The severe drought that struck California from 2011 to 2015 had an obvious impact on rivers, forests, and wildlife. Now, a new study shows it also had some surprising effects on the state’s notorious air pollution, adding new wrinkles to the state’s efforts to clear the skies. Researchers have long known that plants can both help create and cleanse one dangerous air pollutant: ground-level ozone, which causes breathing problems and exacerbates lung damage. Plants can scrub ozone from the air by absorbing the pollutant through their stomata, or pores. But certain plants also emit volatile organic compounds (VOCs) that react with other atmospheric chemicals to create ozone.