Trees in Los Angeles are Now an Unexpected Air Pollution Source

While trees and green spaces are often celebrated for their contributions to cleaner air and a healthier environment, recent research has highlighted a surprising drawback: they can also be significant sources of air pollution. A study mapping volatile organic compounds (VOCs) over Los Angeles revealed that botanical emissions, particularly from trees, are responsible for around 60% of the potential formation of secondary organic aerosols (SOAs) at the beginning of summer .

To map emissions, a team of U.S. researchers conducted nine airborne surveys over Los Angeles in June 2021, directly measuring varying concentrations of volatile organic compounds (VOCs), which can originate from plants and act as precursors to particulate and ozone pollution.

Unlike previous methods that either estimated emissions from known sources or modeled their movement, this recent airborne approach allowed for direct measurement of airborne pollutants multiple times per second using an on-board mass spectrometer. This method provided unprecedented detail on over 400 types of emissions.

Botanical VOC Contributions to Secondary Organic Aerosols in LA Summer

By combining these results with temperature data at a resolution of 4 square kilometers (about 2.5 square miles), the team found that botanical sources of VOCs, such as isoprene, monoterpenes, and sesquiterpenes, contributed to around 60% of the potential formation of secondary organic aerosols at the beginning of the LA summer.

These botanical emissions tend to increase with hot weather and drought, suggesting the problem could worsen as the summer progresses. The researchers warn that this is an issue to monitor as the global climate warms.

Despite efforts to reduce toxic emissions from transportation and industry, ambient air pollution remains a significant health problem worldwide. Microscopic solid particles increase the risk of heart disease and low birth weights, while ozone exposure is linked to respiratory illnesses and increased mortality.

Crucial Agents in Toxic Material Formation and Atmospheric Reactions

VOCs are key to the formation of these toxic materials. They are a diverse group of chemicals that not only affect health directly but also react in sunlight and the atmosphere to form particulates and gases like ozone.

With an estimated 4.2 million premature deaths annually attributed to ambient air pollution, particularly in urban areas, health authorities are eager to identify and mitigate VOC sources in major cities.

There is an endless array of sources for these ubiquitous compounds, with items such as pesticides, hair products, car upholstery, and cleaning agents all emitting substances that can generate harmful compounds in minute amounts. Consequently, it’s unsurprising that volatile chemical products now account for up to half of the fossil-fuel VOC emissions in industrialized cities.

Green Spaces Contribute 16% to VOC Mass Flux through Terpenoids

It might be surprising, but green spaces, which symbolize clean living, actively produce compounds like terpenoids. The analysis revealed that these contribute about 16% to the measured VOC mass flux.

People have significantly debated the importance of biogenic versus industrial sources, especially when considering higher temperatures

“Monoterpene and sesquiterpene emissions typically increase exponentially with temperature, whereas isoprene emissions increase with temperature and light but eventually decline beyond a certain temperature threshold,” the researchers explain in their study.

Recognizing that urban gardens can contribute to pollution should not lead to a reduction in green spaces, as they also provide benefits like cooling the air and improving health in other ways. Some plants even remove certain VOCs from the air.

To fully harness the advantages of green spaces, it is important to understand how factors like drought may escalate large-scale biogenic VOC emissions, how the fallen blooms of plants such as the widely abundant but non-native jacarandas in Los Angeles contribute organic precursors, and which plant species might be lower emitters as global temperatures rise.

Read the original article on: Science Alert

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