One of several reasons why the air around St. Louis is poor are the many coal-fueled power stations based on both the Missouri and the Mississippi Rivers. This one, the Labadie Power Plant, is on the Missouri, slap in the middle of an otherwise pristine view from Klondike Park, Defiance.
Add these to large chemical and metal industry both in St. Louis and across the river in East St. Louis and you have a lot of particulate matter in the atmosphere.
All the more reason, price of oil and global warming aside (!), for the development of clean power technologies.