Some of the Trees Cities Love to Plant Have an Air-Pollution Problem

A tree can provide shade, cool a street, store carbon and still be the wrong tree for the air above it.

Researchers measuring volatile organic compounds, or VOCs, across Beijing found that vegetation produced only about 10% of the city's total VOC emissions during the study period. Vehicles, industry and other human sources produced most of the rest.

But quantity was not the important part.

When the team measured how readily those chemicals participated in reactions that lead to ozone formation, vegetation accounted for roughly 52% of the organic reactivity. Isoprene, a compound released strongly by trees including poplars and weeping willows, dominated the plant contribution.

Heat amplified the effect dramatically.

At 20°C, or 68°F, vegetation accounted for about 21% of the reactivity. At 35°C, or 95°F, that share climbed to 74%.

Trees do not produce ozone on their own. Ozone forms through reactions involving VOCs, nitrogen oxides and sunlight. But the species a city plants can strongly affect how much reactive chemistry is available.

That matters because cities often favor fast-growing, heat-tolerant species that also happen to release lots of isoprene. Roughly 35% of Beijing's trees fall into that category, and the researchers found similarly high or higher proportions in other megacities.

The shade is easy to see. Atmospheric chemistry is not.

Read the full story at Nature, August 20, 2026

Hot Take: The generally good advice to “plant more trees” in cities could use an asterisk. Maybe avoid planting isoprene factories with roots.