Urban trees are quietly worsening city ozone pollution — and heat makes it worse
Many tree species planted in cities for their fast growth and climate resilience — including weeping willows and poplars across Beijing — are emerging as a major contributor to urban ozone pollution, according to a study published in Science Advances and covered by Nature.
By Source Reporters Newsdesk
Thu, 27 August 2026 · 2 min read
Many tree species planted in cities for their fast growth and climate resilience — including weeping willows and poplars across Beijing — are emerging as a major contributor to urban ozone pollution, according to a study published in Science Advances and covered by Nature (Nature).
Ozone forms when volatile organic compounds (VOCs) released by vegetation, vehicles and industry react with nitrogen oxides (NOx), mostly from traffic and industry, in sunlight. Ground-level ozone can inflame or damage airways and worsen breathing difficulties, particularly in people with asthma or lung disease.
The research team, led by atmospheric chemist Bin Yuan of Jinan University in Guangzhou, set out to measure human contributions to Beijing's ozone problem but was surprised to find that urban vegetation played a far larger role than expected. "When we got this data, we did not believe it in the beginning," Yuan said (Nature).
Air sampling across Beijing between May and July 2021 showed vegetation accounted for about 10% of the city's total VOC emissions — yet because plant-derived compounds such as isoprene react especially readily in the atmosphere, plants contributed an estimated 52% of the chemicals that go on to form ozone. Roughly 35% of Beijing's trees are isoprene emitters, led by willows (Salix spp.) and poplars (Populus spp.).
The problem is not unique to Beijing. An assessment of tree species in 24 megacities found predicted isoprene emissions higher than Beijing's in many of them — about 65% of Sydney's trees emit isoprene, for example. Environmental chemist Ian Jamie of Macquarie University in Sydney noted that vegetation's share of VOCs grows proportionally as vehicle emissions, once the dominant organic-compound source, continue to fall (Nature).
Rising temperatures linked to climate change could amplify the effect: the study found the hydroxyl reactivity rate with plant-emitted VOCs at 35 °C was seven times higher than at 20 °C, pushing vegetation's contribution to ozone-forming chemistry from 21% to 74% (Nature).
**Sources:** Nature
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