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

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

Trees and shrubs emit more chemicals during heat waves that can react to form ozone

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5:33pm, May 17, 2017
Trees in Berlin

FEELING THE HEAT  Tree-lined streets in Berlin, shown, offer a lot of benefits for city residents. But during heat waves, the trees, along with other plants, can also contribute to poor air quality, a new study suggests.

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

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