Weeping willows drooping over midsummer park ponds help cool the heat but appear to make air quality worse. That is because compounds emitted by willows generate air pollutants. It means cities worldwide could cut air pollution significantly by choosing the right tree species for urban greening.
A team led by Professor Bin Yuan at Jinan University's Institute for Environment and Climate Research said on the 20th in the journal Science Advances that "an analysis of volatile organic compound (VOC) data collected in Beijing showed that trees such as willows that emit isoprene contribute significantly to ozone (O₃) pollution in megacities."
Ozone is a gas made of three oxygen atoms that blocks harmful ultraviolet rays in the stratosphere about 10 kilometers up, but near the surface it is a pollutant that irritates the respiratory system. Inhaling ozone can damage or inflame the airways. It can cause breathing difficulties, especially for people with asthma or lung disease.
◇Trees cause half of urban ozone pollution
Millions of weeping willows and poplars grow in Beijing. Yuan concluded that compounds released by the trees that turned this megacity green are a main cause of air pollution in hot summers. The study began to gauge how human activity affects ozone pollution, but soon found that urban trees contribute far more to ozone emissions than expected.
The researchers collected air samples across Beijing to measure concentrations of volatile compounds and simultaneously gathered ozone data from observation sites around the city. Between May and July 2021, trees accounted for about 10% of Beijing's VOC emissions. The rest came from human activities such as vehicle exhaust and industrial chemicals.
Although volatile compounds from human activities such as cars and factories were absolutely greater than those from trees, their contribution to ozone formation was much lower than that of plants. That was because tree-emitted volatile compounds more readily triggered the chemical reactions that create ozone.
Ozone forms when volatile organic compounds react in sunlight with nitrogen oxides (NOx) emitted by cars or factories. Volatile compounds first react with hydroxyl groups (-OH) in the atmosphere to produce hydroperoxy radicals (HO₂), which are highly reactive with other substances. When these radicals react with nitrogen oxides, they generate ozone. Tree-emitted VOCs reacted with hydroxyl groups more readily than vehicle or industrial emissions.
According to the study, 52% of the chemicals that generate ozone originated from trees. Paints, cleaners, and chemical products and automobiles, which account for 50% and 24% of VOC emissions, contributed only 21% and 17%, respectively. Among tree-emitted VOCs, isoprene was the main culprit behind ozone formation. Willows and poplars widely planted in cities around the world emit large amounts of isoprene. The researchers found that about 35% of the trees in Beijing are sources of isoprene emissions.
◇Willows and poplars are ozone sources in midsummer
The fact that trees contribute to ozone pollution was the same in other megacities. The team also examined species planted in 24 megacities worldwide to estimate isoprene emissions. As a result, most of those surveyed were predicted to have higher isoprene emissions than Beijing.
In China, Changchun and Harbin were estimated to have higher tree isoprene emissions than Beijing. Overseas, Sydney and Melbourne in Australia and Atlanta in the United States were predicted to be higher than Beijing. In Sydney, about 65% of trees were found to emit isoprene.
Trees in megacities were found to drive ozone pollution especially in midsummer. The study found that at 35 C, the reaction rate between tree-emitted volatile compounds and hydroxyl groups was seven times higher than at 20 C. As a result, the tree contribution in the chemical reactions that create ozone rose from 21% at 20 C to 74% at 35 C.
In summer, heat from air conditioner outdoor units and vehicle exhaust lingers in city centers, raising temperatures higher than in suburbs. That is the urban heat island effect. In midsummer, the heat island effect can further spur tree isoprene emissions. The researchers estimated that on summer days with a daily high of 32 C, when downtown temperatures are 0.8 C higher than in the suburbs, the amount of isoprene emitted by trees increases by 12%.
Robyn Schofield at the University of Melbourne said, "Because of the heat island effect, urban trees experience greater stress and emit more volatile compounds than the same species in suburban forests." Yuan's team projected that as global warming progresses, VOC emissions from urban trees could increase further.
◇Choosing tree species is also an urban air pollution measure
Ian Jamie at Macquarie University said that as other VOC sources such as automobiles have declined sharply over the past decades, the share from trees has grown relatively. Still, the researchers noted this does not mean cities should stop planting trees.
The researchers said, "Trees already provide substantial benefits by cooling urban heat, absorbing carbon dioxide, a greenhouse gas, and supporting biodiversity," and added, "It simply means species should be chosen carefully so as not to worsen air pollution."
For example, the team said that if 10% of isoprene-emitting trees in Beijing were replaced with other species, isoprene emissions could be cut by up to 29%. Yuan said, "With climate change driving higher temperatures and more intense heat waves, factoring ozone formation into proper species selection should be part of urban planning."
The paper lists Min Shao, head of Jinan University's Institute for Environment and Climate Research, and Thomas Karl at the University of Innsbruck as co-corresponding authors. Researchers from the University of California, Irvine; the University of Colorado; and the University of Helsinki in Finland also took part. In Korea, Min Gyeong-eun and Nam Woo-hee of the Department of Environmental and Energy Engineering at the Gwangju Institute of Science and Technology (GIST) were listed as co-authors.
References
Science Advances (2026), DOI: https://doi.org/10.1126/sciadv.aee5583