The Shape of the Probability Density Function of Short-Term Concentration Fluctuations of Plumes in the Atmospheric Boundary Layer

The shape of the probability distribution of a set of high-resolution concentration fluctuation measurements from ion plume is studied using order statistics and certain selected quantities derived from them. A number of graphical techniques are applied, from underlying distribution of concentration. These graphical techniques are applied, from both a descriptive and a computational point of view, to elucidate the underlying distributional shape of concentration and to assess the characterization efficacy of the probability distributions that have been proposed as models for concentration fluctuations. None of the commonly used models for the concentration probability distribution is able to accurately characterize the extreme upper end of the concentration frequency distribution i.e., the end of the distribution that is critical for the prediction of the probability of exposure to peak levels. The clipped- normal distribution is shown to provide a reasonably conservative model for the prediction of the exceedances of critical concentration levels. The g and h distribution yields a bimodal form for the total probability density function for concentration whereas the clipped-normal distribution provides a unimodal form. It is shown that the bimodal form of the total concentration probability density function is consistent with both the data and certain theoretical results.