Stochastic Reconstruction of Intermittent Zero Concentration Periods in Plumes for Accidental Toxic and Flammable Releases

Abstract

Most plumes in the atmosphere are highly intermittent, with varying intervals of zero concentration separating bursts of high intensity concentration fluctuations. Experiments in the atmosphere and in laboratory scale models have shown that these concentration fluctu-ations are not a simple first order Markov process with a purely random time derivative of concentration, but instead exhibit a strong correlation of the variance of the time derivative with the instantaneous concentration level. A method is presented for generating realistic concentration fluctuation time series using stochastic methods with specified levels of mean concentration, fluctuation intensity, zero concentration intermittency fraction and fluctuation integral time scale. Comparisons between simulated and measured concentration time series are made using data from water channel experiments of point-source plumes dispersing in a rough-wall boundary layer designed to simulate neutral atmospheric conditions. The stochas-tic simulation gave reasonable estimates for the probability of observing zero concentration intervals shorter than the average zero interval, but overpredicted the duration of zero periods longer than the average.