The fate of airborne materials released in an urban area is affected by flows on many scales, including the synoptic-scale background flow and small-scale flows induced by buildings. When the city is located in mountainous or otherwise complex terrain, local scale topographically generated flows increase the complexity of the transport of the material. We present SF6 tracer data and Doppler lidar radial wind measurements from three nights when the tracer was released in downtown Salt Lake City (SLC). The larger-scale wind flow affected the formation of local-scale flows differently each night, which in turn affected the pattern of tracer dispersion in a different way each night.

SLC, UT, lies within the northeast quadrant of a basin bounded by mountain ranges to the west, south, and east, and the Great Salt Lake to the north. The wind flow in this basin has a diurnal cycle of northerly flow during the day and southerly flow at night. The simplicity of this cycle is altered by canyon outflows and downslope flows from the bounding mountain ranges. Buildings in the downtown region create complex wind flows on the urban scale.

The DOE's VTMX field campaign of October 2000 was designed to obtain meteorological measurements in the SLC basin well beyond the scope of the traditional network already in the basin. The purpose was to understand how these complex wind systems in the basin may contribute to the formation or destruction of the cold pool that forms in the basin in the fall and winter months. In conjunction with the VTMX campaign was a second field campaign, the DOE's CBNP Urban 2000 experiment. This experiment involved releases of SF6 tracer in the downtown area during the night, to be detected by a dense array of sensors downtown, plus three partial arcs of samplers placed 2 km apart, starting 2 km from the city.

There were ten IOPs during the experiment, occurring under different synoptic conditions. In this paper we will present a comparison between Doppler lidar measured winds and tracer data. The lidar measurements clearly indicated the basin-scale winds and the presence or absence of local terrain-generated flows. The tracer data indicated how fast the tracer was advected out of the city, and in what direction. The three different IOPs show very different scenarios regarding the behavior of the tracer. During IOP 10 the southerly basin-scale winds dominated, terrain-generated flows existed briefly and intermittently, and the tracer was transported out of the city, toward the northwest. During IOP 7, local-scale terrain-induced circulations formed, and the tracer lingered in the downtown area, implying that small urban-scale circulations also formed. Our final case is IOP2, during which synoptically-driven easterly winds flowed over the Wasatch Range, resulting in strong downslope (easterly) flow into the basin. Because of the complexities of the terrain and the position of the city relative to the canyons, local circulations formed such that either the tracer was confined to the downtown area, or it was transported to the east.