Strategic placement of grass, ivy and other plants in cities can reduce the street level concentrations of NO2 and PM by 40 and 60 percent, respectively. There are multiple ways trees help to make urban air cleaner by filtering out pollutants:
Lowering temperatures reduces the movement of harmful ambient particles and prevents more pollutants from evaporating into the air. Trees create a great cooling effect by shading homes and streets, breaking up urban heat islands, and releasing water vapor into the air through their leaves. More tree crowns mean less dark surfaces like parking lots and paved streets being exposed to sunlight and emitting heat. Tree canopy cover in Los Angeles has decreased over the last 50 years, and a corresponding 6°F increase has been measured. Depending on the tree placement, trees can cool a city by up to 10°F, reducing the concentration of PM and other air pollutants with each degree.
Removal of pollutants
The first way trees remove air pollution is by particle interception: trapping pollution particles on their leaves and bark. Once the particle has been removed from the air, it is usually washed off the tree by rain or falls onto the ground with leaves and twigs. Studies have shown that in one urban park, tree cover removed 48 pounds of PM, 9 pounds of NO2, 6 pounds of SO2 and 100 pounds of carbon — daily. Silver birch trees in particular have been studied for their particle interception abilities: They have been found to reduce concentrations of PM by more than 50 percent.
A more complex way that trees filter the air is through gas uptake by leaf stomata. The stomata are tiny pores on tree leaves, and they absorb air to collect CO2 in order to perform photosynthesis. During that uptake of air, they also absorb gaseous pollutants in the air. Once inside the leaf, the gas diffuses throughout the leaf’s pores. It is then absorbed by films of water inside the leaf where it will either form acids or react with inner-leaf surfaces to become less toxic. It is estimated that one tree can absorb almost 10 pounds of polluted air through its leaf stomata every year.
Energy effects on buildings
Now that we understand trees’ chemical abilities, we can factor in their physical ones. Trees shade buildings in the summer and block winds in the winter, so it makes sense that they reduce building energy use for both heating and cooling purposes. Minimizing energy needs lowers the amount of fuel combustion necessary and therefore reduces the amount of pollution from power plants entering the air in the first place.
Once seen as an aesthetic window dressing, trees have never seemed as important in cities as they do now that we know their full potential. We can’t decide to stop breathing when we walk down a city street, but we can decide to plant and maintain healthy trees and hedges in cities to support the cause for greener cities. The air we breathe is a little bit cleaner thanks to each and every tree. Let’s keep it that way.
For more information on this topic, please visit the new Vibrant Cities Lab website.