While representatives from some 40 nations have agreed, in principle, to curb production of chemicals that may threaten our planet's ozone shield, there is new doubt about the severity and imminence of the threat itself. Scientists with the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado at Boulder have found that volcanic dust in the stratosphere has confused satellite monitoring of the ozone layer.
In a statement issued by the university, LASP researcher Todd Clancy explained: ``[The instrument] has been seeing brightening that may be explained by sunlight scattered by aerosols. This brightening is currently being attributed to a large, global decrease in upper stratospheric ozone [above 40 kilometers or so].'' Dr. Clancy's colleague Bruce Jakosky adds, ``There are serious problems in measuring anything in the presence of aerosols.''
Stratosphere ozone absorbs biologically harmful ultraviolet solar radiation. The chemicals that threaten it belong to the chlorofluorocarbon (CFC) family. People use them as working fluid in air conditioners and refrigerators, in making plastic foams, and as propellants in spray cans. When CFCs escape and migrate to the stratosphere, they may take part in complex chemical reactions that lead to ozone destruction.
Some geochemists have thought they saw the beginnings of such destruction already. They're particularly concerned by what, in recent years, has seemed to be a year-to-year drop in ozone concentrations over the Antarctic in spring -- the so-called ozone ``hole.''
Such concerns, which seemed to confirm theoretical predictions of ozone destruction, underlay the consensus reached last week in Geneva on the need to freeze production of CFCs. The meeting was a follow-up to the March 1985 conference at which 28 countries signed the Vienna Convention for the Protection of the Ozone Layer. The representatives expect to meet again in February to consider an action plan.
However, there has been considerable scientific disagreement as to whether there is evidence of CFC-induced ozone destruction. The November issue of Geophysical Research Letters, which is devoted to Antarctica's ozone hole, reflects wide uncertainty. The editors concluded that ``no clear link between man-made pollutants and ozone depletion over the Antarctic has been established.''
Now the team from Colorado, which reported its findings at the fall meeting of the American Geophysical Union, is warning that failure to take volcanic dust into account has confused satellite-derived ozone data. The dust has been thrown up by such recent eruptions as that of Mexico's El Chichon, Mt. St. Helens in Washington state, and a Colombian volcano.
Under the circumstances, the Colorado researchers say, no one knows what's going on in the Antarctic stratosphere. ``Right now,'' observes team member Ron Thomas, ``there are more than enough theories to go around.''