Both of them are quite right. Aermod has both commercial and non commercial versions. The only difference ia the ease of use. I started using Aermod long back with the non commercial available from USEPA. There are several guides available with USEPA to learn the model. But be aware that a model performance is just as good as the input data. Be very cautious in selecting the input data otherwise it will be like Garbage In Garbage Out kind of situation. Lastly assessment if model output is the key to atmospheric dispersion modeling in the light of input data, baseline data, assumptions and limitations