Tree Species, Not Just Greenery, Shape Ozone Risk in Beijing, Study Finds

Urban trees generally clean the air, but researchers in China have found that the specific species planted can significantly affect how much they contribute to ground-level ozone. A study led by researchers at Jinan University, working with the University of Innsbruck in Austria and published in Science Advances, measured emissions directly from a 102-meter platform on the Beijing Meteorological Tower between May and July 2021.

Trees release biogenic volatile organic compounds (BVOC), which react with sunlight and nitrogen oxides to form ozone. The researchers found that these biological sources made up only about one-tenth of measured volatile organic compound emissions in Beijing but contributed nearly half of the total chemical reactivity driving ozone formation, with the compound isoprene alone responsible for more than 90% of that biogenic reactivity.

Temperature sharply amplified the effect: between 20C and 35C, the reactivity of tree-emitted compounds rose seven to eightfold, compared with only about 1.4-fold for human-made emissions, pushing vegetation's share of total reactivity from 21% to 74%. On days when ozone was most sensitive to these compounds, observed ozone levels rose with temperature at nearly the same rate as the reactivity itself. About 35% of Beijing's trees, including weeping willow and Chinese white poplar, are isoprene-emitting species, and Beijing's isoprene emissions were the highest among more than 20 cities compared in the study.

"The findings do not call for less urban greening or the removal of mature trees," said co-author Thomas Karl, an atmospheric physicist at the University of Innsbruck, adding that they instead suggest adding emission potential to tree-selection criteria. The researchers calculated that replacing high-emitting trees in one-tenth of Beijing's urban tree population with low-emitting species during routine renewal could cut isoprene emissions by at least 29%, though controlling nitrogen oxides remains essential since ozone formation requires both.