Go to the EPA website and you will find the latest dispersion models used. WASP used to be one a decade ago but Im sure there are new ones. Empirical models can be made using open-path optical remote spectroscopy and the EPA OTM-10 methods.
I have used "Screening Air Dispersion Model" 'SCREEN3' it was easy to manipulate and in out put you have more then the model of dispersion. In fact, by using this model plumes type as wel as estimate concentrations under different conditions of meteorology can be provided.
@Paolo : If you need any help I can assist with you.