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A lengthy California drought left trees and plants parched and influenced their contributions to smog.

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Drought is not just about water. It affects air pollution, too

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.

Understanding how drought influences these two processes can be tricky. Dry conditions could cause ozone levels to rise, because plants shrink their stomata to prevent water loss, reducing their ability to remove pollution. But drought might also reduce ozone levels, because the stress could cause plants to produce fewer ozone-forming VOCs.

California’s lengthy drought, and the state’s extensive network of air pollution sensors, gave researchers a rare opportunity to see what happens in the real world. The team, led by atmospheric chemistry Ph.D. candidate Angelique Demetillo and environmental science professor Sally Pusede at the University of Virginia (UVA) in Charlottesville, examined more than a decade’s worth of satellite and sensor data that documented atmospheric conditions over Bakersfield and Fresno, two California cities that suffer from ozone pollution.

The drought’s impact on air quality changed over time, the researchers report this week in Environmental Science & Technology. Plants did remove less ozone, with absorption dropping by about 15% during the most severe years of the drought. But during the early years of the drought, trees and other plants were able to maintain their production of one key ozone-forming VOC, isoprene. The chemical helps plants like oak trees withstand heat stress, and it appears the trees draw on carbon stores to sustain isoprene production. “It’s like a person exercising; when you’ve burned through your recent consumption the body switches over and starts burning fat,” says Manuel Lerdau, an organismal ecologist at UVA and a co-author of the study.

After about 4 years of drought, however, the stress took its toll. In 2013, plant isoprene levels fell dramatically, by 65% in Bakersfield and 54% in Fresno. Overall, that meant up to a 20% dip in ozone pollution. And even after the drought ended, isoprene levels didn’t immediately rebound.

The plant VOC reduction might sound like good news for reducing California’s smog. But the complexity of atmospheric chemistry means droughts could actually complicate clean air efforts. That’s because, currently, regulations mostly focus on controlling nitrogen oxides (NOx) from sources like cars and factories, which react with VOCs to form ozone. When VOC levels are higher, those NOx controls help choke off smog-creating reactions. But reduce the VOCs, and NOx limits go from “being very effective to less effective,” Pusede says.

Such findings offer yet another complication for U.S. states and cities struggling to meet federal clean air standards, especially in drought-prone western states. Regulators have little ability to control VOC emissions from plants, notes Pusede, and “I don’t know if we’d want to even if we could.” But the study could help regulators do a better job of factoring drought into their air pollution models.

The work also “provides a road map for better quantifying these impacts in other places,” says atmospheric scientist Jessica Neu of NASA’s Jet Propulsion Laboratory in Pasadena, California, who reviewed the paper. And because much of the needed data can now be collected by satellites, she says it opens the door to investigating “drought impacts on air quality globally.”