A three-dimensional Lagrangian pollutant dispersal model for passive contaminants has been modified to allow simulation of dispersal of heavy gases and moist, buoyant plumes, e.g. cooling tower plumes. The model uses output from a two-dimensional Navier-Stokes solver for description of the atmospheric boundary layer. For heavy gas dispersion modeling, Lagrangian markers representing the emission are released into the flow field and are displaced by mean winds, turbulent velocities derived from the Langevin equation, and buoyancy forces resulting from density differences between the pollutant and the atmosphere. In the case of moist, buoyant plume dispersal, the Lagrangian markers represent water in both liquid and vapor phases and carry both dry energy (sensible heat) and latent heat of condensation/evaporation, and accelerations due to buoyancy are related to temperature differences. Model evaluations were made using experimental tests of passive and heavy gas dispersion in simulated atmospheric surface layers. Predicted concentration fields are in accord with measurements. Model simulations of cooling tower plume dispersal in the planetary boundary layer were made using field data from the single natural draft cooling tower at the Philippsburg site in West Germany and the French energy central at Bugey. Predicted plume rise and visible plume outline are in good agreement with observations for various meteorological and source conditions.