Urban trees are a triumph. Their shade helps battle heat by keeping asphalt and concrete cool, their roots help limit flooding by absorbing excess stormwater, and their boughs help humans coexist with other animals by providing a natural habitat. But new research suggests that not every tree species is cut out for city living. Some, in fact, emit large volumes of volatile organic compounds (VOCs) that create ozone, possibly contributing to poor air quality (Sci. Adv. 2026, DOI: 10.1126/sciadv.aee5583).
There are many things in city air that can create ozone. The gas is produced at ground level on warm sunny days when ultraviolet rays drive reactions between VOCs and nitrogen oxides (NOx). Cars and trucks spew both types of compounds from their tailpipes, and the city environment is rife with VOC-emitting chemical products.
But plants also emit VOCs. So, to understand how trees might influence urban air quality in megacities, Jinan University’s Bin Yuan and his colleagues first teased apart the biggest emitters in Beijing.
To find the culprits, Yuan’s team collected VOC flux measurements from a platform on a meteorologic tower in the heart of the city. “Flux is the amount of a species that is emitted or deposited for a unit area per unit time,” Yuan explains. In other words, it’s a rate measurement.
From these data, Yuan’s team determined that anthropogenic chemical products contributed 50% of the measured VOC flux. But emission rate is only part of the picture. Chemical reactivity is equally important to calculate, particularly when trying to understand the formation of secondary pollutants like ozone. And that’s where trees had the most impact.
Despite contributing to only 11% of the overall VOC flux, biogenic emissions—those primarily from trees—represented 52% of the reactive VOC species in the area. And one chemical component stood out above the rest: isoprene.
Isoprene reacts incredibly quickly in the atmosphere, especially on sunny days, Yuan says. This means that essentially all the emitted isoprene reacts, creating an abundance of ozone. About a third of the trees in Beijing release large amounts of isoprene. And the city isn’t alone: the researchers identified many other cities around the world that have superemitters lining their streets.
So, should all city trees get the chop?
Of course not. “If we identify the trees with the highest emission potential, we need to replace those first,” Yuan says. In China, for example, many cities’ officials have planted numerous willow and poplar trees, both of which are known major isoprene emitters. Based on the researchers’ models, if officials replaced 10% of these trees with trees that do not emit isoprene, Beijing would see at least a 29% reduction in isoprene emissions overall. This would subsequently limit possible ozone production.
The work makes it clear that prioritizing low-emitting trees has the potential to improve air quality in Beijing and other cities pursuing urban greening initiatives, atmospheric chemist Hannah Kenagy of the University of Minnesota writes in an email. She was not involved in the study and notes that such an approach will be especially important “under climate change since biogenic emissions are highly temperature-dependent.”
Importantly, ozone production requires both VOCs and NOx, Eva Pfannerstill, an atmospheric chemist at the Jülich Research Center who was not involved in the research, notes in an email. In her view, the most important takeaway is not that trees can cause poor air quality. Rather, “the presence of trees (which we need for many reasons, shade, cooling, uptake of pollutants, etc.) causes a need for even faster reduction of (fossil fuel burning-related) NOx emissions.”
By carefully considering what trees are planted and continuing to regulate anthropogenic sources of NOx and VOCs, city officials can help residents breathe easier in the years to come.
