yes, Trek, you're right, it all blows around, but when there's an inversion, bad air gets trapped in an area, particularly when the emissions of that area are out of control.

from today's news:

Air pollution shrouds Beijing on eve of Olympics

By TINI TRAN – 3 hours ago

BEIJING (AP) — The wall of gray haze around the National Stadium and across the city cut visibility down to a mile. On the eve of opening ceremonies, Beijing's polluted air took center stage Thursday as the most visibly pressing problem for Olympic organizers who had promised to clean up the Chinese capital.