Overheated trees are contributing to urban air pollution

An aerial survey of Los Angeles reveals that high temperatures cause plants to emit more compounds that can contribute to harmful ozone and PM2.5 air pollution.

Air pollution in Los Angeles from cars and human activity is getting a boost from the city’s plants, some of which emit chemicals in response to rising temperatures and drought.

“Since it’s hard to control the plant emissions, it’s even more important to control the [human-caused] part,” says Eva Pfannerstill at Forschungszentrum Jülich, a research institute in Germany.

From jacaranda trees in bloom to fragrant eucalyptus, many plants emit a class of compounds called terpenoids. For the plants, these act as chemical signals and can serve as antioxidants. But in the atmosphere, such volatile organic compounds (VOCs) react with other pollutants to produce harmful ozone and particulate pollution called PM2.5.

In places where human sources of air pollution have declined thanks to cleaner vehicles and power generation, emissions from plants have come to play a larger role. But how large is unknown.

Pfannerstill and her colleagues measured VOC emissions in Los Angeles by flying a plane above the city over several days in June 2021. The researchers used an on-board mass spectrometer to identify concentrations of more than 400 types of VOCs in the air. Simultaneous 3D measurements of wind speed enabled them to isolate the molecules rising from the city from those blowing in from elsewhere.