Everyone 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 the effect of trees on air quality extensively. Trees clean the air in basically four different ways and Dr. Nowak has organized these as an easy to remember acrostic: T-R-E-E.
“T” is for Temperature.
Who hasn’t sought the cool air under a shade tree or enjoyed the coolness of a walk in the woods? Transpiration from trees (that is the process of moisture evaporating, primarily from leaves) and the canopies themselves not only affect air temperature directly, but also heat storage, wind speed, relative humidity, surface roughness and more. These factors work together to reduce mid-day air temperature under trees in a lawn type setting by almost 2 degrees F cooler than the same lawn with no trees. In effect, trees create a different, i.e. 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 undesirables happening at that moment when you get in and crank up the AC.
Forest Service researchers in Davis, California found trees in parking lots made air 3 degrees cooler. Those few degrees difference reduced vehicle surface temperatures up to 36ºF, inside (cabin) temperatures by 47 degrees and, importantly, gas tank temperatures by 7 degrees! Fewer hydrocarbon emissions result from gas that evaporated 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?
“R” is for Removal of air pollutants.
Airborne particles, or particulate matter, is removed (at least temporarily) from the air by mechanical interception, primarily on leaf surfaces. While these bad guys float around in the air, their microscopic size enables them to be breathed deep into our lungs, potentially causing serious health problems or just a temporary irritation. In either case, reducing the amount of airborn particles and gaseous pollutants in areas above the acceptable level is a good thing.Trees are able to remove both gaseous pollutants and airborne particles. Gaseous pollutants include ozone, sulfur dioxide, nitrogen dioxide, and carbon monoxide. These are taken out of the air by trees primarily through leaf stomata. Stomata are small “windows” on green leaves which let carbon dioxide in and oxygen out. Thankfully, these stomata also accept gaseous pollutants in small amounts where it diffuses into spaces between leaf cells and from there is handled various ways.
“E” is for Emission of Volatile Organic Compounds (VOCs) and tree maintenance emissions.
You may remember some time ago, trees were reported to contribute to ozone formation. This misleading fact contains only part of the truth. Most trees do emit biogenic VOCs such as isoprene and monoterpenes which can contribute to the formation of ozone and carbon monoxide. The other side of this story is that in areas with low nitrogen oxide concentrations, such as more rural areas, VOCs are believed to remove ozone. Additionally, since trees lower air temperature, the net effect of increased trees in urban areas is an overall lowering of VOC emissions and therefore ozone formation.
Trees in urban areas require energy inputs for planting, maintaining and removing. Because we burn fossil fuels (which emit CO2, SO2, N, CO and VOCs) in all these activities, we also need to factor that into the Trees + Air equation. In this case, it tips the scale a bit to the net loss side… but not for long!
“E” is also for Energy effects on buildings.
Well-placed trees can significantly lower temperatures in buildings by shading them. On the other hand, poorly placed trees can increase energy needs by shading in the winter or blocking summer breezes. For trees to have a net positive effect on energy usage in buildings, proper placement is critical. With proper placement, however, the savings are quite significant. In fact, it is a double win. Homeowners get a lower energy bill while we all benefit from the reduced energy demand. When energy demand decreases, pollutant emissions from the power plants supplying that energy also decreases, and that generally improves air quality.
When we put all these letters back together we get the whole word. Likewise, we must consider each of these effects as cumulative and interactive to understand the overall impact of trees on air pollution.
The research on trees and air pollution is rather new and still ongoing. Currently, the best picture we can get is by running simulation models. The general conclusion is that increasing urban tree cover reduces ozone concentration. Economically, it’s probably one of the most efficient ways to do so. And that’s to say nothing of the many other benefits these aesthetically appealing air cleaners provide.
Adam Downing is a faculty member of Virginia Tech, who has worked as the Natural Resources Extension Agent serving Virginia Cooperative Extension’s Northern District, a 19 county area generally described as the Northern Piedmont of Virginia. His professional expertise is in providing informal education regarding forestry and natural resources to address current issues in rural, urban, and rural/urban interface areas to home & landowners, professionals, decision-makers, and the general public. His mission is to enable people to make best decisions regarding the forest & natural resources, within their realm of influence, resulting in environmentally sustainable management, growth, and quality of life for that person and that community.