SALT LAKE CITY, Utah — When it comes to its chronically poor winter air, Salt Lake City already has topography working against it. Steep mountain ranges on either side of the city create a giant bowl, where dreaded winter temperature inversions trap cold air that stagnates with dust, factory smoke, and diesel exhaust from the city's expanding web of freeways.
At least a million people have moved to the valley since 2000, and the air on some days looks — and actually can be — dirtier than Mexico City's.
