Concentrations and deposition of reactive nitrogen in Grand Teton National Park
Nitrogen deposition is an issue of increasing importance in national parks in the western United States. In 2011 a network of sites in and around Grand Teton National Park was operated from April to September to better understand concentrations and spatial gradients of reactive nitrogen species and deposition fluxes in the park. Grand Teton Reactive Nitrogen Deposition Study (GrandTReNDS) measurements included precipitation chemistry and atmospheric concentrations of gases (NH3, HNO3, NOx, NOy) and particles (NH4+, NO3-, organic nitrogen).
A strong gradient in ammonia concentrations was observed, with higher average concentrations to the west (0.6 µg/m3) and lower average concentrations to the east (0.3 µg/m3), consistent with the presence of large agricultural operations west of the park. Concentrations of nitric acid, nitrate, and ammonium did not exhibit any clear spatial trends. In comparison to previous measurements in Rocky Mountain National Park, ammonia concentrations were higher in Grand Teton National Park while PM2.5 nitrate and ammonium concentrations were similar in the two regions. Average nitric acid concentrations were similar between the two parks as well, with the exception of one high elevation Grand Teton National Park site where higher concentrations were observed. Wet deposition of ammonium and dry deposition of ammonia were the largest reactive nitrogen deposition pathways in GTNP followed by wet deposition of nitrate and wet deposition of organic nitrogen.
Previous ecological assessments established a critical load for wet deposition of inorganic nitrogen to GTNP. Our observations reveal that the critical load is currently being exceeded. It is important to recognize that substantial additional inputs of reactive nitrogen are also occurring through dry deposition of ammonia and wet deposition of organic nitrogen. Neither pathway is currently considered in the U.S. critical load framework.