How do scientists measure the thickness of the ozone layer in the atmosphere?

Concern about the depletion of atmospheric ozone really took off in the mid-eighties when scientists of the British Antarctic Survey first detected an ozone “hole” over their research station on Halley Bay.

They found that the springtime quantity of ozone over Antarctica had decreased by more than 40 percent between 1977 and 1984. The “hole,” or area of extreme ozone depletion, was even wider than Antarctica and, extending from about seven and a half miles above earth to a little over fifteen miles above, spanned most of the lower stratosphere where atmospheric ozone is concentrated.

Ozone is simply a form of oxygen with a slightly different chemical composition: it comprises three atoms (03) rather than two (02). Ironically, this substance is toxic to human beings and other animals when it occurs at the lowest levels of the atmosphere.

In the stratosphere, however, ozone is essential to maintain life of any kind on Earth. The ozone layer not only protects human beings from skin cancer by screening out harmful ultraviolet radiation, but it also protects the most elementary organisms in the food chain (bacteria, algae, protozoa) from lethal damage to their genes.

The largest concentration of ozone is found in the lower stratosphere, from about nine and a half to nineteen miles above earth (fifteen to thirty kilometers), where it attains more than a thousand times the normal peak concentration in the air we breathe. Chemicals that dissociate ozone to oxygen by knocking off its extra molecule, namely nitrogen oxides and chlorine, each atom of which has the ability to destroy multitudes of ozone molecules, can remain in the stratosphere for years, devastating the ozone layer.

How have scientists learned so much about the ozone layer? Basically, through instruments on the ground, in balloons, in airplanes, and on orbiting satellites.

Ground-based equipment measures solar radiation at slightly different wavelengths, some of which are known to be readily absorbed by ozone, and some of which are known to pass through it. If, over time, scientists find there are more of the wavelengths that can be absorbed by ozone than the kind that cannot, they deduce that ozone is decreasing. Conversely, if absorbable-wavelength radiation decreases, ozone has increased. From such measurements the thickness of the ozone layer is calculated.

In 1987 a map of the ozone cover and hole over the Antarctic was created on the basis of data collected on board NASA’s Nimbus 7 satellite by the Total Ozone Mapping Spectrometer (TOMS).

At the same time, 150 scientists drawn from around the world participated in the Airborne Antarctic Ozone Experiment in which specially outfitted, medium-altitude, long-range DC-8 and high-altitude NASA-ER2 airplanes measured the ozone-depleted region’s size and chemistry. In addition to the renowned “hole,” they found more widespread ozone loss over the Southern Hemisphere and markedly higher amounts of chlorine monoxide (a constituent element of the chlorofluorocarbons, CFCs, that are believed to be major culprits in ozone destruction) than those at other latitudes.

They also found severely depressed levels of nitrogen oxides that can protect ozone by hindering chlorine’s ability to attack it. A polar-orbiting satellite continues to track the development of the ozone hole over time, relaying its information to the Goddard Space Flight Center in Greenbelt, Maryland.

Long-term records show that ozone levels over the Arctic icecap have dropped roughly 5 to 6 percent over the last seventeen years. Recently a hole has been observed over the most northern latitudes in the spring.

A team of one hundred scientists, with over $10 million funding supplied by NASA and NOAA (National Oceanic and Atmospheric Administration), has just completed a study of this hole and the role of man-made CFCs in causing it. With the aid of airplane- and balloon-borne instruments, the research team found that patches of stratospheric air contained up to fifty times the normal amounts of CFCs’ chemical constituents and a significant absence of the protective active nitrogen.

On the basis of the vast bodies of data that have been painstakingly accumulated by scientists, NASA experts now predict that the hole in the Antarctic ozone layer could repair itself by the year 2100. They stipulate, however, that for such repair to occur there would have to be a worldwide phaseout of practically all emissions of CFCs and halon gases by the year 2000.

Although the European Community countries are fully behind this and the United States almost as much so, several large industrializing nations have yet to commit themselves.