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We describe a new high-resolution sampling technique which can be used to measure concentration fluctuations simultaneously at several points in space. The technique has been used to measure the probability distribution function as a function of the detector location relative to a continuous and steady source. Results are compared to previous experiments and theoretical predictions. The spectra of the concentration fluctuations are analyzed and their behaviour as a function of downwind distance from the source is described. IntroducrionThe frequency distribution of the concentration of a passive scalar in a turbulent field is an important subject from theoretical and experimental points of view. The information obtained in these studies is relevant to air quality models, screen obscuration studies, combustion problems and other applied areas. It also has a theoretical aspect on information regarding the probability of characterization of the mechanism of the process which is responsible for these fluctuations. However, very few experimental studies have been carried out which may supply data for theoretical investigations. Experiments in a wind tunnel were carried out by Fackrell and Robins (1982). In these experiments, the concentration fluctuations were measured with a resolution of 300 Hz; and the probability distribution function for a ground-level source and an elevated source was measured, as well as the spectra. Field experiments which were carried out by the U.S. Army for observation purposes, were analysed by Hanna (1984). In these experiments, the detection system's response time was only 1 s and therefore results were limited to cases where time average of the order of 1 s are interesting. Similar information was collected and analysed by Sawford et al. (1985). In his experiments, he uses, in addition to the smoke-plume, an SF6 tracer. Sawford's experiments were carried out at distances of up to 100 m downwind from the source. Measurements in the cross-wind direction were taken as well, but not simultaneously at distinct points. However, it was argued (Csanady, 1973;Townsend, 1976), that the concentration of a plume measured with a rapid response detector appears as short bursts of high concentration with longer intervals of zero concentration. This phenomenon is more prominent near the source. Consequently, high-resolution sampling techniques are required. Such a Boundary-Layer Meteorology 45 (1988) 157-175.

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