Urban air pollution caused by overheating trees

In Los Angeles, air pollution from cars and human activity is exacerbated by the city's factories emitting chemicals in response to rising temperatures and drought.

“It's difficult to control emissions from factories, [human-caused] “Some.” Eva Fannerstill At the Forschungszentrum Jülich, a German research institute.

From blooming jacaranda trees to fragrant eucalyptus, many plants emit compounds called terpenoids. To plants, these act as chemical signals and function as antioxidants. But in the atmosphere, these volatile organic compounds (VOCs) react with other pollutants to create harmful ozone and particulate pollution called PM2.5.

In areas where cleaner vehicles and electricity generation have reduced man-made sources of air pollution, industrial emissions have begun to play a larger role, but by how much is unclear.

Vannerstill and her colleagues flew planes over Los Angeles over several days in June 2021 to measure the city's VOC emissions. Using an on-board mass spectrometer, the researchers identified concentrations of more than 400 VOCs in the air. Simultaneous 3D measurements of wind speed allowed them to separate molecules rising from the city from those blowing in from elsewhere.

The researchers found that terpenoids made up the majority of VOC emissions in many parts of the city, especially in areas with the most vegetation and on the hottest days. When temperatures exceeded 30°C (86°F), terpenoids made up the majority of emissions, even in the downtown area, where vegetation was sparse and people were more likely to be present.

It's unclear exactly why plants release more terpenoids when temperatures rise, but it could be a response to heat or water stress, Pfannerstill said. Higher temperatures also increase VOC emissions associated with anthropogenic sources, such as gasoline, paint, and even scented personal care products like deodorant and hairspray, possibly due to increased evaporation rates.

The contribution of these personal-care products in particular spikes with population density, suggesting a small but direct link between urban air pollution and citizens' beauty habits. “It's measurable,” Vannerstill says.

The observed relationship between heat and emissions also suggests a pathway through which climate change could exacerbate air pollution: in Los Angeles, the researchers found that a 3°C warming predicted for the city by mid-century could double the impact of VOCs on ozone formation; and its impact on PM2.5 pollution could increase by 40%.

“This direct observation is crucial for building good models to predict what air pollution will be like tomorrow or a few years from now.” Matthew Cogon Jonathan Myers, environmental secretary at the National Oceanic and Atmospheric Administration, said the increase in emissions from factories underscores the importance of further reducing man-made sources of air pollution that react with VOCs, such as nitrous oxide, which comes from burning fossil fuels.

The types of plants planted in cities also have an impact. Roissyne Comanet It's an important consideration as more cities pursue urban greening initiatives. “Vegetation is important,” she says at Columbia University in New York.