According to this new report (on the Weather Channel Web site, courtesy of Climate Central), warmer temperatures in the U.S. mean more "stagnant" air quality days. Stagnant means little wind, lots of sun, high temperatures, not much rain (or none at all).
Now, the warmer temperatures might be related to global climate change. (That depends on the length of time we're talking about for the warmer temps, trend-wise, but things are getting warmer, generally, and generally agreed upon.)
Even though bad ozone days might be decreasing because of better emissions controls, the report says that stagnant air days are increasing over much of the lower 48.