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Vegetation & Air Quality
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- Vegetation & Air Quality
Planting urban trees and vegetation improves air quality in several ways. Trees and vegetation absorb pollutants from the air through leaf uptake and contact removal. By shading buildings, they lower demand for air conditioning energy thereby decreasing emissions from power plants. If widely planted throughout a community, trees can cool the air and slow the temperature-dependent reaction that forms ground-level ozone pollution.
Improving Air Quality Through Direct Removal
Trees and vegetation remove pollutants from the air by absorbing gases or attaching particles to their leaves. Trees absorb gaseous pollutants by uptake through leaf stomata, which are tiny pores in the leaf surface. Inside the leaf itself, gaseous pollutants diffuse and may react with inner-leaf surfaces.
In contrast, particulate pollutants are removed from the air by contact and retention. Because particles can be washed away by rain or dislodged by leaf motion, vegetation serves only as a temporary holding place for airborne particulate matter.
Common pollutants that trees and vegetation can remove
include nitrogen oxides, sulfur oxides, particulate
matter, and ground-level ozone.
Research shows that large trees remove
considerably more pollution than smaller ones: a healthy
tree with a trunk-diameter of 30 inches removes about 70
times more pollution than a tree with a three-inch trunk
Improving Air Quality Through Shade and Reduced Temperatures
Trees and vegetation reduce air conditioning demand by shading homes and buildings – which reduces heat transfer to the indoors – and by cooling the air through evapotranspiration (the evaporation of water from leaves). Using less energy to cool homes and buildings decreases air pollutant emissions from power plants.
In addition, by reducing ambient air temperatures, trees and vegetation can slow the formation of temperature-dependent pollutants, such as ground-level ozone.
Similarly, shade trees planted in parking lots reduce
evaporative
emissions
of
volatile
organic compounds (VOCs)
– precursors to ground-level ozone – from
parked cars. Scientists predict that increasing tree
cover from 8% to 50% in Sacramento parking lots may
reduce evaporative emissions by 2%.
Vegetation and VOCs
Some species of trees and vegetation emit VOCs. The formation of these "biogenic" emissions varies greatly and is affected by sunlight, temperature, and humidity. Even related species of trees may show wide variation in the amount of VOCs they emit.
Biogenic emissions may represent a sizable portion of
the total VOC emissions in an area. As a result, areas in
nonattainment
for ground-level ozone may want to consider low VOC
emitting species along with factors such as hardiness,
disease and resistance, and irrigation needs, when
investing in large-scale tree planting initiatives.
Low VOC emitting species include certain types of pine
and maple trees. High VOC-emitting trees include
eucalyptus, sycamore, willow, and certain oak varieties.
For more information on biogenic emissions, visit the
tree species
database
maintained by the Urban Forest Ecosystems Institute at
Cal Poly State University. The database has an emphasis
on species found in California.
While it is important to be aware of VOC contributions
from trees and vegetation, the air quality improvements
gained from direct pollutant removal, reduced energy use
and power plant emissions, slower rates of ground-level
ozone formation from lower air temperatures, and other
benefits generally outweigh the negative impact of
biogenic emissions.