The cause of the seasonal thinning of the ozone shield over Antarctica is still elusive, but ongoing research is narrowing the list of suspects.

The discovery in 1985 of a "hole" in the atmospheric ozone layer near the South Pole has focused worldwide interest on what is happening to ozone, a small yet vitally important constituent of the earth's atmosphere. It has also raised concern about possible health effects, particularly an increase in the skin cancer rate.

A question dating back to the 1970 controversy about the effects of supersonic transport aircraft has resurfaced: To what extent are human activities producing ozone changes? Over the last fifteen years, the focus of investigation has shifted from one probable cause to another. Today, the emission of chlorofluorocarbons (CFCs) into the atmosphere has raised fears of their effect on the ozone layer and has led to demands that the production of these extremely useful chemicals be curtailed or even abolished.

Of course, the "hole" isn't really a hole at all, but a temporary thinning in the ozone layer between 10 and 25 kilometers above sea level. This phenomenon takes place for a few weeks each year, around October, in the region of the Antarctic. Research so far has discovered no long-term changes in ozone elsewhere, although the evidence is not conclusive. The ozone hole was discovered by scientists of the British Antarctic Survey operating an observation station on the Antarctic continent. After they reported their findings in 1985, NASA scientists searching their records of satellite data confirmed the effect. Indeed, the hole has been around since the mid-1970s, and getting larger every year, reaching a depletion of about 50 percent. Concern has centered both on the extent of the hole and on our inability to explain the reasons for its geographic and cyclic occurrence.

This was the situation in the summer of 1986, as a large scientific expedition set off to investigate the mystery. The National Ozone Expedition, which took measurements of the Antarctic stratosphere from July through October, found that the effect was present in October 1986, but was not as large as in 1985 the first indication that the hole may not continue to grow. Also, by tackling the problem simultaneously with ground observations, satellite observations, and balloon experiments, scientists for the first time had enough data to at least eliminate the theory that the hole was related to the 11-year solar cycle.

OZONE THEORY

Ozone is a small molecule composed of three atoms of oxygen. Its unique contribution to the welfare of our planet is its ability to absorb ultraviolet light. An invisible component of the sun's rays, ultraviolet radiation can damage human skin, causing skin cancers, and seriously harm other life as well. The presence of sufficient ozone in the atmosphere thus shields life on the earth's surface from this damaging and potentially deadly radiation.

The ozone layer was discovered in the upper atmosphere around 1880. Ground-based observers noted that ozone molecules effectively absorbed the solar near-ultraviolet radiation, that is, the portion of the ultraviolet spectrum adjacent to the visible part. By measuring the intensity of solar radiation at sea level, scientists have deduced the level of ozone overhead. By analyzing the data in more detail, they have even obtained a rough idea of the altitude distribution and deduced that the bulk of the ozone is located in the stratosphere.

Ozone is created by solar ultraviolet radiation from a further region of the ultraviolet spectrum. Since Polar stratospheric clouds hover over McMurdo Station in Antarctica site of the U.S. National Ozone Expedition' which conducted experiments in four teams between August and November 1986.

Ozone acts as an ultraviolet shield, most ozone is formed at the upper boundary of the ozone layer, where the incoming ultraviolet radiation is strongest. That radiation is powerful enough to split oxygen molecules into their two constituent oxygen atoms. Eventually an atom can find another oxygen molecule and form an ozone molecule. This is the ozone theory elaborated by the distinguished British geophysicist Sydney Chapman. He also described a destruction mechanism--solar ultraviolet radiation breaking the molecules apart--which keeps the ozone level in balance.

The earliest measurements of ozone distribution relative to altitude, carried out in Germany in the 1930s with balloon-borne instruments, supported the Chapman theory. After World War IT, U.S. research rockets extended the measurements to much higher altitudes The first such experiments were carried out by John Hopfield and Harold Clearman at the Applied Physics Laboratory of Johns Hopkins University around 1947(footnote). These and other, more refined experiments have shown that the Chapman theory is only approximately correct. The most important removal mechanism of stratospheric ozone is a chemical process rather than, the direct dissociation of the ozone molecule by solar ultraviolet radiation. Indeed, the main ozone depleters are nitrogen oxides, transformed in the stratosphere by solar radiation from naturally occurring nitrous oxide that percolates upward from bacterial sources on the earth's surface

The nitrogen oxide theory developed as part of another controversy more than a decade ago. In 1970' the United States had an ongoing program to construct two prototypes of a supersonic transport [SST] aircraft. But questions were raised about the environmental impact of a fleet of 500 SSTs, releasing their combustion products into the stratosphere. Attention first concentrated on the water vapor from the engine exhaust, and to what extent water would interact with ozone. The chemical reactions involving these molecules, in the presence of sunlight, were thought to lead to some destruction of ozone.

Concurrently, the emotional issue of skin cancer first gained public attention. Certain forms of Skin tumors, basal cell and squamous cell carcinomas have a much greater incidence at lower latitudes. They are more than twice as common in south Texas than in Minnesota--presumably because of the greater ultraviolet exposure received in Texas [see box]. With ozone weakened, more ultraviolet light would reach the earth's surface, causing more tumors. The skin cancer issue more than anything else persuaded Congress to cancel the SST program, even though the two prototypes were nearly complete.