Trees May Become the Biggest Air Pollution Contributors in LA

A new study can help the city plant better trees.
Golden sunlight bathes Los Angeles street, with cars, pedestrians, marquees, and some trees.
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Krystal Vasquez, Contributor

(Inside Science) -- Cars have been the main source of air pollution in Los Angeles for many decades, but according to a study published this spring in the journal Environmental Science & Technology, this pollution source might soon be replaced by trees.

In fact, atmospheric chemists Ron Cohen and Clara Nussbaumer from the University of California, Berkeley estimated that trees might be contributing to up to one-quarter of the city's fine particulate matter. Inhaling these particles is associated with a number of health problems, such as asthma attacks and respiratory disease.  

Even though trees have long been touted for their ability to clean the air, they can also contribute to the formation of particles that are 2.5 micrometers in size or smaller, referred to as PM 2.5. During metabolic processes like photosynthesis, plants release chemicals from their leaves, called volatile organic compounds (VOCs), which react in the atmosphere to form new sets of molecules that can stick together and form semisolid clumps.

VOCs can also come from a variety of other sources, including motor vehicles and personal care products. Historically, most of the VOCs in Los Angeles came from cars. That was until California's air quality regulations successfully reduced the amount of this pollution present across the state.

Yet, despite recent reductions to the pollution emitted by cars, concentrations of PM 2.5 in the air have stayed roughly constant since 2012, according to Cohen and Nussbaumer.

"PM 2.5 in California is still very high. It's pushing the federal standards and has real health effects for people," said Joost de Gouw, an atmospheric chemist at the University of Colorado Boulder who was not involved with this study.

The researchers also noticed that an increasing number of high pollution episodes were occurring during hotter weather. This trend poses especially bad news for an area that expects to have more frequent heat waves due to climate change.

However, this odd relationship with temperature provided a clue that allowed Cohen and Nussbaumer to narrow down which VOC sources Californians need to worry about next.

"[Tree emissions] are one of the things we know increase exponentially with temperature," said Cohen. The fact that more PM 2.5 was observed on hotter days hints that trees are a likely culprit, he explained.

But only trees that emit large amounts of certain types of VOCs, known as isoprene and monoterpenes, can produce enough particles to influence our air quality. This includes oak trees and Mexican fan palms, which happen to be some of the most abundant trees in Southern California.

However, Brian McDonald, a research scientist at the National Oceanic and Atmospheric Administration Chemical Sciences Laboratory, noted that other sources might also be contributing to this problem. Cleaning supplies, personal care products, and other household chemicals can all emit large amounts of particle-producing VOCs.

"There's a lot of sources of VOCs in the urban environment that we didn't pay as much attention to before," said McDonald, who was not involved in the study.

In other words, while trees might be responsible for some of LA's PM 2.5 pollution, more work needs to be done before scientists can conclusively say that they've become the city's dominant source.

But even if trees aren't the current leading source of PM 2.5, they could still become important in the future as temperatures rise or as these other VOC sources become better regulated. But what will this mean for LA, a city that has set the ambitious goal of planting 90,000 trees by the end of 2021?

Well, cutting down all these new trees is probably not the answer. While trees might have once been planted haphazardly for aesthetic purposes, "there has been a shift ... to move them from being ornamental to a vital piece of our infrastructure that provides benefits needed by residents of cities," says Rachel Malarich, the City Forest Officer of Los Angeles. For example, trees can be used to combat some of the effects of climate change by sucking up carbon dioxide or providing a cooling effect in sweltering neighborhoods. Many also do a much better job at filtering out pollutants (as opposed to creating them) and can double as homes for local species of birds and other urban animals.

Still, Malarich doesn't plan to ignore the fact that some trees can cause pollution and hopes to actively incorporate research like Cohen's and Nussbaumer's when deciding which trees to plant next.

"I want [air quality researchers] to help me understand what we can be doing better," Malarich said. "Because we are investing public funds in tree planting and tree maintenance, we want it to be providing additional support for healthy communities."


Editor's Note: This story was produced in collaboration with the NPR Scicommers program.

Author Bio & Story Archive

Krystal Vasquez (@caffeinatedkrys) is a freelance science writer in Los Angeles, California who enjoys writing about earth and environmental science. She is also currently finishing up her Ph.D. in atmospheric chemistry at the California Institute of Technology.