Urban Trees Emit VOCs that Boost Ozone Levels, Study Finds

Across Beijing, millions of weeping willows and poplar trees give the megacity a lush appearance. However, a new study reveals that compounds released by these trees are a significant source of air pollution.

The findings extend beyond Beijing. Many urban‑planted species, selected for fast growth and climate resilience, are increasing ozone emissions, according to research published in Science Advances. Warmer temperatures are likely to intensify the problem.

Ozone forms when volatile organic compounds (VOCs) from vegetation, vehicles and industry react with nitrogen oxides (NOx). Sunlight drives the series of reactions that produce ozone, which can cause breathing difficulties, especially for people with asthma or lung disease.

Co‑author Bin Yuan of Jinan University in Guangzhou explained that the team originally aimed to assess human impacts on ozone. The data showed that urban vegetation contributed far more than expected. “When we got this data, we did not believe it at the beginning,” Yuan said.

Tree species differ in VOC emissions; willows (Salix spp.) and poplars (Populus spp.) are high emitters of isoprene, a VOC released during photosynthesis. The study estimates that about 35 % of Beijing’s trees emit isoprene.

Measuring emissions

Researchers collected air samples across Beijing to quantify VOC concentrations and gathered ozone data from monitoring stations. Vegetation accounted for roughly 10 % of total VOC emissions between May and July 2021, with human activities supplying the remainder.

Because VOCs do not directly create ozone, the team measured their atmospheric reactivity. VOCs react with hydroxyl radicals to form peroxy radicals, which then combine with NOx to produce ozone‑forming compounds. The analysis showed that vegetation contributed about 52 % of the chemicals that lead to ozone formation, with isoprene being the dominant contributor.

Although human‑derived VOC emissions were higher in absolute quantity, their share of ozone‑forming chemistry was substantially lower than that of plants.

Heat intensifies

VOC and ozone production accelerate with temperature. The study found that at 35 °C the hydroxyl‑VOC reaction rate was seven times higher than at 20 °C, raising vegetation’s share of chemical reactivity from 21 % to 74 %.

The researchers also evaluated isoprene emissions from tree species across 24 megacities. They projected that cities such as Sydney and Melbourne could emit more isoprene from urban trees than Beijing. In Sydney, roughly 65 % of trees are isoprene emitters, according to the model.

“Vegetation’s proportion of VOC sources is growing as other emissions decline,” says Ian Jamie, an environmental chemist at Macquarie University in Sydney.

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