Some City Trees Make Ozone Pollution Worse, Study Finds

Trees in cities are supposed to clean the air. In many cases they are doing the opposite. A team at Jinan University and the University of Innsbruck, using direct measurements from a 102-meter meteorological tower in Beijing, found that biogenic emissions from urban trees accounted for only a tenth of total volatile organic compound output but nearly half of its chemical reactivity, the metric that actually drives ozone formation. The reason is isoprene, a single compound that made up over 90% of the reactive biogenic emissions. The temperature sensitivity is what makes this politically uncomfortable. Between 20 degrees Celsius and 35 degrees, the ozone-forming potential of tree emissions jumped seven- to eight-fold, while human-made VOCs barely changed. On the hottest days, vegetation's share of total atmospheric reactivity climbed from 21% to 74%. Beijing is an extreme case, partly because roughly 35% of its urban canopy consists of high-isoprene species like weeping willow and Chinese white poplar. But the data challenge is familiar: models built for natural forests have been applied to cities without sufficient direct measurements to back them up. The paper, published in Science Advances, does not argue for removing trees. It argues for choosing them more carefully. Swapping out high-emitting species in just a tenth of Beijing's urban tree population during routine replanting could cut isoprene emissions by nearly 30%. The approach matters as AI-assisted urban planning gains traction in cities scrambling to adapt to hotter summers. As climate regulations squeeze anthropogenic VOC sources, the relative importance of what trees emit will only grow.