Dispersion from an elevated source in a convective boundary layer

Abstract

This paper proposes a model for dispersion of pollutants emitted from an elevated source into a convective boundary layer. In our model we incorporate some of the most recent understanding of dispersion in the mixed layer. For example, we relate the horizontal plume spread σ y to the convective velocity scale w ∗. Our formulation for σ y emphasises the role of the mixed layer height z t and the associated time scale z t/w ∗ in controlling dispersion in the vertical direction. We have also attempted to account for the effects of self-induced plume turbulence on plume spread. An important ingredient of our model is the concept of plume impingement distributed around a mean distance x t at ground-level. The computation of x t has been guided by the recent work of Lamb (1979) and that of Briggs (1975). The model has been tested by comparing predictions with concentrations measured around the Dickerson and Morgantown power plants in Maryland. The results of the verification are very encouraging. This study shows that we can obtain rough estimates of ground-level concentrations by assuming that the pollutants are well mixed through the PBL. Then, the expression for the concentration C can be simply written as C = 0.9Q w ∗z tx , where Q is the emission rate and x is the distance from the source.