Count on trees to improve air quality

Date published: 6/30/2006

E VERYONE KNOWS that trees clean the air. But do you know how they do it? Dave Nowak, a researcher with the USDA Forest Service in the Northeast, has studied extensively the effect of trees on air quality. Trees clean the air in four ways, and Dr. Nowak has organized these as an easy-to-remember acronym: T-R-E-E.

"T" is for temperature. Who hasn't sought the cool air under a shade tree or enjoyed a cool walk in the woods? Transpiration from trees (the process of moisture evaporating, primarily from leaves) affects not only air temperature directly, but also heat storage, wind speed, relative humidity and more. These factors work together to make the midday air temperature under trees in a lawn-type setting almost 2 degrees Fahrenheit cooler than on the same lawn with no trees. In effect, trees create a cooler microclimate.

So, cooler air is nice, but how does it help air quality? The emission of many pollutants and/or ozone-forming chemicals increases with temperature. A great example of this takes place in parking lots every day. You know how hot your car is after just a few minutes of sitting in the open sun? Well, hot seats and door handles are not the only undesirable things happening at that moment when you get in and crank up the AC.

Forest Service researchers in Davis, Calif., found trees in parking lots made air 3 degrees cooler. Those few degrees' difference reduced vehicle surface temperatures by up to 36 degrees, inside (cabin) temperatures by 47 degrees and, importantly, gas-tank temperatures by 7 degrees! Fewer hydrocarbon emissions result from gas that evaporates out of tanks and hoses with reduced temperatures.

At present, most of our parking lots are a place where high temperatures increase pollutant emissions and thus the formation of smog and ozone. Gives new meaning to "made in the shade," doesn't it?

ADAM DOWNING is an agent in Virginia Cooperative Extension's Madison office, War Memorial Building, Main Street. He specializes in forestry and natural resources. Contact him by phone at 540/948-6881; by fax at 540/948-6883; or by e-mail at adowning@vt.edu.

Date published: 6/30/2006