Abstract
A wind tunnel study of short-term concentration fluctuations and peak concentrations under three stratification conditions, unstable condition, neutral condition and stable condition, in an urban street canyon was conducted. Tracer gas was emitted from a point source in the canyon and was measured with a fast-response concentration detector.
The peak concentrations under the stable condition in the canyon showed a value that was two times higher than the neutral condition and unstable condition. The values of the standard deviation of the fluctuating concentration in stable condition were also higher than the other two stabilities, however the values of the normalized concentration fluctuation intensity did not depend on the stability conditions. The peak-to-average concentration ratios of the inside canyon in the stable condition had a range of 3-5 and the peak-to-average concentration ratio in the stable condition was smaller than that of the neutral and unstable conditions.
An auto-correlation analysis showed that longer residence time of tracer gas in the canyon was measured in the stable condition, due to the low wind speed. The cumulative density functions of concentration under the stable condition was only slightly changed by smoothing the time series with averaging times up to 13.5s compared to the other two stabilities. This shows that the instantaneous eddies in the stable condition were less fragmented than those in the other two conditions.
