METEOROLOGICAL CHARACTERISTICS AND SPATIAL EXTENT OF UPSLOPE EVENTS DURING THE 1993 TROPOSPHERIC OH PHOTOCHEMISTRY EXPERIMENT

A detailed study of mountain-valley circulation at the Front Range, we st of Boulder, Colorado, during the transition period from summer to f all has been conducted using an extensive chemical and meteorological data set. This set was collected during the Tropospheric OH Photochemi stry Experiment (TOHPE) in August and September 1993. One major object ive of TOHPE was to compare chemical trace gas concentrations measured simultaneously by a long-path (10.3 km) absorption spectrometer and b y various in situ methods that were located at the ends of the long-pa th beam. Generally, the study area experienced clean westerly flaw cha llenging the detection limits of the trace gas measurement techniques and limiting the dynamic range of the instrument comparison. Occasiona lly, however, the westerly flow was replaced by thermally driven upslo pe flow that brought in polluted air masses out of the Denver-Boulder urban corridor from the east. The trace gas signatures of these upslop e events are used to characterize the frequency and spatial extent of the specific flow mechanisms. Upslope flow reached regional scale at s unset, providing short instances of the best meteorological conditions for comparing the trace gas measurements at higher concentration leve ls. The concept of a generally easterly moving layer of regionally pol luted air masses applies and agrees well with these observations. Two pollution events on September 28 and 29, 1993, are studied in detail a nd strongly support this concept. These complex upslope transport mech anisms may have the potential for mixing anthropogenic pollutants into the free troposphere.