As the ozone-hole season unfolds in Antarctica, a fleet of new satellites is watching over Earth's ultraviolet-absorbing shield.
This summer, the United States and Japan orbited instruments that complement an ozone probe working since January on Europe's Earth observing ERS-2. It's an unprecedented international effort to learn what exactly is going on in the stratosphere's chemical cauldron, where ozone is formed and destroyed.
Scientists have gained this hawk-like view just in time to catch this year's complete South Pole ozone cycle, which runs through the spring and summer. David Hofmann, with the National Oceanic and Atmospheric Administration climate monitoring unit in Boulder, Colo., says changes in air circulation and some increases in ozone-eating chemicals should result in a little deeper ozone hole than last year. But he notes that, because of restrictions on their use, concentrations of ozone-destroying chemicals in the stratosphere are beginning to peak. He expects year-to-year differences in the seasonal ozone loss to be relatively minor from now on.
Dr. Hofmann explains that "in order to detect the ozone layer's recovery, we're going to have to keep track of these small changes." He adds that the new satellites will be helpful in doing that.
Scientists will likewise have a more comprehensive view of what happens to the ozone layer in the Arctic during next year's northern spring. The 20 to 40 percent ozone drop in the Arctic in recent years is less severe than the 90 percent loss in the Antarctic. Nevertheless, Neil Farris of the European Ozone Research Coordinating Unit in Cambridge, England, reported that record-setting losses last spring "caused serious ozone depletion [even] in mid-latitudes, including Britain."
There's more going on in the stratosphere than the ozone-eating chemistry scientists have already uncovered. It is clear that chlorine from chlorofluorocarbon (CFC) refrigerants and bromine from fire extinguisher halon compounds and methyl bromide pesticides attack the ozone. Traces of a variety of substances, however, join in the destructive chemical dance. Their poorly known interactions determine the details of what happens between 10 and 50 kilometers (6 and 30 miles) above the earth's surface where the ozone lies. The new instruments in orbit will help scientists decipher the choreography of that dance.
Take the instrument called GOME on the European Space Agency's ERS-2. It can pick up mere whiffs of chemicals with concentrations of just a few parts per trillion. Paul Crutzen, director of the Max Planck Institute for Chemistry in Mainz, Germany, - who shared the 1995 Nobel prize for the original CFC work - is enthusiastic about this new ability to get at the nitty-gritty of ozone destruction. "It is finally possible to measure on a global scale not only ozone values but the trace gases that make them fluctuate," he says.
The ultimate danger of a thinning ozone layer is the increase in the amount of biologically damaging ultraviolet radiation (called UV-B) from the sun that a thinning ozone shield lets through. Jay Herman at the Goddard Space Flight Center in Greenbelt, Md., has found a way to use ozone monitoring information to track this. Data from the TOMS (Total Ozone Mapping Spectrometer) flown on the US National Aeronautics and Space Administration's (NASA) defunct Nimbus-7 satellite show evidence of regional UV-B increases.
From 1979 to 1992, Dr. Herman discovered that average annual UV-B exposure rose 8 percent per decade at 55 degrees north latitude and by 9.9 percent per decade at 55 degrees south latitude. Major populations in England, Germany, Russia, and Scandinavia as well as southern portions of Argentina and Chile live in those regions. UV-B changes are smaller over North America below 55 degrees latitude. Near the Canadian border, they show about a 4 percent increase per decade.
This finding indicates that a widespread ozone layer thinning, which the TOMS data indicate, is beginning to have an effect at the planet's surface. Thus, Robert Harriss, director of NASA's Mission to Planet Earth office in Washington, says that the most important upshot of the study is that it shows information gathered to monitor ozone concentrations can be used to track "long-term global surface ultraviolet radiation levels." The new satellites ensure that scientists will have that ability.
Nimbus-7 was shut down in May 1993. A second TOMS on Russia's Meteor-3 satellite died in December 1994. The agency is preparing another TOMS for flight on a Russian spacecraft in 2000. But there will be no gap in coverage now. On July 2, a Pegasus rocket launched the first satellite dedicated specifically to ozone-layer research. This satellite - called the TOMS Earth Probe - now has returned its early images. These are being processed to show both the global ozone distribution and the global pattern of UV-B reaching the surface.
Meanwhile in mid-August, Japan sent up its multipurpose Advanced Earth Observing Satellite (ADEOS). Besides carrying a NASA TOMS, it has two other instruments to measure ozone. While TOMS maps global ozone distribution, one of the Japanese instruments tracks how those concentrations vary with altitude over the north and south polar regions. The third instrument will trace less detailed ozone profiles elsewhere. Taken together, these data should provide the most detailed 3-D picture of the ozone shield yet available.
Government officials as well as scientists need to know stratospheric chemistry at this level of detail. Many nations consider the present incomplete knowledge persuasive enough to justify restricting ozone-eating chemicals. To know how strictly to enforce these restrictions on the bromine chemicals for which there are no presently approved substitutes, governments need more realistic assessments of the dangers than they now have. The new satellites will help provide that realism.