Across Beijing, millions of weeping willows and poplar trees create a green city, but they also release chemicals that make air pollution worse. A study finds these compounds are adding to ozone levels.
This isn’t just happening in Beijing. Many fast-growing tree species planted in cities across the world are releasing chemicals that increase ozone pollution. Rising temperatures from global warming could make this problem even bigger.
Ozone forms when certain chemicals, called volatile organic compounds (VOCs), released by plants, cars, and industries mix with nitrogen oxides (NOx) from traffic and industry. These reactions, powered by sunlight, create ozone, which can harm people’s breathing, especially those with asthma or lung problems.
Scientists were surprised to find that trees contribute more to ozone than expected. For example, willows and poplars release high levels of isoprene, a VOC that helps form ozone. In Beijing alone, 35% of the trees are isoprene emitters.
The study measured air samples in Beijing and found that trees accounted for about 10% of all VOC emissions during summer 2021. While human activities like car exhaust and industry still release more VOCs overall, plants contribute more to ozone formation because their chemicals react faster with NOx.
Researchers also studied 24 big cities, including Sydney and Melbourne, and found that isoprene emissions from trees could be even higher than in Beijing. For example, about 65% of trees in Sydney release this chemical.
As temperatures rise, the problem grows worse. At hotter temperatures, tree chemicals react more quickly with NOx to form ozone. This means plants may contribute up to 74% of ozone-forming reactions at high heat, compared to just 21% at cooler temperatures.
This growing issue highlights how nature itself can sometimes harm air quality, especially as the climate changes.