This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Amid current heat waves, 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 (335-foot) 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.
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% of the chemical reactivity of biogenic VOC emissions.
Temperature sharply amplified this effect. Between 20°C (68°F) and 35°C (95°F), the chemical reactivity of VOCs emitted by city trees increased seven- to eightfold, compared with only about 40%, or 1.4-fold, for VOCs from human activities. Vegetation's share of total VOC reactivity consequently rose from 21% to 74%.
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