On the Trail of Carbon Monoxide ... 138 Miles High

THE orbiting astronauts are ``meeting scientists' greatest expectations'' in mapping of Earth's visible features with radar, according to mission controllers. They are also tracking an environmental factor you cannot smell or see.

It's carbon monoxide gas. This is not only an important pollutant, it also plays a significant role in climate. Scientists need to know where it comes from, where it goes, and how its concentration varies over the planet and over time.

Burning organic fuels of any kind produces carbon monoxide (CO). It's a familiar toxic emission from cars and furnaces. The burning of fields and forests, especially in the tropics, is a major CO source. There also are natural sources.

CO's role as an undesirable air pollutant is obvious. Its climate role is more subtle. It doesn't affect climate directly. It does help determine the concentration of certain heat-trapping ``greenhouse'' gases, especially methane. Once in the air, it has more heat-trapping power than its chemical cousin, carbon dioxide.

The air cleans itself of methane and similar trace gases by chemical reaction with a compound of hydrogen and oxygen called the hydroxyl radical, or ``OH'' in chemist's shorthand. The more OH (produced naturally) the atmosphere has, the quicker it cleanses itself of methane.

This is where carbon monoxide comes in. CO also reacts with the OH radical. Thus, the more CO pollution there is, the less OH will be available to enable the atmosphere to get rid of methane. And the more methane accumulates, the greater is its contribution to possible global warming.

Until recently, CO concentrations had been rising at a globally averaged rate of about 1 to 2 percent a year, as inferred from 30 years of Northern-Hemisphere measurements. Atmospheric chemists were concerned that this might limit the availability of the main atmospheric cleanser.

However, Paul Novelli of the University of Colorado at Boulder and several colleagues have found that this trend reversed a few years ago. They reported last month in the journal Science that CO concentrations have dropped at a rate of 6.1 percent a year averaged around the Northern Hemisphere from June 1990 through June 1993. The Southern Hemisphere decrease averaged about 7 percent a year.

As Dr. Novelli and his colleagues noted, scientists don't know what is going on. Mt. Pinatubo's eruption in the Philippines may have thinned Earth's stratospheric ozone enough to let in more ultraviolet radiation. This, in turn, would have made more of the OH chemical that destroys carbon monoxide. There also may have been less biomass burning in the tropics.

The scientists findings are based on air samples taken at 27 sites between latitudes 71 degrees N and 41 degrees S. Scientists would like a bigger picture. This is what the astronauts can begin to give them with an instrument called MAPS (Measurement of Air Pollution from Satellite.) It measures CO concentration between altitudes of 2 and 10 miles along the orbiter's course. Like Endeavour's radar that charts geography, MAPS could one day fly permanently on an unmanned satellite.

MAPS has flown on the shuttle before. Its November 1981 observations showed more CO concentration over the tropics than over the Northern Hemisphere in the atmospheric layer surveyed.

Its October 1984 observations confirmed biomass burning in South America and southern Africa as a major carbon monoxide source, according to the National Aeronautics and Space Administration. The new data will be compared with these older findings.