More trees mean cleaner air, right? Not necessarily, suggests a new study looking at the wooded areas next to roadways. Instead, lines of trees known as “greenbelts” might actually trap a common pollutant from vehicle exhaust—nitrogen dioxide (NO2)—boosting on-the-ground levels of the gas up to 21%. That could make breathing hard for bikers and pedestrians with asthma or other respiratory diseases.
For decades, cities from Shanghai, China, to London have lined their avenues with trees to beautify downtown areas—and, planners hoped, cut air pollution. To test that idea, urban ecologist Heikki Setälä from the University of Helsinki and his team measured air quality in and around 10 greenbelts in the Helsinki metropolitan area in the summer and winter of 2016, focusing on NO2. The greenbelts were all downwind from heavily trafficked roadways, including some major freeways. As a control, the team recorded levels of NO2 in adjacent open fields at the same distance from the road that were also downwind.
After taking 40-plus days of measurements, the team found that along busy roads in front of greenbelts, summertime NO2 levels were on average 14% higher than in the fields. Inside the greenbelts they were about 12% higher. In the winter, the roadside concentration of NO2 was 21% higher and the interior levels were 10% higher, they reported here this month at the annual meeting of the Ecological Society of America.
In the study, which also appeared online 9 August in Urban Climate, Setälä suggests that lack of wind might be the culprit. The dense tree canopies likely cause it to stall out, making it more difficult for pollutants to disperse—a situation that gets worse in winter. That scenario makes sense, says physiological ecologist Tom Whitlow of Cornell University, who has conducted wind tunnel and field experiments on how particle pollutants interact with leaves and trees.
The effect resembles a smoker holding an umbrella that traps rising cigarette fumes. The higher levels alongside roadways are particularly worrisome, Setälä says, because sidewalks and trails are often built there. “Nobody has scientific evidence that this increased [long-term] concentration [of NO2] is really harmful,” he says, “but of course if they’re elevated, that’s not a good sign.” Laboratory studies on the short-term effects of much higher concentrations of the gas suggest it can harm the lungs of asthmatics and those with a chronic form of lung disease.
Sara Janhäll, an urban air researcher from the Swedish National Road and Transport Research Institute in Linköping, says the team did a good job of designing its study. However, she wishes they had quantified leafy surfaces within greenbelts to clarify how well they passively captured the gas. Measuring the conversion of NO2 into nitrogen monoxide, a sunlight-driven process, would have also indicated how much more of the gas could be present in shadier greenbelts near pollution sources.
When it comes to pollution, trees aren’t all bad. Another study led by Setälä’s team found that trees in 25 urban forests and parks in Baltimore, Maryland, reduced ground-level ozone by about 9% compared with adjacent open spaces, they report in an upcoming issue of Atmospheric Environment. However, when it came to NO2 concentrations, the wooded settings—which were far from busy roads—were no better than those in open spaces. Together, Janhäll says, the results serve as a reminder that—in the right place—plants really can improve air quality. “We can use vegetation much more than we do, but the design can be totally wrong, and then you won’t get an air pollution-reducing effect.”