Trees are causing air pollution in Los Angeles, finds study
A recent study has challenged traditional beliefs about the role of trees in urban ecosystems, suggesting that they may be contributing to Los Angeles's consistent air pollution levels. Despite efforts to reduce traffic exhaust and enhance environmental protections, the city's smog levels have remained steady. This led scientists from Colorado and South Korea to propose in a 2022 study that "secondary" sources of pollution could be responsible for these persistent rates.
Trees and shrubs: Unexpected contributors to pollution
The research, published in the journal Science, identifies trees and shrubs as potential contributors to air pollution. The study focuses on terpenoids, organic compounds found in plants that can exacerbate pollutants' harmful effects when released into the atmosphere. These terpenoids transform into volatile organic compounds (VOCs), which react with air pollution to create ozone and fine particulate pollutants. The study also discovered that plants emit more VOCs due to rising temperatures and drought conditions, both common in Los Angeles.
Terpenoids: The largest source of VOCs
A multi-institutional team of researchers used a mass spectrometer aboard a plane flown over Los Angeles to measure VOC concentrations. They utilized 3D measurements of wind speeds to identify pollutant sources and found that terpenoids were the largest source of VOCs. This effect was most noticeable in vegetation-rich areas and on the hottest days. When temperatures exceeded 86 degrees Fahrenheit, terpenoids caused the worst emissions, even in densely populated areas with fewer plants like downtown LA.
Human-Related VOCs also increase with higher temperatures
While it remains unclear which plants contribute most to emissions, the researchers noted that pollution from human-related VOCs also increased with higher temperatures. These VOCs originated from various sources, including gasoline and personal hygiene products like deodorants. In densely populated areas, beauty products had a small but "measurable" impact on smog, according to Eva Pfannerstill, the first paper author and an atmospheric chemist at Germany's Forschungszentrum Julich research institution.
Importance of controlling human-caused emissions
The researchers emphasized the need for correct interpretation of their findings. Pfannerstill stated that controlling the human-caused emissions is crucial, especially given the difficulty in controlling plant emissions. Science editor Jesse Smith added that successful mitigation of urban air pollution needs to consider how climate warming will significantly alter emission amounts and composition. The study does not suggest that trees are harmful but serves as a stark reminder of the severe impact humans have had on their environment.
Another study supports how certain trees affect the air quality
A 2023 study reveals that rising temperatures cause certain trees to emit a chemical that worsens air quality. During hot weather, trees like oaks and poplars produce isoprene, a compound that assists plants in coping with heat stress. In urban environments, isoprene interacts with vehicle emissions to form ozone, a harmful pollutant. Scientists at Michigan State University exposed young poplar trees to high heat and increased CO2 levels to study climate change's potential impact on tree isoprene production.
Climate change amplifies tree isoprene emissions, study finds
Researchers found that at 95 degrees Fahrenheit, high temperatures greatly increase isoprene production in trees, overriding the dampening effect of elevated CO2 levels. Lead author Abira Sahux described the situation as a significant increase in isoprene emissions. Published in the Proceedings of the National Academy of Sciences, these findings will help predict future tree isoprene emissions and their impact on air quality.
