Here's a link to an article from the National Parks Conservation Association, and here's the relevant text:
"Ozone levels in the Smokies violated federal health standards more than 175 times since 1998, threatening the health of hikers and damaging 30 species of plants, the environmental groups said.
The report used a plant ozone-exposure standard of 60 parts per billion for comparison purposes. Ozone exposure numbers were computed by adding the concentrations for all hours experiencing 60 parts per billion or greater for the months of April through October.
According to the study, the Smokies’ average annual ozone exposure of 133,200 ppb-hours exceeded that found in Atlanta, Knoxville, Tenn., and Charlotte, N.C. Only one city in the study’s analysis, Los Angeles with more than 180,000 ppb-hours, exceeded the Smokies."
Most of the ozone and pollution comes from upwind coal-fired power plants.