Frequency distribution of the concentration on the center of mean plume axis in the surface layer.

Abstract

Concentration of NOx was measured several tens of meters downwind from a continuous NOx source by the fast respondent analyzer. Horizontal and vertical distributions of averaged NOx concentration were also measured in the same periods using bag samplers. The frequency distribution of instananeous concentration observed on the mean plume axis agrees well with the sharp cut off power low given by Gifford's fluctuating plume model. But the discontinuity of the frequency distribution at the maximum concentration is not sharp. It was confirmed that the frequency distribution of concentration averaged over a short period within 200 sec also followed Gifford's model. The ratio of the maximum short period concentration to the long period (2,700 sec) concentration was about 5 to 1 when the averaging time of the short period was 4 sec. The ratio descreases with the averaging time until about 100 sec. It becomes almost independent of the averaging time and not so different from 1 when the averaging time is longer than 100 sec.