The Ozone Hole: Sorting Out the Facts

by Forrest M. Mims III

In recent years, much attention has been given to the "ozone hole" over Antarctica. This phenomenon is observed each year in October during the Antarctic spring. After several weeks, the Antarctic vortex, a whirling weather system that encircles and isolates the South Pole during winter, breaks up and ozone levels rapidly rise.

In meterological terms, the Antarctic ozone hole is a significant ozone minimum and not a literal "hole" through the entire ozone layer. Nevertheless, for a brief time ozone levels within the hole can plummet to 100 DU. (Normal levels are about 300 DU.) At the same time, the ozone levels in a broad belt encircling the hole are the highest on earth.

Because various scientific studies have concluded that the ozone hole is caused in part by chlorine believed to come from manufactured chemicals–especially CFCs–some scientists, politicians, and government agencies have sounded an alarm about the prospect of severe ozone depletion leading to ozone holes elsewhere. In a widely publicized statement two years ago, then-Sen. Albert Gore raised the possiblity that an ozone hole might appear over Kennebunkport, Maine. Although a prominent NASA scientist discounted this possibility, other scientists held a press conference to express alarm about possible serious ozone depletion over the Arctic. Developments like these led to many scary reports in the media.

Fortunately, the Antarctic hole is a phenomenon preceded by the very cold temperatures and darkness found inside the winter Antarctic vortex, which is much stronger than the Arctic vortex.

Long before the ozone hole was identified in 1985, G.M.B. Dobson, inventor of the Dobson spectrophotometer, discovered something very different about Antarctic ozone. In a paper titled "Forty Years' Research on Atmospheric Ozone at Oxford: A History" (Applied Optics, March 1968), Dobson described an ozone monitoring program that began at Halley Bay, Antarctica, in 1956.

When the data began to arrive, "the values in September and October 1956 were about 150 [Dobson] units lower than expected. . . . In November the ozone values suddenly jumped up to those expected. . . . It was not until a year later, when the same type of annual variation was repeated, that we realized that the early results were indeed correct and that Halley Bay showed a most interesting difference from other parts of the world."

The ozone decline reported by Dobson was not nearly as severe as the one that characterizes the Antarctic ozone hole today. However, two scientists who reviewed old ozone records recently reported that the ozone amount in the spring of 1958 fell to only 110 DU at the French Antarctic Observatory at Dumont d'Urville.

In Annales Geophysicae (November, 1990), P. Rigaud and B. Leroy observed that in 1958, the Antarctic vortex, where the most significant ozone depletion occurs, was centered over Dumont d'Urville, on the opposite side of the South Pole from Halley Bay. They reported that the concentration of CFCs in the atmosphere in 1958 was much lower than it is today and concluded that natural phenomena, such as volcanic aerosols in the stratosphere, may also lead to ozone destruction.

Writing in a recent issue of Science, however NASA scientist Paul Newman has convincingly refuted Rigaud's and Leroy's ozone measurement methods. He pointed out that Rigaud and Leroy relied on photographic plates, an unreliable method for measuring stratospheric ozone.

Meanwhile, the chemistry and dynamics of the atmosphere inside the Antarctic and Arctic vortices remain the subjects of extensive research using various kinds of ground-based instruments, instrumented balloons, high-flying aircraft, and satellites.

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File Date: 5/30/95
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