From the Associated Press (reported in the Asheville Citizen-Times):

... It’s a problem in a handful of the nation’s 52 parks that are monitored constantly for ozone, including Joshua Tree National Park in California’s Mojave Desert and North Carolina’s Great Smoky Mountains National Park. There were 12 days monitors recorded violations in the Smokies last year, putting the park at No. 3 on the list. ...

Smog is created when the sun’s rays hit pollutants such as oxides of nitrogen and volatile organic compounds that are in motor vehicle exhaust, solvents, pesticides, gasoline vapors and decaying dairy manure.

“There is no simple answer to ozone pollution,” said Thomas Cahill, a researcher at the University of California, Davis, who studies air problems in Sequoia and across California. ...