I am looking for software that helps in teaching undergraduate students how to study the effects of the transport, dilution, and dispersion of air pollutants.
You should take a look at the NOAA Hysplit model. I think you will find it very useful.
"The HYSPLIT model is a complete system for computing simple air parcel trajectories, as well as complex transport, dispersion, chemical transformation, and deposition simulations."
https://www.ara.com/products/multiple-path-particle-dosimetry-model-mppd-v-304 -- is free avaiable soft for analizing the deposition of aersol particlies (various sizes) in different areas of the respiratory tract (and lungs) -- as additional information for understanding -- what kind of the dust aersols are most dangerious for individuals
As Bjorn said, you should definitely look at NOAA HYSPLIT. Additionally, you can try out FLEXPART, which is another transport and dispersion model. If you're interested in teaching about single-source Gaussian plumes, we can use US EPA's SCREEN 3 or AERSCREEN.
The Operational Street Pollution Model (OSPM) (http://envs.au.dk/videnudveksling/luft/model/ospm/) is a model to calculate the air pollution in street canyons as a function of building geometry, meteorology, and traffic. It has a graphical user interface, and is thus very easy to setup and run.