Courtesy of: EarthTalk®

Dear EarthTalk: Do urban trees really help reduce pollution and clean the air? —John Alderman, Washington, DC

Back in 1872 Frederick Law Olmsted, the granddaddy of American landscape architecture and the designer of New York's Central Park, proclaimed that trees were the "lungs of the city." While Olmsted's statement may have been more philosophical than scientific, researchers have since found that city trees do indeed perform important environmental functions like soaking up ground-level pollutants and storing carbon dioxide, which helps offset global warming.

Each year in Chicago, for example, the windy city's urban tree canopy removes 15 metric tons of carbon monoxide, 84 metric tons of sulfur dioxide, 89 metric tons of nitrogen dioxide, 191 metric tons of ozone and 212 metric tons of particulates, according to David Nowak, project leader of the U.S. Forest Service's Urban Forest Ecosystem Research Unit. Trees absorb these gaseous pollutants via the tiny pores in their leaves and break them down into less-harmful molecules during photosynthesis.

This article appears in Apr 7-13, 2011.