Urban trees planted for quick growth and climate resilience are driving ozone air pollution in major cities, according to research reported by Nature.
The study, published in Science Advances, analyzed tree species across Beijing, including weeping willows and poplars. These species release high amounts of isoprene, a volatile organic compound produced during photosynthesis. Roughly 35% of the trees in Beijing emit isoprene. Between May and July 2021, the study period, vegetation accounted for about 10% of total VOC emissions in Beijing, while human activities like industrial chemicals and vehicle exhaust generated the remaining 90%.
Plant emissions proved far more reactive than human sources in generating ground-level ozone. Ground-level ozone damages airways and causes breathing problems, especially for people with asthma or lung disease. To assess the contribution of plants, researchers measured how fast VOCs react with atmospheric hydroxyls to form peroxy radicals, which then react with nitrogen oxides under sunlight. The study determined that vegetation generated 52% of the chemicals that react to form ozone in Beijing, with isoprene acting as the dominant driver.
Bin Yuan, an atmospheric chemistry researcher at Jinan University in Guangzhou, said the team originally set out to evaluate human-driven pollution. “When we got this data, we did not believe it in the beginning,” Yuan said. The team also examined tree species in 24 megacities, finding that projected isoprene emissions in cities like Sydney and Melbourne were higher than in Beijing. In Sydney, approximately 65% of trees emit isoprene.
Ian Jamie, an environmental chemist at Macquarie University in Sydney, said vegetation has become a proportionally larger VOC source because vehicle exhaust has declined substantially in recent decades. Warmer temperatures also increase these emissions. The researchers found that at 35 °C, the hydroxyl reactivity rate with plant VOCs was seven times higher than at 20 °C, pushing the contribution of vegetation to chemical reactivity from 21% at 20 °C to 74% at 35 °C.
