I big chunk does come from cars, but you can go to the EPA Echo database (google: EPA Echo) and search for pollution sources by zip code or explore the map.  You can find all the large power plants and see the total pollution they emit.  It's pretty eye opening when you see the thousands and thousands of lbs of NOx, SO2 and other major pollution sent into the atmosphere.  It'll take some work on infrastructure and how we think about car to completely get rid of the smog in Denver.  But starting with major power coal power stations along the Front Range is the low hanging fruit.  As long as it doesn't result in significant rate increases I'm all for it.