Trees may serve a major role in the control of smog, and their continued destruction in cities means that despite reductions in automobile and industrial pollution, smog will get worse, not better, in the future, according to research soon to be published by scientists at the Georgia Institute of Technology.
The researchers suggest city planners spare as many trees as possible, because greenery cools the city and reduces a phenomenon known as the "urban heat island," in which concrete and asphalt absorb and store heat, creating a kind of solar oven that sets off a chain of chemical reactions to produce smog.
The Amazon is not the only place being deforested. Atlanta, for example, has lost about 20 percent of its trees in the past 15 years. During that period, average summer temperatures in Atlanta have climbed almost 4 degrees Fahrenheit, a whopping increase compared to surrounding rural areas, according to Georgia Tech's William Chameides and colleagues, whose report is to appear later this summer in the Journal of Geophysical Research.
The heat island does two things: It boosts emissions of man-made hydrocarbons, mostly from the evaporation of fuel such as gasoline; and it increases the production of natural hydrocarbons, which react with nitrogen oxides to form ozone, the principal ingredient in smog. The natural hydrocarbons come from vegetation. The hotter it is, the more a tree sweats natural hydrocarbons.
Chameides found that even though trees may contribute to smog, their destruction creates even more smog. He argues that by sparing the ax, cities can improve their air quality and lower summer temperatures significantly.