There are plenty of good air dispersion models and reviews into their usage. You could try HYSPLIT, it's an easy to use software that can model air dispersion. You'd be able to find all information by googling HYSPLIT and accessing the NOAA site.
Article A review of dispersion modelling and its application to the ...
Article NOAA’s HYSPLIT atmospheric transport and dispersion modeling system
While this seems outdated, it may be more important to look for a best practice guide from the authorities that have jurisdiction over your project. (For me these best practice guides are published in German.) The website of the environmental agency is a good place to start.
A good book on the subject is Guidelines for use of Vapor Cloud Dispersion Models, by Steven Hanna and Peter Drivas. There are also many models which perform neutrally buoyant particulate cloud/plume release predictions in addition to gas releases. EPICode from the USA's Lawrence Livermore National Laboratory (LLNL) is one such code which is widely used.