Trees in Beijing are not just victims of the city's notorious air pollution. They are also contributors, according to a new study by Chinese scientists that found urban greenery is actively producing ozone smog.
The research, published by a team in China, reveals that certain tree species release compounds that react with pollutants from traffic and industry to form ground-level ozone. This means the very plants planted to clean the air may be adding to one of its most stubborn problems.
The hidden chemistry of urban forests
Scientists measured emissions from common Beijing tree species and tracked how those chemicals interacted with the city's nitrogen oxides. The result was a significant spike in ozone formation, especially on hot, sunny days when trees are most active.
The study focused on the complex reactions between biogenic volatile organic compounds, or BVOCs, emitted by leaves and the man-made pollutants that blanket the capital. Ozone, unlike particulate matter, is not emitted directly but forms when these ingredients cook together in sunlight.
Why locals should pay attention
For residents of Beijing, ozone is a familiar summer hazard. It can trigger asthma, reduce lung function, and damage crops. The new finding complicates efforts to control it, because planting more trees, a popular policy move, might inadvertently worsen the problem in certain conditions.
The researchers stressed that not all trees are equal. Some species release far more of the reactive compounds than others, suggesting that tree selection could be a tool for managing ozone levels. The study does not call for cutting down forests, but it does urge city planners to think carefully about what they plant and where.
The discovery adds a fresh layer to the puzzle of urban air quality. It shows that even the most natural elements of a city can play an unexpected role in the chemistry of its air, and that solving pollution requires looking at the whole system, not just the obvious sources.
