Observations of atmospheric reactive nitrogen species in Rocky Mountain National Park and across northern Colorado

Increasing rates of nitrogen deposition are a concern in many protected ecosystems. Understanding the sources influencing these regions can be a challenge as there are often few observations available to understand the transport of key species. Several field campaigns were conducted in and around Rocky Mountain National Park (RMNP) from 2006 to 2009 to assess the impacts of various reactive nitrogen sources and regional meteorology on reactive nitrogen deposition. Measurements of ammonia, ammonium, nitric acid, and nitrate at ground-level sites across northern Colorado were used to examine spatial gradients in atmospheric reactive nitrogen concentrations, the influence of wind direction on reactive nitrogen transport in the regional atmosphere, and anthropogenic contributions to reactive nitrogen concentrations in RMNP. The highest concentrations of reduced nitrogen occurred on the eastern plains of Colorado while oxidized nitrogen concentrations were highest along the Front Range urban corridor. Both regions lie east of RMNP. Upslope (easterly) winds associated with mountain–valley wind patterns and larger, synoptic scale forcing, transport emissions from these sources westward up the eastern slope of the Rockies and into RMNP; the highest ammonium and nitrate concentrations in RMNP were clearly associated with this upslope transport pattern. Concentrations of key reactive nitrogen species east of the Continental Divide in RMNP were, on average, more than 50% higher than those observed at a background site located west of the park, further indicating large contributions from sources east of the park. These observations highlight the need to focus on controlling reactive nitrogen emissions east of the park as part of ongoing efforts to reduce reactive nitrogen deposition in RMNP.