April 2013 had unusually high levels of Nitrogen wet deposition at two National Acid Deposition National Trends Network (NADP/NTN) sites in Rocky Mountain National Park, Colorado.  Both sites, Beaver Meadows and Loch Vale, have long data records dating to the 1980s, making it possible to examine annual and seasonal trends in concentrations, deposition, and meteorology to better understand the conditions that existed during various levels deposition.

Hourly five-day ensemble back trajectories were generated for Beaver Meadows and Loch Vale for 1980-2015 to examine transport pathways under various sets of conditions including, all times, Aprils only, hours with and without precipitation, various levels of Nitrogen deposition and combinations of these.  In addition to the standard statistical and graphical analyses of back trajectories, a new technique was developed to better visualize transport differences under various conditions.  This is to generate a map of how unusual it was to receive air masses from upwind areas as compared to average conditions.  This allows easy visualization of areas that contribute in both common and rare ways during various conditions and uses an easy to understand metric, the Z score, for how unusual that transport was.  The Z score is the number of standard deviations from the mean transport condition.

Results show that April and July have the greatest precipitation at these sites, with April more likely to have synoptic scale storms and July more likely to have convective storms. Only about 7% of hours at Beaver Meadows and 12% at the higher elevation, Loch Vale, have precipitation.  Though the predominant wind direction at both sites is northwesterly, when precipitation occurs, there is more likely to be transport from the east than the west. This is evident from both the 10-m tower meteorological data at nearby Longs Peak and from back trajectory analyses. April 2013 had unusually frequent transport from eastern Colorado, even compared to the April average.  Other Aprils with higher than average Nitrogen deposition also tended to have higher than average transport from the east, while Aprils with lower average deposition had more transport from other areas, usually to the west and south.  Some analyses were completed for both 2008-2015 when there are higher resolution meteorological data available and for 1980-2015 with a lower resolution, but longer time period.  Results were consistent with both sets of trajectories.