Air

Increasing the urban tree canopy can dramatically improve air quality. Trees absorb pollutants and intercept particulates such as dust, ash, pollen and smoke, while releasing oxygen into the air. Trees shade asphalt surfaces and parked cars, significantly reducing hydrocarbon emissions, and moderate air temperatures through shade and evaporation. They also reduce power plant emissions by decreasing energy needs in buildings.

Smog is created by chemical reactions between nitrogen, oxygen and volatile organic compounds (VOCs) in sunlight. Sources of VOCs include vehicle exhaust, gasoline vapors, solvents and emissions from industrial facilities. High summer temperatures increase the rate of this reaction and lead to ozone alert days. Automobile exhaust is a well-known source of air pollution. However, many people do not realize that plastics and vinyl in parked cars, heated in the sun, are the source of 16% of hydrocarbon emissions (McPherson, et al, 2002), one of the principal components of smog.

The air in cities can be 6 – 8°F hotter than surrounding rural areas throughout the year (Pomerantz, et al, 2002). Scientists call this phenomenon the urban heat island effect. Heat islands develop as vegetation is replaced by asphalt and concrete for roads, buildings, and other structures. These surfaces absorb – rather than reflect – the sun's heat, causing surface temperatures and overall ambient temperatures to rise.

Higher temperatures lead to increased air pollution, including ozone. Trees reduce ozone and can cool urban heat islands by 10 – 20°F (Pomerantz, et al, 2002). All three counties in Delaware fail to meet basic EPA air-quality standards for ground-level ozone. The resulting air pollution has significant impacts on public health, on local ecosystems and on our economy. Failure to meet air quality standards eventually will affect Delaware’s allocation of federal highway dollars.

Rising levels of carbon dioxide are accelerating global warming. Large, healthy shade trees have the greatest potential for improving air quality and reducing greenhouse gases due to their higher carbon storage capacity (CUFR, 2004). Through the process of photosynthesis, trees remove carbon from the atmosphere and generate oxygen.

As trees grow, they sequester more carbon in their trunks, branches, leaves, and roots. One hundred mature trees can remove 5 tons of carbon dioxide, 1,000 pounds of pollutants, 400 pounds of ozone and 300 pounds of particulates per year, including the compounds that cause acid rain. An acre of trees absorbs the amount of carbon produced by driving a car for 26,000 miles.