Urban smog control: a new role for trees?

Urban smog control: A new role for trees?

Trees can serve as a major source of the hydrocarbons contributing to smog ozone (SN: 9/17/88, p. 180). But trees can also help cool the air, thus slowing the heat-driven photochemical reactions that brew ozone from hydrocarbons and nitrogen oxides. Indeed, because smog production is so temperature-sensitive, trees that cool cities may do more to limit ozone than to foster it.

This conclusion, drawn from new computer simulations, suggests that sprawling urban growth may take an unnecessarily large toll on air quality if planners don't spare as many trees as possible, says study coauthor William L. Chameides, an atmospheric chemist at the Georgia Institute of Technology in Atlanta. He believes city planners and air-pollution-control managers should join forces to "think about how they want their [region] to grow" -- and especially "where they want to leave green spaces."

Noting that most U.S. ozone-mitigation strategies focus on limiting hydrocarbons, Chameides says the new findings also reinforce the importance of shifting to approaches that place at least as much emphasis on nitrogen oxides.

Growing cities sacrifice many trees to development. Atlanta, whose population has increased 30 percent each decade since 1970, has lost about 20 percent of its trees over the past 15 years, Chameides says. During that same period, average summer temperatures have climbed steadily in both the city and its surroundings. But Atlanta's increase, totaling almost 4[degrees]F, dwarfs those of its rural neighbors--in one case by a factor of about 5. In a paper to appear in the JOURNAL OF GEOPHYSICAL RESEARCH later this summer, Chameides and Georgia Tech colleague Carlos A. Cardelino argue that the urban temperature increase probably stems from a deforestation-fostered "heat-island effect," in which asphalt and dark-roofed structures become massive heat reservoirs.

On the basis of Atlanta's 1985 tree cover of about 57 percent, Chameides and Cardelino computed likely ozone concentrations for a typical summer day in Atlanta under a range of scenarios. Each scenario explored some facet of the city's changes over the past 15 years, such as tree loss or an estimated reduction of as much as 50 percent in hydrocarbon emissions from human activities, including traffic and industrial facilities.

Comparisons of these simulations suggest that the sharp, steady rise in Atlanta's summer temperatures over the past 15 years has resulted in a large net increase in hydrocarbon emissions from vegetation--despite the loss of one-fifth of the city's tree cover. This apparent contradiction reflects the fact that a tree's hydrocarbon-emission rate increases dramatically as temperatures climb. Indeed, Chameides and Cardelino say the estimated vegetative-hydrocarbon increase from Atlanta's remaining trees would have canceled the city's decrease in automobile and industrial hydrocarbon emissions.

Arthur H. Rosenfeld, a leading analyst of urban heat-island effects, has long advocated tree planting to reduce urban heating and to sequester the carbon dioxide emissions that threaten to initiate a global warming (SN: 5/7/88, p. 297). "But i hadn't even thought about that [vegetative-hydrocarbon feedback on ozone from tree cutting], so I'm happy to have [the Georgia Tech team] publish it," says Rosenfeld, who directs the Center for Building Science at Lawrence Berkeley (Calif.) Laboratory.

The new simulations make "an important contribution," adds John R. Holmes, director of research at California's Air Resources Board (CARB) in Sacramento. The results also dovetail with observations made by CARB -- a state agency responsible for some of the toughest auto-emissions regulations in the nation. During the 1970s, when CARB's vehicle-emissions controls focused on hydrocarbons, Los Angeles experienced a reduction in ozone, but only downtown, where traffic was highest and vegetation was lowest. Ozone levels continued to increase in the areas where they had always been highest--downwind. It wasn't until "we had large reductions in both hydrocarbons and nitrogen oxides that ozone levels went down rapidly all over," Holmes says.

Gary Z. Whitten, who models urban ozone at Systems Applications, Inc., in San Rafael, Calif., says the Atlanta report makes a good argument for controlling nitrogen oxides, but he points out that the computer model failed to account for several factors that could greatly affect ozone production, such as a heat-driven increase in atmospheric mixing. Whitten views the analysis as "interesting" but only a "first start" at defining the complex interactions among trees, temperature and smog.