Models can be described as simple or advanced based on the assumptions used and the degree of sophisticated with which the important variables are treated. Advanced models have been developed for such problems as photochemical pollution, dispersion in complex terrain, long-range transport, and point sources over flat terrain. The most widely used models for predicting the impact of relative unreactive gases, such as SO2, released from smokestacks are based on Gaussian diffusion.
In Gaussian models, the spread of a plume in vertical horizontal directions is assumed to occur by simple diffusion along the direction of the mean wind. The maximum ground level concentration is calculated by means of the following Equation.