Study of pollutant dispersion in an urban street canyon

Abstract

Pollutant dispersion from an urban street canyon was studied to determine the process by which pollutants emitted at street level are transported from the canyon and to quantify the net rate of pollutant exchange between the canyon and the upper air. The experiments were performed in a street canyon with a building height to street width ratio of 1.5 and with ambient winds perpendicular to the street. To examine pollutant transport within the canyon, mean wind velocities were determined by analyzing the trajectories of tracer balloons that were released in the canyon and photographed in rapid sequence. The trajectories indicate the presence of a primary vortex cell, provided the ambient wind velocity exceeds 1.5 to 2.0 m s/sup -1/. The measurements indicate that while pollutant mixing within the canyon occurs rapidly by mean wind advection, the final transport of material normal to the roof-level plane is accomplished by turbulent motions. To determine the net rate of pollutant exchange between the street canyon and upper air, a tracer study was conducted. The tracer study was performed by releasing a controlled amount of SF/sub 6/ gas near the street. From the measurement sof tracer decay, retention times, defined as the length ofmore » time for a pollutant concentration to reduce to e/sup -1/ of its original value, were computed. The retention times were found to be of the order of minutes for ambient winds between 1.7 and 4.5 m s/sup -1/. The vertical pollutant flux near midblock was quantified by a ventilation velocity representing the average velocity of pollutant transported out of the canyon. The ventilation velocities were computed from the retention times and found to range between 15 and 80 cm s/sup -1/. Meteorological measurements of the ambient flow were also taken and a semi-empirical formula for predicting street-level pollutant concentrations was developed from the data. 47 references, 25 figures, 2 tables.« less