There is no cure for every disease. Nor for all the harms the Earth endures. Even trees don’t always serve as a miraculous remedy against urban air pollution. In fact, under certain conditions some trees can worsen air quality.
This finding comes from a master’s project led by Donato Kofel, an Environmental Science and Engineering graduate from the Federal Polytechnic School of Lausanne, known as EPFL. Kofel examined both the positive and negative influences of trees on urban air in the Swiss canton of Geneva.
The study dives into geographic information systems, or GIS, a powerful mapping tool. “These maps compress a lot of information into a single image, letting people grasp it all at a glance,” Kofel explains.
To map tree effects, Kofel drew on an inventory of roughly 240,000 isolated trees—trees not part of forests. These are trees along boulevards, in parks, and scattered across the city. Isolated trees account for about a quarter of Geneva’s total tree population, and the inventory records species, location, trunk height, trunk diameter, and crown diameter.
Visual outputs from this data helped create maps of total leaf area, serving as a proxy for how well trees can filter particles from the air. At the same time, the study explored another key process: how trees contribute to the formation and buildup of ozone.
Oaks Under the Spotlight
The benefits of trees are well recognized: cooling urban heat islands, filtering fine particles, capturing CO2, and releasing oxygen. Yet trees also have drawbacks. They emit biogenic volatile organic compounds, BVOCs, at rates that vary with species, air temperature and humidity, sunlight, and tree health. When BVOCs mix with pollutants from human activity, they can drive ozone formation.
Oak trees emit compounds that turn into ozone harmful to humans
BVOCs can react with other airborne compounds to form ozone through photochemical oxidation. This ozone can affect health and the environment. The environmental engineer calculated the ozone-generation potential of tree emissions and found that under certain conditions, trees may partly degrade air quality.
The team began by reviewing literature on Geneva’s 51 most common tree species, using that data to estimate hourly BVOC emission rates. The analysis revealed that some oak species exhibit the highest BVOC emissions, and thus the greatest potential for ozone formation. Oaks are among the most common street and park trees in Geneva.
With help from Romana Paganini and Ilann Bourgeois, the team estimated how much particulate matter and ozone urban trees remove each year to illuminate their positive impact. Detailed maps showed that urban trees remove about a quarter of the particulate matter produced by human activity in Geneva.
Additional Air Pollution Considerations
Yet the research also showed that the ozone-forming potential of these trees is roughly ten times their pollution-destroying capacity. The study estimates BVOC emissions at about 130 metric tons per year, equivalent to roughly 18% of VOCs emitted by road traffic. The findings suggest that human activity provides enough nitrogen oxides for BVOC-driven reactions to generate ozone.
One takeaway: reducing ozone formation from trees would benefit from lowering nitrogen oxide emissions, depending on the exact mix of BVOCs and oxides present in the air. Translation: there is no simple answer about whether city trees are universally good for air quality, since their effects depend on local emissions and conditions.
City planners can still use this method to guide projects. Larger planting programs, when paired with strategies to cut traffic and other emissions, tend to be more effective in improving urban air quality.
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Note: the study highlights the need for deeper research to answer outstanding questions. In the meantime, it shows that trees can contribute meaningfully to cleaner urban air, but they are not a cure-all in every scenario.
Officials emphasize that addressing air pollution requires action at the source, balancing road traffic with other emission sources. The methodological approach demonstrated in this work offers a way for cities to tailor tree-planting initiatives to local conditions and goals.
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