AT a time when human damage to Earth's ozone layer is sharply clear, the United States and Soviet Union are cooperating to keep a crucial aspect of ozone research going. For scientists, it could be a just-in-time reprieve.
They rely on satellite data for a global perspective. The 12-year-old Total Ozone Mapping Spectrometer (TOMS) on the American Nimbus-7 satellite could fail at any time. The United States won't send up a replacement before 1993. The Soviet Meteor-3 weather satellite is scheduled to carry a new version of the TOMS instrument into orbit later this year - probably in August.
This should ensure the continuity of data that ozone investigator Susan Solomon considers ``crucial.''
An atmospheric chemist with the United States National Oceanic and Atmospheric Administration (NOAA), Dr. Solomon notes that, scientifically speaking, the fact that pollution - especially certain chlorofluorocarbon (CFC) chemicals - is attacking the ozone layer is not in question. She explains: ``The ozone database has been very carefully looked at. No matter how you slice it, there's a downward trend - greatest in high latitudes, less in midlatitudes, and least in the tropics. The question now is how much worse will it get?''
Scientists are concerned about the ozone layer's integrity because the gas - a compound with three oxygen atoms per molecule - absorbs much of the sun's biologically damaging ultraviolet radiation. Ozone forms in the stratosphere under action of sunlight. Various natural processes destroy it. The natural balance between formation and destruction had maintained a steady-state ozone concentration that varied from year to year, season to season, and geographically with latitude.
Atmospheric chemists were used to this natural variability. But pollution that upsets that balance caught them off guard. This was especially true of the famous ozone hole that develops over Antarctica in the spring - August through October - when ozone concentration can drop by 50 percent or more.
Reviewing the past five years' awakening in Nature magazine last September, Solomon noted that ``the possibility that the stratospheric ozone layer could be depleted by half at certain latitudes and seasons would have been deemed a preposterous and alarmist suggestion in the early 1980s.''
Even though the ``preposterous'' suggestion now is a widely acknowledged fact, scientists still feel somewhat naive about ozone layer damage. New facets continue to appear.
Earlier this month, for example, Mark R. Schoeberl of the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center in Greenbelt, Md., and Dennis Hartmann of the University of Washington in Seattle published a study in Science magazine that suggests the Antarctic ozone hole could double in size.
Chemical changes take place on particles in stratospheric clouds that form in the cold winter darkness of polar regions. These reactions put chlorine in the CFC chemicals into forms that promote ozone destruction when sunlight returns in the spring.
Drs. Hartmann and Schoeberl point out that, so far, the area of depletion chemistry hasn't spread over the entire polar vortex - the air mass more or less isolated by winds that circle Antarctica in winter. They warn that, if ozone depletion does grow to fill that vortex, then ``the ozone hole could double in size.''
Not all meteorologists believe the vortex is tightly isolated. If air flows through it to and from higher latitudes, the Hartmann-Schoeberl analysis may not be valid. This is one of the important scientific questions that scientists can't resolve without more data.
Another major uncertainty is what is happening in the Arctic. Aircraft measurements have shown that the Arctic also is chemically primed for ozone depletion. Yet it hasn't shown dramatic losses.
Some scientists have wondered if the Arctic stratosphere simply hasn't been frigid enough to encourage the winter cold cloud chemistry. NOAA aeronomist Michael H. Profitt and colleagues reported in Nature last September that aircraft data suggest there is significant seasonal Arctic ozone loss but that it is partly masked by inflow of ozone-rich air from lower latitudes. Again, more monitoring is needed to clear up a key question.
Solomon points out that the disturbing aspect of the present state of knowledge is that ``everything you can think of [that might be happening] would only tend to make the Antarctic ozone hole worse.'' She notes, for example, that the rise in atmospheric carbon dioxide concentration that may warm the lower atmosphere would have the opposite effect in the stratosphere. It would cool the air there and could expand the area of Antarctic cold-cloud, ozone-destroying chemistry.
All told, the scientific unknowns about the ozone layer put a premium on continued detailed monitoring. At the same time, the proven fact that industrial chemicals are destroying ozone has encouraged political action. The 1990 Antarctic ozone hole underscored this point. Stratospheric ozone concentration over the South Pole reached a record low of a little less than half its normal value. Moreover, the hole persisted longer than usual, lasting into December.
The European Community now has agreed to ban ozone-threatening CFCs by mid-1997. That's three years earlier than the international ozone-layer protection treaty requires.
In the United States, the newly amended Clean Air Act imposes schedules for phasing out suspect chemicals, including some not covered by the treaty. US producers must cut output of CFCs and of halons - chemicals used in fire extinguishers - by 15 percent this year.
Even when ozone-threatening chemicals no longer enter the atmosphere, the pollution already there will persist for many decades. This means that the need to monitor the ozone layer has become a long-term feature of human life on Earth.