From the American Lung Association via WebMD:
10 with Cleanest Air Year-Round
1. Cheyenne, Wyo.
2. Santa Fe-Espanola, N.M.
3. Honolulu
4. Great Falls, Mont.
5. Tucson, Ariz.
6. Anchorage, Alaska
7. Farmington, N.M.
7. Bismark, N.D.
9. Albuquerque, N.M.
10. Rapid City, S.D.
And the 5 Sootiest
1. Los Angeles-Long Beach-Riverside, Calif.
2. Bakersfield, Calif.
3. Pittsburgh-New Castle, Pa.
4. Visalia-Porterville, Calif.
5. Fresno-Madera, Calif.
Follow the link above to see cities ranked through number 25 for both categories and to learn more.