Reactive Nitrogen Composition and Origin in the United States Rocky Mountains

Excess reactive nitrogen (Nr) deposition is occurring in sensitive ecosystems in the Rocky Mountains. In some high alpine lakes in Rocky Mountain (RMNP) and Grand Teton (GTNP) national parks, this increased deposition is causing biogeochemical changes. Nr is a rich mix of inorganic and organic oxidized and reduced N compounds originating from an array of natural and anthropogenic sources. Current routine monitoring programs do not measure important Nr compounds including dry deposited ammonia and dry and wet deposited organic N (ON). These can be large contributors to the total Nr deposition budget. This missing fraction could hamper the ability to develop effective management goals and control strategies, and track progress towards these goals to remediate damaged and maintain healthy ecosystems. To better understand the Nr composition and origin, field studies were conducted in RMNP in 2006 and 2009, and in GTNP in the summer of 2011. Large contributions of wet-deposited ON and dry-deposited ammonia were measured during all seasons with 17% and 17% annual contributions of these two species respectively, to total Nr at RMNP, and 11% and 28% summer contributions at GTNP respectively. At RMNP, the Nr originated from sources within Colorado (>40%) and from sources throughout the western United States including ~10% from California. Forty five percent of the dry deposited ammonia originated from sources outside of Colorado, indicating regional contributions to a pollutant often considered as having predominantly local origin. However, at GTNP it was estimated that about three quarters of the ammonia originated from sources in the Snake River Valley, an intensive agricultural region directly upwind of the park. The large ammonia dry deposition was estimated assuming a one-way dry deposition flux. Any reemission of previously deposited ammonia would reduce this input. The importance of reemission remains an open and important question. Measurements of the total ambient ON compounds are still problematic, though the water soluble particulate ON fraction was found to be significant. In addition, limited measurements suggest that during biomass burning events reduced ON gases are equal to or greater than ammonia concentrations.