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ANR Research Clarifies Role of Tree Species in Air Pollution

The Issue

Trees and shrubs help clean the air. They absorb carbon dioxide, release oxygen and provide surfaces for the deposition of airborne particles and unhealthful gases such as ozone. Also, water evaporating from tree leaves cools the air and shade from trees cuts energy consumption, reducing the need for air-polluting energy generation. However, there is another side to the story. Some trees and shrubs emit high rates of certain volatile organic compounds (VOC), which react with nitrogen oxides (NOx) and sunlight to form ozone, a ground-level pollutant. Other plants emit very little VOC.

What Has ANR Done?

A team of researchers representing UC Cooperative Extension, UCLA and UC Riverside measured isoprene emission rates for more than 60 California trees and shrubs. Although isoprene is not the only compound released by trees, it is often dominant and therefore was selected for the project. Of the species studied, the researchers found that the highest isoprene emitters were certain oak, poplar and eucalyptus species. Plants that emitted very low levels of isoprene included those in the rose family such as almonds and apricots, ash trees, cotton, lilac and euryops daisy.

Emissions from trees might be compared to vapors from gasoline spilled at the pump. For example, a medium-sized liquidambar tree on a warm summer day emits about one gram of isoprene per hour. For a very large planting of 10,000 trees, the isoprene emission would be about 10 kilograms per hour, an amount comparable to the evaporative emissions caused by spilling about 12 gallons of gasoline per hour.

The Payoff

UC's information aids air pollution control

The California Air Resources Board is using the UC study results to develop state implementation plans that strike the proper balance between NOx versus VOC reduction.

The research is also a valuable tool for landscape planners, who when selecting trees and shrubs consider a wide variety of characteristics such as size, shape, color, growth rate, etc. Now, the VOC emission rates also can be considered.


Supporting Unit: UCCE Kern County

John F. Karlik
UC Cooperative Extension
1031 S. Mt. Vernon Avenue
Bakersfield, CA 93307
(661) 868-6220 jfkarlik@ucdavis.edu