PORTLAND, OREGON—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.