Antarctic ozone hole sinks to a record low.

Atmospheric pollutants have again left their mark in the skies over Antarctica as this year's ozone hole reached maturity last week. The atmospheric concentration of ozone dwindled to its lowest recorded level, according to satellite measurements. The dramatic ozone depletion marks the third year in a row that a severe ozone hole has developed over Antarctica, in contrast to the 1980s when major holes formed only in alternate years.

"Three years in a row of low ozone leads one to wonder that maybe most years will be low in the future," says Charles H. Jackman of NASA's Goddard Space Flight Center in Greenbelt, Md.

The term ozone hole refers to a marked reduction in the number of stratospheric ozone molecules. Such a hole has formed over Antarctica each September since the late 1970s. Energized by the first faint rays of sunlight in the austral spring, chlorine chemicals in the atmosphere destroy ozone in a layer between 12 and 23 kilometers in altitude.

Although winds from the north replenish the lost Antarctic ozone in November and December, the ozone hole's growth each year may be contributing to a general thinning of the global ozone layer, which satellite and ground-based instruments have detected over the past decade. Ozone blocks out harmful ultra-violet radiation from the sun, and scientists say the weakening of this shield will increase the risk of skin cancer in humans and will harm animals and plants.

On Oct. 6, the concentration of ozone over parts of Antarctica dropped to 110 Dobson units, beating a record of 121 Dobson units, set in 1987, says NASA's Arlin Krueger. The measurements were made by the Total Ozone Mapping Spectrometer on board the Nimbus-7 satellite. Normally, ozone levels for this time of year would be about 225 Dobson units, Krueger says.

This year's ozone depletion signals a clear departure from a pattern established in the 1980s, when severe ozone holes developed only on odd-numbered years. Weaker ozone holes appeared on the alternate years because the polar atmosphere turned more turbulent than normal, allowing warm winds from the north to penetrate into the polar region. In contrast, severe ozone loss occurred when the Antarctic atmosphere remained stable and cold during winter. After two strong ozone holes in 1989 and 1990, some scientists thought 1991 might bring a weaker hole, but the atmosphere remained cold and stable.

Jackman speculates that the intense ozone loss of the last few years may account for the development of a major depletion this year. Such a feedback could occur because ozone destruction helps maintain the cold polar temperatures needed to form the ozone hole. Normally, when sunlight returns to Antarctica in the springtime, ozone in the polar stratosphere absorbs the light and warms the upper atmosphere. But when half the ozone has disappeared from the polar region, the stratosphere stays much colder than normal even into the next year -- a condition that could lead to severe ozone loss year after year, Jackman says.

"I don't think it's a good indication for ozone in general," he adds.

Balloon measurements made at the South Pole reveal an unusual character in this year's ozone depletion, one not seen in previous years, says Samuel J. Oltmans of the National Oceanic and Atmospheric Administration in Boulder, Colo. Along with the familiar pattern of loss in the lower stratosphere, ozone also disappeared from a layer between 27 and 29 km in altitude. The significance of the upper thinning remains unclear, says Oltmans, who is now attempting to determine when it first appeared.