THE EFFECTS OF URBAN TREES ON AIR QUALITY

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Download free The Effects Of Urban Tress On Air Quality.pdf Urban vegetation can directly and indirectly affect local and regional air quality by altering the urban atmospheric environment. The four main ways that urban trees affect air quality area: Temperature reduction and other microclimatic effects Removal of air pollutants Emission of volatile organic compounds and tree maintenance emissions Energy effects on buildings. Temperature Reduction: Tree transpiration and tree canopies affect air temperature, radiation absorption and heat storage, wind speed, relative humidity, turbulence, surface albedo, surface roughness and consequently the evolution of the mixing-layer height. These changes in local meteorology can alter pollution concentrations in urban areasb. Although trees usually contribute to cooler summer air temperatures, their presence can increase air temperatures in some instancesc. In areas with scattered tree canopies, radiation can reach and heat ground surfaces; at the same time, the canopy may reduce atmospheric mixing such that cooler air is prevented from reaching the area. In this case, tree shade and transpiration may not compensate for the increased air temperatures due to reduced mixingd. Maximum mid-day air temperature reductions due to trees are in the range of 0.04oC to 0.2oC per percent canopy cover increasee. Below individual and small groups of trees over grass, mid-day air temperatures at 1.5 m above ground are 0.7oC to 1.3oC cooler than in an open areaf. Reduced air temperature due to trees can improve air quality because the emission of many pollutants and/or ozone-forming chemicals are temperature dependent. Decreased air temperature can also reduce ozone formation. Removal of Air Pollutants: Trees remove gaseous air pollution primarily by uptake via leaf stomata, though some gases are removed by the plant surface. Once inside the leaf, gases diffuse into intercellular spaces and may be absorbed by water films to form acids or react with inner-leaf surfacesg. Trees also remove pollution by intercepting airborne particles. Some particles can be absorbed into the tree, though most particles that are intercepted are retained on the plant surface. The intercepted particle often is resuspended to the atmosphere, washed off by rain, or dropped to the ground with leaf and twig fallg. Consequently, vegetation is only a temporary retention site for many atmospheric particles.

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