Ozone concentrations in U.S. national parks like Yellowstone and Acadia were largely indistinguishable from ozone levels in America's largest metropolitan areas between 1990 and 2014, according to a new analysis. It also suggests higher ozone levels are linked to a decrease in park visitation, during this time. The U.S. National Park Service has raised concerns over high levels of ozone and poor visibility in national parks, which receive more than 300 million visitors each year. Despite the known damages from ozone pollution, recent rules and regulations to control regional haze and ozone have been costly and controversial, and their effects are not entirely clear. Seeking to uncover how ozone levels have changed in U.S. national parks, David Keiser and his colleagues evaluated daily ozone measurements from 33 national parks - including Acadia, Yellowstone and Yosemite - from 1990 to 2014. The authors analyzed maximum daily 8-hour ozone concentrations as well as exceedance days - the number of days in a year where the maximum daily eight-hour ozone concentration exceeds 70-parts-per-billion (ppb) - and compared the levels to those in 20 major U.S. metropolitan areas, such as Los Angeles, New York City and Chicago. Keiser et al. found no statistically distinguishable difference between average annual ozone concentrations in the national parks and metropolitan areas they sampled. Metropolitan areas experienced a 13% decrease in average summer ozone concentrations over the 24-year period, whereas ozone levels in national parks increased from 1990 to the early 2000s and only started decreasing thereafter. The decrease, say the authors, corresponds to regulatory efforts, which have been slower to take effect in parks. In a second part of their study, the researchers compared monthly park visitation data with monthly average daily maximum ozone readings and discovered a significant negative relationship between park visitation and ozone concentrations; this may have been the result of potential visitors staying home following park-issued alerts about air quality, the authors say. Also possible, the authors note, visitors may have decreased the length of visitation after experiencing park days with poor visibility. The results suggest that passage of rules like the EPA Regional Haze Rule in 1999, which sought to improve visual air quality in national parks, could benefit the public, making parks more inviting. Their results thus add support to previous work that documents society's willingness to pay for environmental improvements at national parks.