Since China instituted regulations on particulate matter and other air pollution in 2013, summer surface ozone levels have continued to rise. Now researchers believe they know why: cuts in particulate emissions changed the local air chemistry, allowing ozone-producing radicals to accumulate (Proc. Natl. Acad. Sci., U.S.A. 2018, DOI: 10.1073/pnas.1812168116). From 2013 to 2017, ozone levels in major urban areas in eastern China rose by 5 to 10%, says Daniel Jacob, an atmospheric chemist at Harvard University. Through computer-modeling experiments, he and researchers at Nanjing and Tsinghua Universities have now found an unexpected connection between ozone and airborne particles that are 2.5 µm and finer, called PM2.5. Thanks to regulations, emissions of these particles—which have adverse health effects—have declined rapidly in the country, falling by 40% from 2013 to 2017. The team’s models suggest that these particles can sponge up free radicals that play a key role in ozone production. China’s regulations lowered these radical-sopping particles but didn’t control emissions of the volatile hydrocarbons that react with radicals to form ozone. Together, Jacob says, this seems to have been a recipe for rising ozone levels.