Denver doesn't tend to have much of a smell most of the time, but you damn sure know when it's time for the National Western Stock Show!

Luckily it's held in January, so you don't get heat on top of the smell. And driving across town on I-70 you get hit with the distinct odor of canned dog food when passing by the Purina factory.
The only times I've been to NYC were in the Summer, and it always smelled like Six Flags amusement park to me.