https://phys.org/news/2018-07-air-pollution-negatively-national.html

"The U.S. has spent billions of dollars over the last three decades to improve air quality," said David Keiser, assistant professor of economics at Iowa State. "Given the popularity of national parks, as well as the fact that people go to parks to be outside, we believed it was worth better understanding air quality trends in these areas and whether people, through their actions, respond to changes in air quality in parks."

The study found that ozone levels improved in metropolitan areas starting in 1990; however, national parks improvements have only been apparent since the early 2000s, corresponding to the passage of the Regional Haze Rule, a 1999 EPA regulation that strengthened air quality protections for national parks and wilderness areas.