Annual cycle in reduced and oxidized forms of atmospheric nitrogen species at Rocky Mountain National Park

Atmospheric deposition of nitrogen compounds to alpine ecosystems is a growing concern in the Rocky Mountains. Rocky Mountain National Park (RMNP), adjacent to both urban and agricultural centers in Colorado, is particularly sensitive due to its pristine, nitrogen limited ecosystems. As part of the Rocky Mountain Atmospheric Nitrogen Study (RoMANS), we investigated the annual cycle of nitrogen speciation at RMNP. From November 2008-2009, several high time resolution techniques were deployed at the long-term IMPROVE monitoring site at RMNP. This study focuses on high time resolution measurements of gas-phase concentrations of ammonia and NOy compounds, using modified versions of commercially available instruments. Modifications include enhanced speciation on time scales of minutes to hours, potentially important because of the episodic nature of transport and deposition. At distinct times, oxidized vs. reduced species predominated at RMNP. Reduced nitrogen, identified using two online techniques as gas phase ammonia, was the largest identified contributor to the gas phase nitrogen budget at RMNP. Ammonia concentrations demonstrated a distinct diurnal and seasonal profile showing peaks both midday and in the summer. Nitrogen oxides demonstrated less seasonal and diurnal dependence, responding more on an event basis often associated with easterly upslope flow and precipitation. Ongoing analysis of the results from RoMANS is intended to inform policy efforts aimed at minimizing adverse nitrogen deposition at Rocky Mountain National Park.