A Lagrangian Particle Transport and Diffusion Model for Non-passive Scalars

We present a model of the transport and diffusion of materials whose density differs from the ambient air density or where there is excess momentum above the ambient flow. The model describes the evolution of the material concentration and can be applied to both positively and negatively buoyant pollutants. In order to calculate the transport and diffusion, we use a Monte-Carlo technique to follow the Lagrangian trajectories of many particles. This makes the model suitable for the calculation of transport and diffusion in non-homogeneous areas such as complex terrain or within an urban canopy. Results are compared with wind-tunnel observations of neutral jet releases under different conditions. Comparisons with wind-tunnel observations are made also for releases of negatively and positively buoyant materials. The agreement with observations is good.