Plenty of other good answers.

In addition to the comments about the air effectively being "clearer" at low temperatures etc, there is a known visual or psychovisual phenomena where higher-contrast images or scenes are perceived as being sharper (this is exploited by people trying to sell you new TVs etc).

"Mucky" air will decrease the contrast (as well as perhaps physically blurring) which will make the scene "pop" less. As others have said, the angle of the sun can also dramatically affect the scene contrast.

Related to this, I find general urban street scenes "pop" in the sunshine shortly after rain - the rain clears the air, washes away dust, and if surfaces are wet and shiny the contrast is much higher - it can look "hyper-real".