A detailed study of mountain-valley circulation at the Front Range, west of Boulder, Colorado, during the transition period from summer to fall has been conducted using an extensive chemical and meteorological data set. This set was collected during the Tropospheric OH Photochemistry Experiment (TOHPE) in August and September 1993. One major objective of TOHPE was to compare chemical trace gas concentrations measured simultaneously by a long-path (10.3 km) absorption spectrometer and by various in situ methods that were located at the ends of the long path beam. Generally, the study area experienced clean westerly flow challenging the detection limits of the trace gas measurement techniques and limiting the dynamic range of the instrument comparison. Occasionally, however, the westerly flow was replaced by thermally driven upslope flow that brought in polluted air masses out of the Denver-Boulder urban corridor from the east. The trace gas signatures of these upslope events are used to characterize the frequency and spatial extent of the specific flow mechanisms. Upslope flow reached regional scale at sunset, providing short instances of the best meteorological conditions for comparing the trace gas measurements at higher concentration levels. The concept of a generally easterly moving layer of regionally polluted air masses applies and agrees well with these observations. Two pollution events on September 28 and 29, 1993, are studied in detail and strongly support this concept. These complex upslope transport mechanisms may have the potential for mixing anthropogenic pollutants into the free troposphere. ¿ 1997 American Geophysical Union