Dispersion in Idealized Urban Street Canyons

Abstract

Urban street canyon geometry and pollution source configuration play important roles in determining the level of peak levels of air pollution due to transportation sources. Urban air pollution specialists anticipate that street canyon depth and width, intersection locations, canyon orientation to the dominant wind directions and local building geometries will determine most of the peak pollution incidents which determine air pollution control strategies. Most relevant urban air pollution models are based on limited measurements made at field and laboratory scales. Uncertainty still exists concerning the level of complexity which must be contained in numerical models to reproduce critical aspects of canyon motions, especially at lower wind speeds. Past laboratory measurements may have been biased by inaccuracies introduced by the line source designs used in the experiments. This paper reports the results of visualization, wind field, and dispersion measurements made in a meteorological wind tunnel using an improved line-source configuration. Results suggest that mean concentrations vary inversely with wind speed. Observations also confirm hat the flow within the canyon is highly intermittent, and the street level vortex periodically lifts out of the canyon and then reforms.