IOWA CITY, June 13 -- The Clean Air Act was, on paper, working. Between 2003 and 2015, surface ozone across the United States fell 11 percent as catalytic converters got cleaner, coal plants closed and diesel rules tightened. A study released this month finds that the trend turned a decade ago, that since 2015 American smog has risen about 4 percent, and that the climbing source is not a regulation that was relaxed but a fire season that no longer ends.

The paper, published in the journal Science by a University of Iowa team led by atmospheric scientist Weizhi Deng with co-author Meng Zhou, maps surface ozone in kilometer-by-kilometer grids across the lower 48 states from 2003 through 2024, using satellite observations and machine learni...