What Is Destroying the Ozone?

By MICHAEL D. LEMONICK

Two months ago, 15 scientists flew into McMurdo Station, Antarctica, to investigate a mystery: What causes a thinning in atmospheric ozone above the frigid continent, a phenomenon that has peaked each October since it was first observed in 1983? It was hardly an academic question; the ozone layer is a blanket of oxygen molecules that protects the earth's surface from the sun's harmful ultraviolet radiation, a form of light just beyond the human range of vision. Speculation on the reason for these "holes" has ranged from weather patterns and solar activity to the action of man-made chlorofluorocarbons (CFCs), chemicals used as spray-can propellants, refrigerants and industrial solvents.

Last week the team radioed Washington with its interim findings. Sure enough, the hole appeared in mid-September, right on schedule, and bloomed over the next 20 to 30 days, until the ozone content of the hole had dropped by about 40%. Says Team Leader Susan Solomon, a chemist with the National Oceanic and Atmospheric Administration in Boulder, Colo.: "We suspect a chemical process is fundamentally responsible," although atmospheric dynamics undoubtedly help shape it. The chemicals could be natural -- volcanic, perhaps -- but CFCs might play a role.

Antarctica is not the only spot where ozone levels are low, says Donald Heath, a NASA scientist. Tests over Arosa, Switzerland, since the 1920s have shown an average ozone loss of 3%, mostly in the past ten years. And Heath believes he has found another hole. Centered over Spitsbergen, Norway, 700 miles from the North Pole, it is one-third the size of the Antarctic hole. Heath claims the region's ozone loss has been 1.5% a year for the past six years and says this location fits models of CFC-caused loss.

The reason for concern is that without ozone, life on earth would be impossible. Ozone is oxygen but in an unusual form. Most oxygen comes in two- atom molecules, but external energy -- in this case, the sun's ultraviolet radiation -- can split some of them apart. The single oxygen atoms tend to attach themselves to the remaining molecules, forming an oxygen-atom triplet. The result: a layer, from six to 30 miles up, of ozone-enriched air. Once formed, an ozone molecule is a good absorber of ultraviolet. But when CFCs rise to the ozone layer, sunlight decomposes them, releasing the chlorine they contain. The chlorine is a catalyst, breaking ozone apart without itself being affected. At present, the ozone layer lets enough ultraviolet through to cause sunburn and, in some people, skin cancer. More ultraviolet would increase the effect: the Environmental Protection Agency estimates a 1% drop in global ozone could cause 20,000 additional skin cancers in the U.S. annually.

The Antarctic team's assertions are bold, since the tests are not completed. But even if the ozone change is not induced by man, says NASA Research Scientist Richard Stolarski, "it would do us well to understand, because we're going to have to react whether it's natural or man-made. We need to know if these changes are greater than in the past or if we are just paying more attention to them now. And we should know that as soon as possible."

With reporting by Dick Thompson/Washington