TEHRAN Air pollution can damage trees, but certain varieties of trees can emit air pollutants, creating ozone and fine particulate matter that make it hard to breathe. Planting a peach or an avocado tree can reduce air pollution, but the California Air Resources Board says that not all trees provide cleaner air. Planting a California sycamore, for instance, can add to air pollution. Research by the California Environmental Protection Agency's Air Resources Board shows that it takes informed selection and planting of trees to reduce urban air pollution. In a study presented to the Board July 9, over 1,400 tree species were compiled and rated for their various pollution impacts and pollen production. Air Resources Board Chairman Alan Lloyd said, "People plant trees for many reasons: to beautify their home, provide shade, and reduce energy bills. But, they may not be aware that certain trees can decrease air pollution and pollen counts. Necessary information is now available to choose the most effective smog and pollen reducing species." When considering a new tree, there are three main concerns: intended use, such as shading needs, climate of the area, and the tree's emissions types and amounts. The emissions of trees are classified as low, moderate, or high, based on the sum of the hourly emission rates of the chemicals isoprene and monoterpenes. These compounds are very reactive and play a role in tropospheric ozone formation and aerosol production according to the Washington State University Laboratory for Atmospheric Research. Large scale planting can affect air quality through regional concentrations of ozone and fine particles. To reduce ozone concentrations in urban areas, it is particularly important to use low emitting species. The emissions from deciduous trees such as oak, poplar, aspens and willows are mainly isoprene, whereas coniferous trees such as pines, cedars, redwood and firs emit mostly monoterpene. There are several species which emit both isoprene and monoterpenes such as spruce and eucalyptus. When selected with attention to what each tree emits, trees and other plants can improve local cooling, reduce energy use and slow the chemical reactions that lead to the formation of ozone, or urban smog. The differences in emission rates from one species to the next can vary significantly. Some plant species can release as much as 10,000 times more biogenic, naturally formed, volatile organic compounds (VOCs) than the more atmospherically friendly low emitters. For more information about selecting appropriate trees, see Cal Poly San Luis Obipso's website Selectree. The Air Resources Board recommends that people consider a tree's capacity to emit biogenic VOCs before they plant. These compounds are particularly important because they create ozone and fine particulate matter (PM 2.5), which are damaging to the human respiratory system. Plants positively affect air quality by local cooling and removing some pollutants. But they may negatively affect air quality by emitting biogenic volatile organic compounds and releasing potent allergens. Low-emitters include the Chinese Hackberry, Avocado, Peach, Ash, Sawleaf Zelkova and the Eastern Redbud. A few of the high emitters include the London Plane, California Sycamore, Liquidamber, Chinese Sweet Gum, Goldenrain Tree, and the Scarlet, Red and Willow Oaks. The Air Resources Board aims to promote and protect public health, welfare, and ecological resources through effective reduction of air pollutants while recognizing and considering effects on the economy. The ARB oversees all air pollution control efforts in California to attain and maintain health based air quality standards.

(Courtesy of Environmental News Network)