacity (ANC) of lakes and streams (0-31 kg-S ha-1 yr-1) were chosen. Locations where the deposition is higher than the CL are considered to have deposition in “exceedance” of the CL. Exceedances can be calculated using more than one method 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]) using representative annual deposition estimates for 2017 across the conterminous US. Given that TDep represents a measurement-based adjustment of CMAQ model results, this project evaluates the extent to which those adjustments affect the magnitude and spatial extent of critical load exceedances as compared with using deposition estimates directly from CMAQ for all National Park Service (NPS) units with available data. This analysis showed that selection of the deposition data source was most important in consideration of CL exceedance for lichen species richness. The extent of CL exceedance was less affected by the deposition data source for CLs to protect herbaceous species richness and surface water acidification. Results from this study can be used to guide natural resource management and policy in the context of understanding uncertainty in air pollution effects caused by atmospheric N and S deposition across a broad set of federally protected NPS units. The results can also provide insight into where local deposition measurements elucidate risk to ecosystem health.

Exceedance of critical loads of nitrogen and sulfur deposition across national parks: Comparing 2015–2017 CMAQ and TDep model outputs

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