How Trees Reduce Urban Air Pollution
Trees Lower Air Pollutants, DecreaseTemperatures & Conserve Energy
Sep 25, 2009
Many cities are notorious for their dangerously high levels of air pollution and the resulting health issues, such as asthma and emphysema. Summer heat and vehicle emissions are just a few of the contributing factors that increase air pollution levels. However, healthy trees planted effectively along roads, buildings and in parking lots can decrease overall air pollution levels for city residents.
Trees Remove Air Pollutants
Trees can remove both gaseous air pollutants and particulate matter. Gaseous air pollution enters a tree through leaf stomata and is either held in intercellular cavities or processed into usable nutrients. According to David Nowack’s report, "The Effects of Urban Trees on Air Quality" for the USDA Forest Service, particulate matter usually collects on the surface of trees. The particulates often enter the ground by either being washed off the tree by precipitation or falling to the ground with leaves and twigs. Larger trees tend to be much more effective, by approximately 70%, at removing air pollutants than smaller trees. Over the course of one year, trees removed an estimated 1,821 metric tons of air pollution in New York City.
Trees Reduce Temperature
Trees reduce surrounding air temperature and create their own microclimates through transpiration and the effects of tree canopies. Tree canopies not only provide shade but they also reduce solar radiation absorption and provide heat storage. The larger the tree canopy, the greater the reduction of surrounding air temperature. Decreasing air temperature is effective in reducing air pollution because many of the chemicals that create pollution are temperature dependent. Shade from trees planted in parking lots can create small reductions, 1 to 2%, of the levels of harmful emissions produced from car ignitions.
Trees Reduce Energy Usage
Trees can keep buildings cooler in the summer and more insulated in winter. By blocking solar radiation and creating a cooler microclimate, trees offer buildings protection from summer heat. During winter, trees can provide effective wind breaks and block drafts from entering structures. This lowers the amount of energy used to heat or cool the building and reduces the air pollution created in the production of energy.
Placement and Type of Tree is Important
For trees to be the most effective in improving air quality they need to be placed in appropriate locations. According to Francisco Escobedo in his article, "Urban Forests in Florida: Do they reduce air pollution?" on the University of Florida IFAS Extension website, scattered trees are less effective in reducing air pollution. Stands of trees or dense evergreens provide more effective removal of particulate matter and protection from solar radiation than scattered deciduous trees. Trees surrounding buildings should be placed in areas that provide protection from solar radiation in summer and block winter wind.
The types of trees used are as important as their placement. Healthy, hardy and long lived trees are the best choice. Avoiding trees that require frequent maintenance will lower the amount of pollution emitted by fossil-fueled machines like chainsaws, leaf blowers and wood chippers. Nowack’s studies revealed that mulberry, cherry, linden and honeylocust trees were especially effective in reducing ozone levels.
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