WITH the coming of Antarctica's summer, the infamous ``ozone hole'' is beginning to fill. It leaves researchers with an ongoing puzzle. Ozone loss was extreme this year. It's been ``essentially a carbon copy'' of the record-breaking 1987 pattern, says Richard Stolarski of the National Aeronautics and Space Administration Goddard Space Flight Center. Yet 1988 was a year of only modest depletion.
Finding out why one year's hole is deeper than another year's hole is the puzzle atmospheric scientists have to solve if they are to understand what's going on in the Antarctic stratosphere.
Dr. Stolarski is part of a team that tracks ozone by satellite observations. He explains that atmospheric scientists have a fairly clear picture of what causes the ozone hole in any one year. Complex chemical reactions involving manmade pollutants are an underlying factor. Computer simulations can mimic those processes.
But, Stolarski adds, the mechanisms that determine the year-to-year variability in that ozone depletion are a mystery. They probably involve global interactions within the atmosphere that affect the south polar stratosphere. ``We don't even know how to set up computer models of that,'' Stolarski says.
Ozone is a chemical with three oxygen atoms per molecule. It forms in the stratosphere under action of sunlight between heights of about 15 and 50 kilometers (10 and 30 miles). There, it absorbs harmful solar ultraviolet ``light.''
This ultraviolet shield varies naturally from season to season, year to year, and decade to decade. Certain manmade chemicals - such as chlorofluorocarbons used in refrigerators or halons used in fire extinguishers - can reach the stratosphere and destroy ozone. There's some evidence for a slight worldwide ozone decline. But it's hard to know whether this is natural or due to pollution. On the other hand, there's little doubt of what these chemicals are doing in the Antarctic, where the seasonal ozone drop now is dramatic.
Winds blow so vigorously around Antarctica during the winter that they form a strong vortex. This cuts off the south polar stratosphere from the rest of the atmosphere. During the cold winter darkness, clouds form within that vortex. Pollutants condense on the cloud particles and undergo chemical changes that prepare them to speed ozone destruction when spring sunshine again warms the polar air.
In 1987, about half the ozone vanished over parts of Antarctica. The comparable loss in 1988 was only about 15 percent. This year, again, half or more of the ozone disappeared. Stolarski notes one difference from 1987. In that year, the south polar vortex with its ozone hole persisted through November. This year, it's already weakening.
Atmospheric scientists can't foresee the trend for Antarctic ozone. The hole this year is deep, but no worse than 1987. Stolarski says ozone loss occurs between heights of about 12 to 22 km rather than throughout the south polar stratosphere. Ozone in that 12 to 22 km layer was virtually depleted this year and in '87. So, he says, this may be as bad as the ozone hole can get. However, he adds, until year-to-year changes are understood, the longterm trend will be unclear. This is true for the ozone shield generally.
``It is clear that more needs to be learned about ozone and its variability before we can definitely establish a global trend,'' writes Charles Goddard in Environmental Science & Technology.