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Trees have become a hidden source of air pollution in Los Angeles: ScienceAlert

Vehicle emissions in California have steadily declined over the years as environmental measures and advanced technologies help eliminate traffic emissions.

But since 2010, microscopic particles in the air and ground-level ozone have stubbornly refused to go down. The reason for this is the increase in “secondary sources” – which often include the trees and shrubs that green our streets.

To map emissions, a team of U.S. researchers took to the skies over Los Angeles nine times in June 2021 to directly measure fluctuating concentrations of volatile organic compounds (VOCs), precursors to particulate matter and ozone pollution that can come from plants.

Unlike previous maps that either estimated emissions based on known sources or modelled the movement of emissions, this new aerial approach was able to directly measure pollutants in the air multiple times per second. This was achieved using an onboard mass spectrometer that describes the dispersion of more than 400 emission types in unprecedented detail.

By combining the results with temperature patterns at resolutions down to four square kilometers (about 2.5 square miles), the team found that botanical sources of volatile organic compounds (VOCs), which include compounds such as isoprene, monoterpenes and sesquiterpenes, contributed about 60 percent of the potential formation of secondary organic aerosols at the beginning of summer in Los Angeles.

As these botanical emissions increase during hot weather and drought, The problem could get worse over the course of the summer. Researchers believe that we need to keep this problem under control in light of global warming.

Despite efforts to reduce emissions from transport and industry, air pollution remains a significant health problem worldwide. Fine solid particles measuring just a few micrometers increase the risk of heart disease and low birth weight, while ozone in the air we breathe is linked to respiratory disease and increased mortality.

The key to the creation of these two potentially toxic materials are VOCs – a variety of chemicals that directly affect our health and react in sunlight and the atmosphere to form particles and gases such as ozone.

An estimated 4.2 million premature deaths each year are attributable to air pollution, mostly in urban areas, so health authorities are striving to find better ways to identify and control the sources of volatile organic compounds (VOCs) in our largest cities.

There are countless potential producers of these common compounds, because everything from pesticides to hair care products and car upholstery to cleaning products sweats out some compound that, in tiny amounts, can produce something unpleasant. So it’s no surprise that volatile chemical products now account for up to half of fossil fuel VOC emissions in industrial cities.

Perhaps surprisingly, the green spaces that represent clean living produce their own compounds in the form of terpenoids. The analysis showed that these contributed around 16 percent to the measured mass flow of volatile organic compounds.

The importance of biogenic versus industrial energy sources is hotly debated, especially when higher temperatures are taken into account.

“Emissions of monoterpenes and sesquiterpenes typically increase exponentially with temperature, whereas isoprene emissions are known to increase with temperature and light and eventually decrease above a certain temperature threshold,” the researchers note in their study.

Knowing that urban gardens can contribute to pollution is no reason to reduce green spaces, which in turn keep temperatures lower and improve our health in other ways. Some green spaces can even remove certain volatile organic compounds (VOCs) from the air.

But to achieve the greatest benefit, it would be useful to better understand how factors like drought can increase large-scale biogenic VOC emissions, and how the shed flowers of plants like jacarandas—which, while non-native, are among the most common species in Los Angeles—they themselves contribute organic precursors. Or even figure out which plant species might produce fewer emissions as global temperatures inevitably continue to rise.

This research was published in Science.

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