This paper studies the collapse of the estimators for skewness and kurtosis of concentration onto a near universal curve. This phenomenon is observed for data taken from atmospheric dispersion experiments under a variety of different conditions. By means of careful investigation of the high concentration tails, modelled by means of the generalized Pareto distribution, and the fundamental physics of the problem, a set of envelope curves encompassing the data will be established. The implications of these results for modelling the probability density function of concentration are discussed
