A laboratory study and improved pdf model of fumigation into a growing convective boundary layer

A saline convection tank was used to study the fumigation of a plume as it is entrained into and rapidly mixed through a growing convective boundary layer. Dispersion results in the form of crosswind-integrated concentrations for the whole field were obtained using a quantitative video digitization technique. Good agreement was obtained with a probability density function (PDF) dispersion model when it was modified to properly account for the spatial variability of the local mixed layer height. A wide range of boundary layer growth rates from very slow (0.002w*) to fast (0.15w*, where w* is the convective velocity) was investigated to determine the influence of growth rate on ground-level concentrations (GLCs), duration of the fumigation episode, and thickness of the boundary layer entrainment zone. Within this range of growth rates, maximum crosswind-integrated GLCs were found to be only very weakly dependent on the entrainment rate. The duration of a fumigation episode was found to be proportional to the sum of the entrainment zone thickness and the initial plume spread and inversely proportional to the boundary layer growth rate. The experimental and model results are shown to be applicable to fumigation into a thermal internal boundary layer developing inland from a shoreline as well as to nocturnal inversion breakup fumigation.