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Keeping an Ozone Eye in Orbit


WE'VE heard of the Antarctic ozone hole and about possible global changes in the ozone layer. But we don't hear much about one of the most important aspects of this environmental story - the quality of the data. How do scientists really know what's going on between heights around 15 and 50 kilometers (10 and 30 miles) in the stratosphere, where ozone forms with the help of sunlight? It's a question atmospheric chemists now face as their most important data collector - the orbiting Total Ozone Mapping Spectrometer (TOMS) - comes to the end of its days over the next few years.

They are looking to cooperation between the United States and the Soviet Union to keep the data flowing.

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The TOMS rides on the United States Nimbus 7 satellite that's been in space for some 11 years. It's the last of that satellite series. American ozone mappers are looking for a new bus on which to hitch a ride. They are working out plans with Soviet colleagues to put the replacement instrument on a Soviet Meteor weather satellite to be launched in a couple of years.

This kind of instrument deteriorates over its operating life. So users of its data have had to make sure they don't mistake internal changes of the instrument for real changes in the ozone layer. Now they have the challenge of making a smooth transition to the TOMS replacement. They have to ensure that a change in this important instrument doesn't bring a shift in data that falsely suggests a change in the ozone layer itself.

TOMS team member Richard Stolarski of the NASA Goddard Space Flight Center says he's confident that scientists can keep the data ``honest.''

The degree of concern depends partly on what data are involved. Instrument drift has been only a few percent over the TOMS lifetime. The ozone hole, on the other hand, involves springtime losses of as much as 50 to 60 percent. There's little chance of instrument error causing confusion here. The hole the instrument sees is really there.

Global ozone layer changes are much less dramatic. Dr. Stolarski and his colleagues have found changes - mostly losses - of a few percent over many parts of Earth. Here, instrument drift of a few percent could cause confusion. Scientists have learned how to compensate for drift in the present TOMS. Now, Stolarski says, he and his colleagues are already at work trying to anticipate the effect of bringing a new instrument on line.

Ozone is a form of oxygen with three atoms per molecule. In the stratosphere, it absorbs harmful solar ultraviolet ``light,'' which is why the strength of the ozone layer is important. This and other distinctive physical and chemical characteristics help ozone trackers keep tabs on the chemical's distribution throughout the stratosphere. They can measure it from the stations on the ground. They can sample it with balloons. But the most important ozone data set is the one gathered by the TOMS.

Those are the data that produce the colorful pictures showing the ozone hole coming and going with the seasons. Maintaining the integrity of that kind of database is a high priority for atmospheric scientists. If they manage to do it, as expected, we will have Soviet-American cooperation to thank.

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