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Shi Huang

Beijing’s hundreds of thousands of weeping willows and poplars have been a welcome source of shelter from the fierce summer sun for more than four decades – but they may also be an unexpected source of pollution, according to new research.
Scientists warned that this urban greenery may be “choking” the Chinese capital’s 20 million residents by increasing the levels of ozone in the city’s air.
Smog caused by the ozone released by car exhaust has been recognised as a public health problem since the 1940s, with some severe incidents known to have killed hundreds of people.
Beijing’s ozone levels had puzzled researchers because many cars in the city have gone electric, but they have now identified the compounds released by the trees as the most likely culprit.
An international team led by scientists from Jinan University in Guangzhou initially intended to measure how human activities generated ozone pollution – but they soon discovered that the trees in Beijing generated far more smog than expected, according to research published in the journal Science Advances.
This is because they produce unusually high levels of isoprene, a volatile organic compound (VOC) that produces ground-level ozone when it reacts with the nitrogen oxide typically produced by vehicles or factories.
Although Beijing is not the only city to have this problem, it has an unusually high proportion of species such as white poplars and weeping willows, which make up an estimated 35 per cent of its urban greenery and produce high levels of isoprene.
Other high-emissions trees, such as oaks and eucalyptus, are less common in urban areas.
The city’s authorities began greening the city in 1979 and by the end of 2018, had planted an estimated 284,000 poplar and willow trees in the area encircled by the fifth ring road, which is generally regarded as the main urban area.
The trees have obvious benefits such as rapid growth, an ability to survive the city’s cold winters and the plentiful blind they provide during the summer.

However, even before the release of the latest study, these species were known for causing seasonal problems – they shed their catkins in the early spring, triggering respiratory and allergy problems.
“We conducted a horizontal comparison of Beijing’s observation results with existing urban data from around the world and found that Beijing’s standardised isoprene emission intensity from vegetation is at the highest level among existing urban observations, approaching the levels observed in temperate forests,” the study’s lead author Yuan Bin, from Jinan University, told China Science Daily.
The problem is made worse by the Chinese capital’s hot summers, which cause plants to produce more ozone.
Data showed that when the temperature in Beijing rose from 20 degrees Celsius to 35 degrees (68 to 95 Fahrenheit) the rate at which they broke down in the atmosphere to form pollutants rose by around 700 per cent.
In contrast, pollution caused by human activity increased by only about 40 per cent.
The study concluded that vegetation accounted for about 10 per cent of total VOC emissions, but these went on to form more than half (52 per cent) of the ozone detected in the city.
The study did not call for the immediate removal of poplar and willow trees, but suggested that the proportion of other species planted in the city should be gradually increased.
Researchers calculated that replacing 10 per cent of the high-emissions trees with other species would cut isoprene emissions by at least 29 per cent.
“In addition to urban greening in existing cities, planners of large-scale urban developments … may wish to consider urban tree selection in terms of VOC emissions to alleviate future air-quality management challenges,” Yuan wrote.
They added that it was still important to continue efforts to reduce harmful emissions from human activities, especially during summer.
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The city’s poplars and willows have been identified as the source of an unexpected spike in ozone levels during the heat of the summer


The city’s poplars and willows have been identified as the source of an unexpected spike in ozone levels during the heat of the summer.


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Chinese scientists discover that trees are producing ozone smog in Beijing

.css-1c6uqr6{color:inherit;font-weight:inherit;font-size:inherit;font-family:inherit;line-height:inherit;overflow-wrap:break-word;}The city’s poplars and willows have been identified as the source of an unexpected spike in ozone levels during the heat of the summer

The city’s poplars and willows have been identified as the source of an unexpected spike in ozone levels during the heat of the summer