“I thought Denver was one of the most polluted cities in the nation? “
Decades ago, but hasn’t been that way for a very long time. Colorado implemented strict emissions requirements and strict mandatory vehicle testing, including the big diesels, for the Denver Metro area, which is where the bulk of vehicle traffic is concentrated and at the same time is also located on the eastern foot of the Rocky Mountains, which is one place where temperature inversions occur.
The rare temperature inversion in the winter will still trap warm air close to the surface upon occasion, but then wood-burning restrictions are enforced during that time.
All in all, pretty clean air now except for the occasional local forest fire or a big forest fire in a western state like California where the smoke is blown east.