The Integral Equation of Diffusion

Various theories are now available to describe the dispersion of passive material released from a source into the atmosphere. These theories include the eddy-diffusivity equation of diffusion, statistical theory, similarity theory, higher-order closure solutions of the basic equation of motion and continuity, and rondom walk modelling. All these methods explicitly or implicitly require some knowledge of the Lagrangian character of the turbulence field. Some methods only apply to rather idealised states of flow (e.g. statistical theory only applies to uniform flow with homogeneous turbulence) whereas others are much more versatile — (e.g. random walk modelling). Some are simple in concept and application, others are much more complex. In recent years the virtues of random walk modelling have been recognized and the technique widely exploited.