A puff pollutant dispersion model with wind shear and dynamic plume rise

A puff diffusion model, which includes wind shear and dynamic plume rise, is developed for numerical prediction of pollutant concentrations under unsteady and non-uniform flow conditions. The plume from a continuous source is treated as a series of puffs emitted successively from the source. Each puff is represented by a set of six tracer particles, which define the size, shape and location of the puff. Initially these particles are located at the surface of the source, on arbitrarily chosen orthogonal axes. The location of the particles is computed at each time step by taking into account advection, eddy diffusion, wind shear and entrainment of ambient air during plume rise. The concentration distribution of each puff is determined by fitting an ellipsoid to the cluster of the six particles and assuming a three-dimensional Gaussian distribution, with standard deviation equal to the half-lengths of the principal axes of the ellipsoid. The concentration at a point of interest is obtained by summing the contributions from nearby puffs. The effect of wind shear on the pollutant concentration is investigated by use of a typical wind shear encountered in the atmosphere. The results show that, at 600 m downstream from the source, the present model gives concentrations a factor of 2 higher and lower at one standard deviation below and above the plume center, respectively, than that of conventional models in which no wind shear is considered. The plume-rise formulation is calibrated against the observations compiled by Briggs and the model is used to predict the trajectory of a plume observed by Slawson and Csanady. Excellent agreement between the prediction and the observation can be achieved if an appropriate eddy diffusivity is chosen.