How Urban Trees Can Worsen Air Quality in Cities

Millions of weeping willows and poplar trees help make Beijing a lush urban metropolis. However, new research suggests that chemicals released by these trees may be a significant and previously underestimated source of the city’s air pollution.¹

Beijing is not the only city facing this issue. Research published in 2006 indicated that many trees selected for urban planting because of their rapid growth and resilience to climate change can increase ozone pollution. New findings suggest that rising temperatures caused by global warming could intensify the problem.

Ground-level ozone forms when volatile organic compounds (VOCs) released by vegetation, vehicles and the chemical industry react with nitrogen oxides (NO_x). Sunlight drives a series of chemical reactions that produce ozone. Breathing ground-level ozone can irritate or damage the airways and cause difficulty breathing, particularly for people with asthma or other lung conditions.

Co-author Bin Yuan, an atmospheric chemistry researcher at Jinan University in Guangzhou, China, said the research team initially set out to investigate how human activities affect urban ozone pollution. The scientists were surprised to discover that vegetation made a larger contribution than expected. “When I got this data, I couldn’t believe it at first,” Yuan says.

Many tree species emit VOCs, but some release much higher amounts than others. Willow (Salix spp.) and poplar (Populus spp.) trees are widely planted in cities and emit high levels of isoprene, a VOC produced as a by-product of photosynthesis. The researchers found that approximately 35% of Beijing’s trees emit isoprene.

Measuring plant emissions in Beijing

The researchers collected air samples across Beijing to measure VOC concentrations and used ozone measurements from monitoring stations throughout the city. During the study period, from May to July 2021, vegetation accounted for approximately 10% of the city’s total VOC emissions. The remaining emissions came from human activities, including vehicle exhaust and industrial chemicals.

Because VOCs do not directly produce ozone, the researchers used atmospheric chemical reactivity to estimate how strongly plant emissions contribute to ozone formation. They calculated how quickly VOCs react with hydroxyl radicals in the atmosphere. These reactions create peroxy radicals, which interact with NO_x to produce compounds that form ozone in sunlight.

The study found that plant emissions accounted for 52% of the chemicals involved in ozone formation, with isoprene identified as the main contributor. Although human activities released more VOCs overall than vegetation, those emissions had a considerably smaller effect on ozone formation.

The researchers also analysed the tree species planted in 24 major cities to estimate their potential isoprene emissions. Most of the cities studied, including Sydney and Melbourne in Australia, were projected to have higher isoprene emissions than Beijing. In Sydney, for example, approximately 65% of trees emit isoprene.

Ian Jamie, an environmental chemist at Macquarie University in Sydney, said vegetation has become a proportionally larger source of VOCs as emissions from other sources have declined over recent decades. Vehicle emissions, once a major source of organic compounds, have fallen significantly, he said.

Rising temperatures increase ozone pollution

Higher temperatures can increase the production of both VOCs and ozone. The study found that at 35 °C, the reaction rate between hydroxyl radicals and plant-derived VOCs was seven times higher than at 20 °C. Consequently, the contribution of plants to atmospheric chemical reactivity rose from 21% at 20 °C to 74% at 35 °C.