A laboratory model of diffusion into the convective planetary boundary layer

A laboratory model of the convective planetary boundary layer has provided information concerning the evolution of concentration distributions downwind from a simulated continuous point source located near the ground. Results indicate that a Gaussian plume formulation adequately describes the model γ‐concentration distributions, but is useful in predicting the z ‐concentration distributions only to a distance downstream of about x = 0.5zi/(w*/U), where zi is the mixed layer depth and w*/U is a dimensionless stability parameter. Near this distance an elevated concentration maximum appears at a height above the source release height. The elevated maximum rises to a height of about 0.8zi at x = 1.7zi/(w*/U) and retains its identity until the pollutants become vertically well mixed farther downstream. Use of the stability parameter w*/U permits the model results to be applied to a range of atmospheric conditions encompassing the Pasquill‐Gifford stability classes A and B. Close agreement is found between the laboratory data and the atmospheric observations of ground‐level lateral spread of Islitzer (1961) and Islitzer and Dumbauld (1963), where the latter measurements extended out to x≃ 3 km.