Characterization of the transport and dispersion of pollutants in a narrow mountain valley region by means of an atmospheric tracer

Abstract

Three atmospheric tracer experiments using sulfur hexafluoride (SF6), were conducted in a narrow mountain valley from 29 January-3 February 1982. These tests were performed in order to characterize the transport and dispersion of airborne pollutants around Telluride, Colorado which lies near the head of the San Miguel Valley. Particular attention was given to the influence of flow reversals upon pollutant dispersion, for which adequate numerical simulation is not yet available. Data from surface wind stations, pilot balloons and from an acoustic sounder were used to characterize the local meteorology. In this case, an upslope flow followed by a downslope drainage is required for emissions from a site on the slope to reach the upper valley about 10 km away. On the other hand, a downvalley flow followed by an upslope flow is required for emissions from the upper valley to reach the slope site. Certain aspects of the transport and dispersion characteristics associated with the mesoscale winds in the area appear to be relatively insensitive to changes in the synoptic winds. For example, even though the synoptic wind patterns in Tests One and Two were different, the 24-h average concentrations in the upper valley were found to be about 15 and 17 (kg-mole)-1h-1, respectively at the slope site. Test Three involved releasing the tracer from Telluride. The 24-h average concentration of tracer at Telluride was used, along with an estimate of the particulate emission rate, to predict the average ambient concentration of total suspended particulates. The value predicted was 71 μgm-3, which is in excellent agreement with the measured value of 63 μgm-3. After the downvalley flow gave way to the midday upslope flow, tracer concentrations were measured along a ski lift. The tracer concentration decreased linearly with increasing distance up the slope. Estimates of concentrations based upon the Gaussian model, (the parameters of which were determined from meteorological data), were in general higher than those observed. In addition to allowing direct prediction of impacts of pollutants emitted within this region, these data can be used to test the validity of numerical models. © 1983.