H.Hasheminejad - ph.D. Student
N.Taleb Bydokhti - professor
G.R.Rakhshandero - Assistant professor of civil Engineering Department , Shiraz University

The dispersion of hazardous and toxic gases is an important issue within the framework of risk analysis studies associated therewith. In quite a number of cases, accidental releases of hazardous or toxic gases are complicated by the fact that the gases are heavier than the ambient air. For estimating hazard zones associated with their accidental releases to the atmosphere we can use some practical and well-tested models. One of the models that can be used is 3D models. These models employ 3-D partial differential equations describing conservation of mass, momentum and energy. A 3D model needs a turbulence closure assumption to model the Reynolds – stresses . The mathematical modeling of the dispersion of gases is in most cases done to obtain a tool for the prediction of the behavior of a heavy gas cloud resulting from an escape of some hazardous material. On the other hand, Dispersion id also most important in the pollution modeling especially in air quality modeling . It can be caused by atmospheric turbulence or density difference between pollutants (heavy gas) and air. In this paper we reviewed some of the past and recent dispersion modeling in air quality and we tried to review on part of dispersion models in air quality that considered turbulence and density differences.