Concentration fluctuation measurements in a dispersing plume at a range of up to 1000 m

Abstract A series of tracer experiments studying concentration fluctuations in a dispersing plume of pollutant in the atmosphere at ranges of between 50m and 1000m is described. Experiments were conducted on three different field sites in near‐neutral or slightly convective meteorological conditions. The results show time series which are characterised by the intermittent occurrence of periods of fluctuating non‐zero concentrations, interspersed by periods of essentially zero concentration. The spectrum of concentration fluctuations is found to display inertial subrange behaviour, characterised by a −2/3 power law when nS ( n ) is plotted against frequency n , where S ( n ) is the variance (of the fluctuation) per unit frequency interval. The spectral peak frequency varies with distance from the source. In all cases the clipped‐normal probability density function (PDF) provides a reasonable fit to the concentration PDF. The exponential PDF is less flexible in fitting a wide range of experimental conditions, but is slightly superior for some short range examples. In the alongwind direction it is found that, although there is a rapid initial decrease in fluctuation intensity with distance, the intensity seems to approach an approximately constant non‐zero value at long range. In cross‐sections of the plume the variation of fluctuation statistics is dominated by the varying proportion of time during which the concentration is essentially zero. Conditional statistics, calculated from significantly non‐zero concentrations only, show only slight variations across the plume.