According to new research published in Environmental Science & Technology on May 17th, plants can actually contribute to air pollution in cities, during heat waves. In higher temperatures, up to 60 percent of ground-level ozone is created partially as a result of chemicals emitted by plant life in cities.

University of Texas at Austin urban planning expert Robert Young explains “everything has multiple effects,” and that the research does not mean cities should plant fewer trees. Instead, he emphasized the need for tighter controls on other contributors to air pollution in cities.

Plant life like trees still offers other benefits to urban areas. They can reduce stormwater runoff, provide shade, and convert carbon dioxide to oxygen. But the new research shows that they also release chemicals that react with their environment to contribute to air pollution. For example, isoprene reacts with compounds generated from human activity, such as nitrogen oxides, to create ground-level ozone, which can pose problems for human health. Montoterpenes and sequiterpenes react with the nitrogen oxides as well, creating tiny particles in the air, similar to soot. In urban environments, most of these nitrogen oxides come from automobiles.

The researchers, including Galina Churkina of Berlin’s Humboldt University, compared chemical concentrations emitted from plants in the Berlin area during two summers, 2006 and 2014. In 2006, there was a summer heat wave, while the summer of 2014 saw more standard temperatures.

At the normal summer temperatures, 25 degrees Celsius on average, chemicals emitted from plants accounted for between 6 and 20 percent of ozone formation in simulations. During heat wave temperatures, which averaged near 30 degrees Celsius, plant emissions represented up to 60 percent of the ozone formation. According to Churkina, the researchers were surprised by the counterintuitive results, but said “Its magnitude was, however, quite amazing.”

She said the results suggests to city planners that the addition of trees to urban spaces will improve quality of life only if combined with other measures, such as reducing pollution from automobiles, and increasing the use of clean energy generation.

Many other studies have discussed the positive effects plant life can have on air pollution in cities, clearing pollutants such as nitrogen dioxide and microscopic particulate matter from the air, in normal weather. In many cities, levels of these pollutants exceed the levels considered safe for humans. One study showed that foliage can reduce nitrogen dioxide by up to 40 percent, and particulate matter by up to 60 percent.

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