Air pollution is a major problem in many cities worldwide. Urban trees interact with the atmosphere and surrounding urban surfaces, and affect air quality in various ways. 1) Trees intercept particulate matter and absorb gaseous pollutants such as ozone, sulfur dioxide, and nitrogen dioxide, thus removing them from the atmosphere. 2) Trees emit various volatile organic compounds, such as isoprene and monoterpenes, that can contribute to ozone formation in cities. 3) By transpiring water and shading surfaces, trees lower local air temperatures. This cooling can reduce emission of volatile organic compounds from both biogenic and anthropogenic sources, and can alter ozone-forming processes, thereby reducing local ozone levels. 4) Because trees lower air temperatures, shade buildings in the summer, and block winter winds, they can reduce building energy use and consequently reduce pollution emissions from power-generating facilities.
Results from the Chicago Urban Forest Climate Project indicate that trees in the City of Chicago, Illinois (USA), (11-percent tree cover) removed 591 metric tons (t) of air pollution in 1991. The greatest removal was for particulate matter less than 10 microns (212 t), followed by ozone (191 t), nitrogen dioxide (89 t), sulfur dioxide (84 t), and carbon monoxide (15 t). Average hourly improvement (in-leaf season) in air quality due to pollution removal from trees in the Chicago area was less than 0.4 percent. Maximum hourly improvement was estimated at 1.3 percent for sulfur dioxide, though local improvements in air quality can reach 5 to 10 percent or greater in areas of high tree cover.
Key words: air pollution, urban forestry.
Correspondence: David J. Nowak, USDA Forest Service, 5 Moon Library, SUNY-CESF, Syracuse, NY 13210 USA
Telefax: +1-315-4883216