Excess reactive nitrogen deposition at Rocky Mountain National Park (RMNP) impacts sensitive ecosystems there, especially at high altitudes.  For this reason, the National Park Service, State of Colorado and U.S. Environmental Protection Agency have collaborated through a Memorandum of Understanding to work toward reducing impacts in the park.  Progress is tracked using five-year rolling averages of annual wet inorganic nitrogen deposition measured at the Loch Vale (CO98) NADP wet deposition sampler.  When a trend is seen in either concentrations or deposition, it can be difficult to know the reason for it.  We would like to know if trends are due to meteorological factors such as changes in precipitation or transport directions or if they are due to changes in emissions.   While mesoscale chemical transport models could theoretically be used for this task, in practice they are too expensive to run for decades.  An alternative tool using back trajectories is being developed for this purpose.

Initial analysis is with data from Loch Vale (CO98) and Beaver Meadows (CO19), both in RMNP.  Hysplit back trajectories were calculated hourly for 1980-2017 from a near surface height and an elevated height that is nearer to cloud level.  The positions of the trajectories, or endpoints, are determined for each hour for one and two days back in time.  Then the number of endpoints in each of four upwind areas, eastern Colorado, western Colorado, eastern U.S., and western U.S., were tallied and time matched with the weekly NADP observations.  NADP daily precipitation was used to screen out days with no precipitation at the measurement site.  Concentrations and deposition can then be weighted by the fractions of endpoints in each of the four upwind regions, allowing a transport direction- weighted trend to be calculated.  Because there are seasonal differences in both precipitation and transport direction at both sites, trends can be calculated for individual seasons as well as full years. If the technique proves useful, smaller, more numerous source regions could be used.  Preliminary results will be presented at the meeting.