Abstract
Gifford's (1959) fluctuating plume model is extended to obtain an expression for the autocorrelation function for concentration fluctuations. This is then used to derive results for the reduction in sample variance as the averaging time is increased. A simple exponential shape assumption for the autocorrelation function is shown to give reasonable results, but the integral time scale for the concentration field is very much shorter than the Eulerian integral time scale of the velocity field for highly intermittent plumes. A simple expression for the time scale in terms of the ensemble mean and variance of the concentration is proposed.
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