Amid current heatwaves, urban greening is once again moving to the center of public debate. Greener cities are generally expected to have cleaner air because trees capture airborne particles, absorb gaseous pollutants and cool urban neighborhoods. But trees also emit biogenic volatile organic compounds (BVOCs) that, in the presence of sunlight and nitrogen oxides (NOx), can contribute to ground-level ozone formation.
Human activity also produces volatile organic compounds that react with nitrogen oxides to form ozone. Anthropogenic VOCs (AVOCs) are found in solvents, paints, cleaning products, and fuel vapors, among other sources. As these are increasingly regulated to curb climate change, measurements show a decline in AVOC levels. As the climate warms and cities grow greener, controls on human-made emissions may further increase the relative importance of this overlooked source from trees.
Yet direct measurements under real-world urban conditions remain rare, and current estimates rely largely on models developed for natural ecosystems. To move beyond model estimates, researchers at Jinan University in China set up a direct emissions monitoring system on the 102-meter platform and in a laboratory at the base of the Beijing Meteorological Tower. From May to July 2021, the system tracked chemical signals and turbulence fluctuations 10 times per second, capturing the city's chemical "breath" in real time. Working with colleagues at the University of Innsbruck, the researchers quantified VOC emissions and traced them to sources including trees, traffic, chemical products, cooking and household activities.
Isoprene most reactive, heat the decisive factor
The measurements revealed a striking mismatch between emission amount and chemical effect. Biogenic sources made up only about one-tenth of measured VOC emissions but contributed nearly half of total VOC reactivity — a measure of how strongly emissions can drive atmospheric chemistry. Much of this effect came from isoprene, which alone accounted for more than 90 percent of the chemical reactivity of biogenic VOC emissions.
Temperature sharply amplified this effect. Between 20°C and 35°C, the chemical reactivity of VOCs emitted by city trees increased seven- to eight-fold, compared with only about 40 percent, or 1.4-fold, for VOCs from human activities. Vegetation's share of total VOC reactivity consequently rose from 21 percent to 74 percent.
On days when ozone formation was particularly sensitive to VOC changes, observed peak ozone rose with temperature at nearly the same rate as VOC reactivity. This close match supports a link between stronger vegetation emissions on hot days and higher ozone levels.
From Beijing to cities worldwide
To understand why tree emissions were so influential in Beijing, the researchers compared their results with direct measurements from other cities. After adjusting for differences in temperature and sunlight, Beijing's isoprene emissions were the highest among the cities observed and approached levels found in temperate forests.
Yet vegetation cover alone could not explain why Beijing emitted several times more isoprene than cities with similar levels of greenery. Comparing tree inventories and vegetation data from more than 20 cities showed that the answer lay largely in which tree species were planted. About 35 percent of Beijing's trees are isoprene-emitting species, including weeping willow and Chinese white poplar. Across the cities studied, isoprene emission levels varied by more than tenfold and closely tracked the proportion of isoprene-emitting trees.
Several cities in Asia and Oceania may face comparable or greater air-quality pressures from tree emissions. "In European cities, we find considerably fewer tree species that release isoprene. Native tree species often emit monoterpenes, which are also ozone-forming, but are released in smaller amounts than isoprene," explains Thomas Karl, co-author and atmospheric physicist at the University of Innsbruck.
Planning greener cities with cleaner air
"The findings do not call for less urban greening or the removal of mature trees. Instead, they suggest adding BVOC emission potential to tree-selection criteria," Karl emphasizes. For Beijing, replacing high-emitting trees in one-tenth of the total urban tree population with low-emitting species during routine renewal could reduce isoprene emissions by at least 29 percent. "As heatwaves become more frequent, tree species composition could become a useful complement to conventional emission controls," the study authors note.
NOx control remains essential
Tree emissions matter, but they are only one part of the ozone problem. Ozone formation also requires NOx and sunlight, and the chemistry is nonlinear. The results show that warming increases reactive VOC emissions from vegetation, making continued controls on NOx emissions essential, especially during hot periods.
