7.No. 5: Phoenix, AZ
Arizona has less cloud cover than any other state. And Phoenix is the state’s second-sunniest city. More sunlight reaching the ground means more ozone. That could be why Phoenix moved up the list two spots from No. 7 last year. To clean things up, local leaders are working to curb the number of drivers on the road and to cap the use of wood-burning fire pits and fireworks.
8.No. 4: Fresno, CA
The Fresno-Madera-Hanford metro area only has 1.3 million residents. They enjoyed the lowest number of ozone days since 2000, but stayed in the same spot on the list as last year. In addition to the number of cars on the road, geography is partly to blame. The bowl-shaped San Joaquin Valley traps vehicle emissions with no way for them to get out.
