This paper presents a new Lagrangian dispersion model for buoyant plumes in the convective boundary layer (CBL). The model couples a Lagrangian stochastic approach, wherein one tracks buoyant particles given the CBL turbulence statistics, with an entrainment model to address plume buoyancy effects. The new features include the extrainment or removal of plume material by ambient turbulence in the entrainment model and a gravity current model to account for the enhanced lateral spread of ``lofting" plumes at the CBL top. The focus is on the lofting situation, i.e., when a plume rises to the CBL top, is trapped there by the inversion capping the boundary layer, and then disperses downwards.

The treatment of the plume by a large number of buoyant particles was intended to improve the modeling of the plume interaction with the elevated inversion and the behavior of the concentration contours near the CBL top. The motivation for including removal of material from the "active" plume core came from snapshots of plumes in recent convection tank experiments. The measurements showed that small segments of plume material were removed from the plume core and dispersed by the large eddies in the CBL. Overall, the model produced plume spatial statistics and mean concentration fields that agreed well with dispersion measurements obtained in the convection tank at the EPA Fluid Modeling Facility in North Carolina. The agreement included the variation of plume variables and fields with both downstream distance and source buoyancy.