A new study finds that trees and plants in Los Angeles are contributing to the city’s air pollution problem. Researchers conducted aerial surveys over the city in June 2021. They found that plants emit volatile organic compounds (VOCs) that react with other pollutants to form harmful ozone and delicate particulate matter. The study found that these biogenic sources of VOCs, such as isoprene and monoterpenes, contributed to around 60 percent of the potential formation of secondary organic aerosols at the start of the summer.
The emissions increase with hot weather and drought, which could worsen the problem as global temperatures rise. Since it’s hard to control the plant emissions, maintaining the human-caused part’s even more critical,” says a researcher at Forschungszentrum Jülich, a research institute in Germany. The researchers used an on-board mass spectrometer to measure concentrations of more than 400 types of VOCs in the air.
They also measured wind speed to isolate the molecules originating from the city and those from outside. The study found that terpenoids dominated VOC emissions in many parts of the city, particularly in areas with abundant vegetation and on the hottest days.
Trees’ impact on Los Angeles air
Even in downtown areas with fewer plants, terpenoids dominated emissions when temperatures exceeded 30°C (86°F). Hotter temperatures also elevated VOC emissions linked to human sources, like gasoline, paint, and scented personal care products such as deodorants and hair sprays. The contribution of these products increased with population density, indicating a direct link between the city’s smog and its residents’ beauty routines.
The researchers project that in Los Angeles, the effect of VOCs on ozone formation could double with the 3°C of warming expected by the middle of the century, and PM2.5 pollution could increase by 40%. The rising emissions from plants highlight the importance of reducing human sources of air pollution that react with the VOCs, such as nitrogen oxides. The species of plants chosen for urban settings also matters, emphasizes a researcher at Columbia University in New York, as more cities pursue greener environments.
Understanding the impact of vegetation on urban air quality is vital as we move towards urban planning that balances greenery with human health. The findings suggest that future air pollution regulations must consider that only 40% of urban VOC emissions, those not tied to biogenic sources, can be mitigated through regulations.
