EARTH'S protective ozone layer, which absorbs the sun's ultraviolet rays, has been thinner than usual this past winter and early spring.
A National Aeronautics and Space Administration (NASA) study finds that globally averaged stratospheric ozone concentration was as much as 2 percent to 3 percent below normal during these seasons. For middle latitudes in the northern hemisphere, where much of the world's population lives, average concentrations ran as low as 11 percent to 12 percent below normal. They have dipped to a little over 20 percent below normal in Arctic regions above 60 degrees north latitude this spring.
These are record seasonal lows for stratospheric concentrations over the northern hemisphere. As such, they catch the eye of scientists concerned about the effect of pollution by ozone-destroying chemicals - such as the chlorofluorocarbons (CFCs) used as refrigerants.
At the same time, it's important to understand that "this is not an Arctic ozone hole," says James Gleason, lead author of the paper reporting the study in the current issue of Science magazine. Dr. Gleason, an atmospheric chemist with the nonprofit University Space Research Association, works as a contractor at the NASA Goddard Space Flight Center in Greenbelt, Md.
Ozone hole is the term given to the massive ozone loss over the Antarctic during the southern hemisphere spring in August, September, and October. This is a serious ozone depletion. It is an ozone low that is attributed to attack by man-made chemicals, principally CFCs.
Gleason explains that, in contrast, what have appeared over the northern hemisphere during the winter and early spring are lower-than-normal ozone highs. There has been no potentially dangerous ozone depletion. Time of annual highs
This is a time of year when both northern hemisphere and global stratospheric ozone concentrations reach their annual highs.
Scientists' estimates of what those concentrations normally should be are based on data from NASA's Nimbus 7 satellite. It has been tracking stratospheric ozone concentrations worldwide since its launch in 1978. Independent measurements with ground-based instruments have confirmed the satellite's unusually low readings for December through March.
Atmospheric chemists are concerned about such trends even though there has been no potentially dangerous ozone loss. They don't fully understand what is going on over the most heavily populated hemisphere. They don't know to what degree the ozone-level fluctuations are natural and to what extent they are due to chemical pollution.
Aerosol particles from the eruption of Mount Pinatubo in the Phillipines also have played a role. They provide reaction sites for chemistry involved in ozone destruction.
Atmospheric chemist Mario Molina at the Massachusetts Institute of Technology says the current low ozone readings are "more or less in agreement with our expectations." He explains that the ozone region has been unusually cold. This would lead to lower ozone concentrations. Pinatubo particles may also be involved, but their influence is fading out now.
Dr. Molina - one of the scientists who first called attention the CFCs' ozone-destroying potential - says he thinks seasonal ozone levels will likely recover next year. Gleason notes that ozone levels were low going ino the winter partly because of the effect of volcanic particles last summer. Hemispheric differences
Stratospheric ozone behaves differently in the northern and southern hemispheres. Average southern hemisphere ozone concentrations have not been as low over the winter and early spring as those in the northern hemisphere. They have been in the lower part of their normal seasonal range.
To illustrate what's been happening over the northern hemisphere, Gleason cites the ozone-concentration average for March over middle latitudes - 30 degrees to 60 degrees north. That's a region extending roughly from New Orleans to Hudson's Bay and from North Africa to north of Scotland. The normal March average for that large globe-girdling region is 375 Dobson units - the standard ozone concentration unit of measurement. For March, it hit a low of 335 Dobson units - nearly 11 percent below normal.
Under international agreement, production of CFCs and related ozone-destroying chemicals should phase out rapidly in this decade. The United States will cease CFC production in 1995. The pollution should peak in the stratosphere in the next 10 to 15 years.
Molina says that "by the middle of the next century, we expect [the ozone shield] will begin to recover, if the regulations work as expected." But, he adds, "it will be slow."