Atmospheric reactive nitrogen in Rocky Mountain National Park
Date
2018
Authors
Shao, Yixing, author
Collett, Jeffrey L., advisor
Schumacher, Russ, committee member
Jathar, Shantanu, committee member
Benedict, Katherine, committee member
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Abstract
The Front Range urban corridor in Colorado, located east of Rocky Mountain National Park (RMNP), includes a variety of urban sources of nitrogen oxides, while high emissions of ammonia are found in agricultural sources on the eastern plains of Colorado. The spatial distribution and temporal variation of ammonia and other reactive nitrogen species in the region is not well characterized. Periods of upslope flow can transport atmospheric reactive nitrogen from the Front Range and eastern Colorado, contributing to nitrogen deposition in the park. Deposition of excess atmospheric reactive nitrogen in Rocky Mountain National Park poses threats to sensitive ecosystems. It is important to characterize temporal variation and spatial distribution of reactive nitrogen in the region to better understand the degree to which emission sources in the northeastern plains of Colorado impact RMNP and how meteorological conditions are associated with transport of ammonia to the park. Mobile and in-situ measurements of reactive nitrogen gases and particles were made between 2015 and 2016 in northeastern Colorado and RMNP. Gaseous ammonia was measured with high-time resolution instruments (a Picarro cavity-ring down spectrometer and an Air Sentry ion mobility analyzer); 24-hr integrated concentrations of trace gases and PM2.5 chemical composition in RMNP were measured by URG denuder/filter systems coupled with lab analysis; wet nitrogen deposition was collected with an automated precipitation collector followed by lab analysis. Model outputs from The Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) was also included for examining transport of ammonia source plumes. Diurnal and seasonal variability of ammonia concentrations and some other reactive nitrogen species were characterized with high time-resolution measurement data. Repeating diurnal cycles were found in Greeley and RMNP. Ammonia concentrations usually increase in the morning and reach maxima around noon in RMNP, while at Greeley ammonia builds up during the night followed by a rapid decrease after sunrise. A seasonal pattern of ammonia levels was also revealed, with higher concentrations observed during summer. When combined with wind data it is clear that elevated ammonia levels in RMNP were associated with easterly transport from the eastern plains of Colorado. The median daily averaged ammonia concentrations measured in Greeley, Loveland and RMNP are 26.2 ppb, 6.3 ppb and 1.1 ppb respectively. Considerable ammonia variability was found in NE Colorado with higher concentrations measured close to CAFOs and source regions. This was particularly clear in mobile NH3 observations where distinct plumes of ammonia were observed away from confined animal feeding operation (CAFOs) sources. Spatial variations, particularly in the north-south direction, were observed to be strongly dependent on meteorology as highlighted by HYSPLIT back trajectories. This study also evaluates the pilot Early Warning System which informs agricultural producers of impending upslope events that are likely to transport nitrogen from eastern Colorado to the park, so that management practices may be implemented to reduce nitrogen emissions. The performance of the meteorological forecasting was evaluated using continuous measurements of atmospheric ammonia concentrations in the RMNP, as well as the wet nitrogen deposition data from 2015. It was found that the model showed skill in capturing some large wet nitrogen deposition events in the park.
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Subject
early warning system
ammonia
wet deposition
reactive nitrogen
Rocky Mountains