Trees have a big effect on urban air quality, reducing pollution in several ways:
- Trees shade buildings, reduce urban heat islands, and cool the surrounding air through evapotranspiration. These cooling effects reduce the need for air conditioning, which is usually generated by fossil fuels. Therefore trees help reduce fossil fuel emissions by reducing the demand for power. The trees in the City of Boulder prevent about 43,000 tons of carbon from entering the atmosphere each year, just by shading buildings and reducing the need for air conditioning.
- Trees act as “carbon sinks,” absorbing carbon dioxide and releasing oxygen back into the air. An average-sized tree can absorb 141 pounds of carbon dioxide per year.
- Trees absorb other pollutants such as ozone, carbon monoxide, sulfur dioxide, nitrogen oxides, and particulates. These chemicals, especially ozone, are ingredients of smog.
Overall, trees reduce air pollution. But some trees have a greater net effect than others. Volatile organic compounds (VOC) from some species of trees can interact with sunlight and nitrogen oxides from fossil fuel emissions to produce ozone and other chemicals in the air. A few species produce almost as much ozone as they absorb.
Our team is trying to find out more about VOC emissions, particularly sesquiterpenes, from particular species of trees under various conditions. Encouraging citizens to plant trees with low sesquiterpene emissions may contribute to improved air quality.
Two reports from the City of Boulder go into detail about the costs and benefits of its urban forest:
- City of Boulder, Colorado: Municipal Tree Resource Analysis.
- Calculating the Value of Boulder's Urban Forest
Researchers on the Trees and VOCs project have published a paper, "Biogenic volatile organic compound emissions from nine tree species used in an urban tree-planting program," that looks at emissions from specific trees in a Front Range urban environment. It provides good guidance for people in the area wishing to plant trees with lower emissions of ozone precursors. Others may wish to use the i-Tree tool from the Forest Service to investigate different tree species.
For more information
Want to know more about the impact of urban trees on air, water, energy conservation, and fire prevention? The Center for Urban Forest Research, at the Pacific Southwest Research Station of the U.S. Forest Service, analyzes and quantifies the value of urban trees.
Their report Northern Mountain and Prairie Community Tree Guide: Benefits, Costs and Strategic Planting (March 2003) includes information specific to the Colorado Front Range and factors in the production of VOCs to its cost/benefit analysis.
This research is supported by the National Science Foundation grant no. ATM 0608582.