Car-emission standards improve rural air.

Regulations aimed at curbing pollution from motor vehicles are achieving a key goal, according to a 9-year study in Virginia's Shenandoah National Park. A decline in the ground-level concentration of carbon monoxide in the park shows that rural air over the eastern United States is getting cleaner, scientists say.

Until around 1989, concentrations of the toxic gas were on the rise, in step with increasing numbers of automobiles on the roads. The new study, however, shows that between 1989 and 1997, carbon monoxide declined by 23 percent at a monitoring station in Shenandoah Park. The researchers report their findings in the Sept. 15 GEOPHYSICAL RESEARCH LETTERS.

"It's a piece of good news," says Russell R. Dickerson, a professor of meteorology at the University of Maryland in College Park and codirector of the study. Wind patterns make the atmosphere over Shenandoah representative of rural air east of the Mississippi.

The earlier rise in carbon monoxide distressed scientists for several reasons. Carbon monoxide undermines blood's ability to carry oxygen and may contribute to heart attacks (SN: 10/14/95, p. 247). It also meddles in the chemistry of the atmosphere. Carbon monoxide makes it harder for the atmosphere to: cleanse itself of chlorofluorocarbons, which chew at Earth's protective ozone layer. At lower altitudes over cities, carbon monoxide can also increase ozone, the toxic main ingredient of smog.

According to Donald H. Stedman, an atmospheric chemist at the University of Denver, carbon monoxide concentrations in Shenandoah are falling mainly because cars are getting cleaner. Federal standards required manufacturers to build the greener vehicles starting in the 1980s, he says.

Joseph Pinto, an atmospheric scientist with the Environmental Protection Agency in Research Triangle Park, N.C., says the trend at Shenandoah echoes what EPA scientists have been seeing at sites in and around cities nationwide.

Most cities no longer exceed the national ambient-air-quality standard for carbon monoxide--9 parts per million averaged over 8 hours--Pinto says. In 1977, by contrast, at least twice per year carbon monoxide at the average American city climbed above this concentration.

At Big Meadow, the 1,100-meter-high plateau where researchers sampled air, the highest sustained carbon monoxide concentration during the study was about 0.7 parts per million.

The study's intent was not to see if the air away from cities had unhealthy amounts of carbon monoxide, says Pinto. Rather, it was to fill a scientific gap. Scientists couldn't be sure that rural and urban air were benefiting equally from improved car emissions. "It's good to have that confirmation," Pinto says.

Meanwhile, the news from Big Meadow is not all good. Ozone concentrations there have stayed about the same even as carbon monoxide has declined.

Yet Dickerson is optimistic. Nitrogen oxides foster smog production, he says, and their concentrations are expected to decline in response to newly implemented standards for industrial emissions. Smog, too, will wane as nitrogen oxide levels drop, Dickerson says.