The expected deviation of observed concentrations from predicted ensemble means

Abstract

This note presents an analysis to estimate the expected deviation of observed concentrations from predicted ensemble means. It is shown that the expected deviation ε is related to the sampling time T by the expression ε 2 = 2( Γ − 1) T i / T In the above equation, Γ is the instantaneous peak to mean ratio at the receptor under consideration and T i is the Eulerian time scale controlling dispersion of the plume. As reasonable estimates of these parameters can be made we can compute ε for various meteorological conditions. For example, for a tall stack emitting pollutants in an unstable boundary layer e can be as large as 100 % at the point of maximum concentration. This points to the importance of knowing ε when comparing predictions with observations.