Beijing's Beloved Shade Trees Turn Out to Be a Hidden Ozone Source

Beijing's hundreds of thousands of weeping willows and poplars have provided shade from the summer sun for more than four decades, but new research suggests they may also be an unexpected source of air pollution. Scientists say the trees could be adding to ozone levels that affect the Chinese capital's 20 million residents. The findings, from an international team led by scientists at Jinan University in Guangzhou, were published in the journal Science Advances.

Beijing's ozone levels had puzzled researchers because many of the city's cars have gone electric, yet the pollution persisted. The team found that the trees release unusually high levels of isoprene, a volatile organic compound that forms ground-level ozone when it reacts with nitrogen oxide from vehicles and factories. White poplars and weeping willows, which produce high levels of isoprene, make up an estimated 35% of Beijing's urban greenery; other high-emission trees such as oaks and eucalyptus are less common there. Beijing began greening the city in 1979 and had planted about 284,000 poplar and willow trees in its main urban area by the end of 2018.

Lead author Yuan Bin, from Jinan University, said Beijing's isoprene emission intensity from vegetation was the highest among existing urban observations, approaching levels seen in temperate forests. The city's hot summers worsen the problem: as temperatures rose from 20 to 35 degrees Celsius (68 to 95 Fahrenheit), the rate at which isoprene broke down into pollutants rose by about 700%, versus only about 40% for human-caused pollution. Vegetation caused about 10% of the city's total volatile organic compound emissions, but these formed more than half, 52%, of its ozone.

The study did not call for removing poplar and willow trees, but suggested gradually increasing the proportion of other species planted in the city. Replacing 10% of the high-emission trees with other species would cut isoprene emissions by at least 29%, the researchers calculated.

It remained important to keep reducing harmful emissions from human activities, especially in summer, the researchers said.