Urban trees that release large amounts of isoprene can make a disproportionate contribution to ozone-forming chemistry during hot weather, according to research published in Science Advances and reported by Nature on August 20. Measurements in Beijing found that vegetation supplied about 10% of volatile organic compound emissions during the May-to-July 2021 study period but accounted for 52% of the measured chemical reactivity that leads toward ozone formation.
Ground-level ozone forms through sunlight-driven reactions involving volatile organic compounds and nitrogen oxides, which largely come from traffic and industry. Unlike protective ozone high in the atmosphere, ozone near the surface can inflame or damage airways and create particular difficulty for people with asthma or lung disease. Trees also provide shade, cooling, carbon storage and pollutant removal, so the researchers say the result concerns species selection rather than an argument against urban greenery.
The team collected air samples across Beijing, combined them with ozone-monitoring data and estimated how quickly different compounds reacted with hydroxyl radicals. Those reactions produce peroxy radicals that interact with nitrogen oxides and ultimately help create ozone in sunlight. Isoprene, a highly reactive compound released as a by-product of photosynthesis by species including willows and poplars, dominated the vegetation-related contribution even though human activities emitted more volatile compounds overall.
About 35% of Beijing's trees were classified as isoprene emitters. The researchers then assessed planted species in 24 megacities and found that several, including Sydney and Melbourne, could have higher potential isoprene emissions than Beijing; Nature reported that roughly 65% of Sydney's trees emit the compound. The comparison indicates that local planting choices can shape air chemistry far beyond the Chinese capital, although actual ozone outcomes also depend on nitrogen oxides, sunlight and weather.
Heat sharply strengthened the effect. As temperature rose from 20 degrees Celsius to 35 degrees, hydroxyl reactivity linked to vegetation compounds increased sevenfold, while vegetation's share of total reactivity climbed from 21% to 74%. The finding suggests that warming and more frequent heatwaves could magnify an air-quality trade-off in cities that are simultaneously expanding tree cover to reduce dangerous heat.
The University of Innsbruck said replacing one-tenth of Beijing's high-emitting trees with low-emitting species during routine renewal could cut isoprene emissions by at least 29%. Researchers are not recommending mass tree removal; gradual replacement, diversified planting and continued reductions in nitrogen oxides and human-made volatile compounds offer a more balanced response. Further observations in different climates will be needed to translate the Beijing measurements into city-specific planting and pollution policies.
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