92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Thursday, 26 January 2012: 9:00 AM
A Year-Long Back Trajectory Analysis of Reactive Nitrogen Measured Continuously At Rocky Mountain National Park, Colorado
Room 353 (New Orleans Convention Center )
Kristi A. Gebhart, National Park Service, Fort Collins, CO; and B. A. Schichtel, M. G. Barna, M. A. Rodriguez, W. C. Malm, J. L. Collett Jr., C. M. Carrico, and K. B. Benedict

During the past 20-30 years increases in wet and dry deposition of reactive nitrogen in the Rocky Mountains has led to concern that sensitive alpine ecosystems may be irrevocably damaged. This led to recent efforts to determine the source types and source regions that contribute to airborne nitrogen concentrations and deposition in this region. Data collected during a 2006 field study at Rocky Mountain National Park (RMNP), Colorado, indicated that during April and July, approximately half of the deposited nitrogen was from sources within the state of Colorado. Potential source types include agricultural activities, mobile sources, oil and gas extraction, fossil fuel combustion, and wildfires. During the fall of 2008, through the summer of 2009 a follow up field study was conducted to collect data during a full year. Several methods were utilized to obtain continuous measurements of both oxidized and reduced nitrogen compounds. These high time resolution data provide a unique opportunity to examine source-receptor relationships both seasonally and by hour of day. Determination of sources impacting RMNP is being conducted as a multi-pronged weight of evidence approach. Several methods of data analysis and both receptor and deterministic modeling are being applied and an attempt will be made to reconcile the results of these varied approaches. This paper discusses some of the receptor-based back trajectory modeling and analyses of some of the measured and modeled meteorological data related to the study. The simplest back trajectory analyses use several statistical techniques to examine where air masses resided prior to arriving at RMNP under a variety of conditions including periods of high and low deposition, concentrations, and precipitation by month and by hour of day. Back trajectories were generated on a 4 km grid resolution using output from the Weather Research and Forecasting (wrf) mesoscale meteorological model. The wrf modeling included observational data assimilation using data from a radar wind profiler installed for the study. A trajectory-based receptor model, Trajectory Mass Balance, was also applied to estimate the relative attributions of several source areas to the measured concentrations. These trajectory models show that the highest concentrations of most nitrogen compounds occur during upslope easterly transport from the heavily populated Colorado Front Range and agricultural areas in northeast Colorado. However, due to the predominantly westerly winds in this region, on average, the trajectory models indicate more of the annual average nitrogen concentration is from areas to the west.

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