Large numbers of power plants are the cause for the smog in this area. There’s been a few attempts to clean up the air, such as the banishment of wood burning and school buses no longer running on diesel. Cincinnati is the eighth-most polluted city by year-round particle pollution.
Hot days help worsen air quality that’s already bad, and pollution is pretty bad due to how Phoenix and its neighboring cities are structured. Dust storms in recent droughts have increased this problem. Phoenix is the 10th-worst polluted city by ozone.