Trees & Air Quality

How trees impact air quality on the individualized level

By the Urban Trees Initiative

Research Goals

The USC Urban Tree Initiative (USC Trees) is committed to enhancing Los Angeles’ urban forest and providing much-needed shade to marginalized communities. As we consider where to plant trees, we also want to understand which types of trees are better suited to create bluer skies and cleaner air.  

What We Asked:

  1. How does air quality vary at the neighborhood level and over time?
  2. How can you measure a tree's impact on air quality?
  3. Does air quality vary in areas with trees vs. without trees?
  4. What do we know about trees and air quality in Boyle Heights?
  5. What do we know about trees and air quality in  University Park?
  6. What do we know about trees and air quality in City Terrace and East LA?

How We Did It

USC Trees designed a series of sensor packages that recorded levels of air pollutants within and around individual trees canopies to study the relative impacts of tree coverage on air quality.

USC Trees deployed 6 of these sensors in Boyle Heights, University Park and City Terrace in May and June, 2022. The sensors were placed at the sites for approximately 48 hours and the sensors tested for three pollutants known to be bad for human health: particulate matter (PM), nitrogen dioxide (NO2) and carbon monoxide (CO). Particulate matter includes soot, acids, metals, dirt, dust and organic chemicals that are small enough to be inhaled. NO2 and CO are gasses that are released into the air from gasoline-powered cars, trucks and buses. To examine Question 2, we built a new style of sensor that takes in air from a tree canopy and from adjacent air outside the canopy, constantly comparing air quality between these two intakes.

What We Discovered 

1. How does air quality vary at the neighborhood level and over time?

Location of Murchison Elementary School indicated by the green pushpin to the north of the Boyle Heights study site.

To understand pollution on a specific, local level, we first sought to establish baseline levels. 

From September 2021 through the present, we have been recording air quality measurments using a sensor at Murchison Elementary School, located north of Boyle Heights. As the graph below shows, air quality can change by season, month or even time of day. The sharp spike in PM levels at the end of the upper chart represents the city-wide increase in PM that occurred on the evening of July 4th due to emmisions from fireworks.

PM2.5 and CO readings collected at Murchison Elementary School from September 2021 to July 2022.

Zoom in closely and you can see PM readings exceeded 20 ug/m3 during certain periods, especially from November 2021 to January 2022. According to various health studies, exposures higher than 20 ug/m3 for a duration of 24 hours or longer pose a health risk for people with respiratory ailments. 

Areas of the graph that appear above the blue horizontal line indicate PM2.5 levels above 20 ug/m3, and therefore pose a threat to human health.

Baseline measurements tell us how local air quality varies and we will use this information when investigating tree species and their environmental impact. 

2. How can you measure a tree's impact on air quality?

Little data is available about how individual species of trees can alter Los Angeles’ air quality. To get the answers we needed, USC specialists designed innovative sensors. Each device is connected to two tubes, one positioned inside the tree canopy and another just outside of the tree canopy. Over about 24 hours, sensors alternated collecting air from one of the tubes every 15 minutes. These measurements can help determine if or how air quality varies when air is in contact with a tree. 

Left image of diagram of the sensor package with two tubes that alternate collecting air from within a tree canopy and outside a tree canopy. Right image of a sensor on USC campus.

3. Does air quality vary in areas with trees vs. without trees?

Yes, but the answer is nuanced. Early findings suggest that not all trees improve air quality. In fact, it seems that some have no impact while others even contribute to pollution. The scorecard below lists tree species used in the study and their effectiveness in removing PM, NO2 and CO from the air. For example, the deodar cedar we studied was a strong emitter of particulate matter, while the live oak removed CO. 

It’s too early to draw firm conclusions from initial measurements, but it is clear that impactful tree plantings need to take into account how to place the appropriate tree in the appropriate place at the appropriate time. 

By developing this scorecard, Los Angeles planning officials can help create healthier communities by planting trees that effectively remove pollutants. 

Scorecard identifying the impacts of tree species on polluntants.

Interpreting the Scorecard

All trees tested were on USC’s University Park Campus and were assigned a rating as follows:

  • "o": Neutral. Air quality inside and outside tree canopy was relatively the same. 
  • "+": Positive. The tree canopy showed lower levels of air pollution compared to nearby uncovered spaces. 
  • "++": A high degree of confidence that the tree plays a role in removing air pollutants from its immediate environment. 
  • "-": Negative. The air within the tree canopy had higher levels of particulate matter, NO2 or CO than surrounding areas. 
  • "- -" A high degree of confidence that the tree produces pollutants compared to surrounding areas. 
  • “±”: Inconclusive. One tree appeared to both emit and remove particulate matter during different parts of the day. 

4. What do we know about trees and air quality in Boyle Heights?

Preliminary research suggests that areas near freeways tend to be more of a CO hotspot than residential areas with trees. For instance, sensors E (blue) and F (pink) were near major freeways and had the highest levels of CO. Interestingly, sensor E, the had lower levels of particulate matter — similar to levels recorded by sensor H (yellow), which was not close to a freeway and was placed close to dense tree cover. 

5. What do we know about trees and air quality in University Park?

Five sensors were placed on the north and west side of USC’s University Park Campus, and results suggest that trees can make small improvements in air quality. For example, sensors F and H (Purple), which were placed on streets that had less than two trees per 100 feet, had the highest particulate matter values. Meanwhile, sensor A (blue), which was placed in an area with more than three trees per 100 feet, provided the lowest particulate matter measurements. 

6. What do we know about trees and air quality in City Terrace/East LA?

Six sensors were placed in City Terrace/East L.A. near the 10 freeway and industrial areas. Sensors C and F, indicated by the purple dots, were set up along City Terrace Drive right next to the freeway and had the highest CO measurements.

 The sensor locations are indicated by the red and purple dots. 

Sensor F was placed in a Chinese elm, a tree that removes pollutants from its environment, yet its CO values were quite high. Sensors A, D and H were just one block farther from the freeway than sensors C and F but did not have high CO levels. However, sensor H had high particulate matter readings. 

A detailed look at the data values for CO, PM 2.5 , PM 10 for all three study sites. Click the icon in the upper right corner to download a data table.

What We Still Do Not Know 

The data offers only a snapshot of how individual trees affect air quality. Some trees behave differently depending on the condition of the tree, or its ‘health’, as well as other environmental factors. To learn more, we need to deploy more sensors for longer periods and in areas with varying densities of tree placement and tree species. 

Cleaner Air for the Future

This pilot project illustrates that variability in air quality is common and that teasing apart the reasons will be a challenging task. The Urban Trees Initiative is proud to have built a sensor package that advances our study of air quality at this specific, local level. We aim to guide tree planting decisions based on rigorous science and hope to improve air quality for all Los Angeles residents. 

Parties involved with the USC Urban Trees Initiative

  StoryMap Author: Jillian Gorman   

 Contributors: Dulce Acosta, Marianna Babboni, Will Berelson, Naman Casas, Eileen Chen, Tarana Chordia, Charlie Curtain, Monica Dean, Jackson FitzGerald, David Galaviz, Emma Johnson, Runzi Kang, Jinsol Kim, Carolyn Koh, Joan Lee, Yixuan Li, Beau MacDonald, Esther Margulies, Alyssa Ng, Eliana Ramirez, Melinda Ramos-Alatorre, Coleman Reardon, Nick Rollins, Katie Vega, Steve Wesson, John Wilson, Aviva Wolf-Jacobs  

Areas of the graph that appear above the blue horizontal line indicate PM2.5 levels above 20 ug/m3, and therefore pose a threat to human health.

Scorecard identifying the impacts of tree species on polluntants.

PM2.5 and CO readings collected at Murchison Elementary School from September 2021 to July 2022.

Parties involved with the USC Urban Trees Initiative