When it’s hot, plants become a surprisingly large source of air pollution

Planting trees is often touted as a strategy to make cities greener, cleaner and healthier. But during heat waves, city trees actually boost air pollution levels. When temperatures rise, as much as 60 percent of ground-level ozone is created with the help of chemicals emitted by urban shrubbery, researchers report May 17 in Environmental Science & Technology.

While the findings seem counterintuitive, “everything has multiple effects,” says Robert Young, an urban planning expert at the University of Texas at Austin, who was not involved with the study. The results, he cautions, do not mean that programs focused on planting trees in cities should stop. Instead, more stringent measures are needed to control other sources of air pollution, such as vehicle emissions.

Benefits of city trees include helping reduce stormwater runoff, providing cooling shade and converting carbon dioxide to oxygen. But research has also shown that trees and other shrubs release chemicals that can interact with their surrounding environment, producing polluted air. One, isoprene, can react with human-made compounds, such as nitrogen oxides, to form ground-level ozone, a colorless gas that can be hazardous to human health. Monoterpenes and sesquiterpenes also react with nitrogen oxides, and when they do, lots of tiny particles, similar to soot, build up in the air. In cities, cars and trucks are major sources of these oxides.

In the new study, Galina Churkina of Humboldt University of Berlin and colleagues compared simulations of chemical concentrations emitted from plants in the Berlin-Brandenburg metropolitan area. The researchers focused on two summers: 2006, when there was a heat wave, and 2014, when temperatures were more typical.

At normal daily maximum summer temperatures, roughly 25° Celsius on average, plants’ chemical emissions contributed to about 6 to 20 percent of ozone formation in the simulations. At peak temperatures during the heat wave, when temperatures soared to over 30°C, plant emissions spiked, boosting their share of ozone formation to up to 60 percent. Churkina says she and colleagues were not surprised to see the seemingly contrary relationship between plants and pollution. “Its magnitude was, however, quite amazing,” she says.

The results, she notes, suggest that campaigns to add trees to urban spaces can’t be done in isolation. Adding trees will improve quality of life only if such campaigns are combined with the radical reduction of pollution from motorized vehicles and the increased use of clean energy sources, she says.