Antarctic ozone hole returns with a bang.

Antarctic ozone hole returns with a bang

'Tis the season for ozone depletion over the Antarctic, and this year's ozone hole is developing at record speed. Ozone has rapidly dwindled in the Antarctic stratosphere despite weather conditions resembling those that checked the growth of an ozone hole in 1988.

'Tis also the season for satellite setbacks, and NASA's main ozone-sensing instrument is threatening to die of old age before the agency can launch a replacement -- a situation that would drastically hamper efforts to monitor ozone loss over the entire globe.

The stratospheric ozone layer protects life on Earth by blocking out much of the sun's damaging ultraviolet radiation before it reaches the planet's surface. Every September since the late 1970s, the Antarctic has experienced a temporary but dramatic loss of ozone. The ozone hole develops because extremely cold conditions in the polar stratosphere help chlorine and bromine pollutants to destroy ozone through chemical processes.

Balloon measurements made above Antarctica's McMurdo Station indicate that stratospheric ozone levels this year have dropped as swiftly as they did in 1987 and 1989 -- two years when ozone reached its lowest recorded level (SN: 10/14/89, p.246), says David J. Hofmann of the National Oceanic and Atmospheric Administration in Boulder, Colo.

In early September, the rate of ozone loss exceeded even that of 1987 and 1989, according to measurements by the Total Ozone Mapping Spectrometer (TOMS) aboard NASA's Nimbus 7 satellite. "It's clear that the ozone is somewhat lower this year than it was last year at the same time," says Arlin J. Krueger of the NASA Goddard Space Flight Center in Greenbelt, Md. TOMS data show that the minimum ozone level over Antarctica plummeted from 200 Dobson units in mid-August to 133 Dobson units in mid-September.

Ozone losses normally continue through the early part of October. Scientists say they cannot predict whether this year's ozone layer will thin to the same extent it did in 1987 and 1989, when levels reached a minimum value of around 103 Dobson units.

The amount of ozone lost so far this year already exceeds the total lost in 1988, a year when several large weather patterns formed in the Southern Hemisphere and disturbed the Antarctic vortex -- a ring of winds circling the pole that seal off the Antarctic stratosphere. In 1988, the weakened vortex let in winds from the midlatitudes, which kept the stratosphere relatively warm and inhibited the development of a strong ozone hole.

Similar but weaker weather patterns developed this year, disrupting the Antarctic vortex to some degree in late August and early September. Because dramatic ozone losses occurred during the same period, scientists now wonder whether weak atmospheric disturbances might sometimes enhance the growth of ozone holes.

At the same time, many researchers express concern that the aging TOMS instrument may be approaching death's door. Launched in 1978 with a planned lifetime of two years, the monitoring device is plagued by a rotating wheel that occasionally drifts out of sync with other parts. The problem has affected TOMS on and off since 1984, but this summer "was by far the most severe episode," says Krueger.

Data collected during these episodes remain usable, but eventually the drift will worsen and TOMS will send only static back to Earth. NASA plans to launch the old design model for TOMS on a Soviet satellite in 1991 as a stopgap until a new instrument goes up in 1993. Researchers want to check the calibration of the current TOMS against another model in order to resolve questions about how much the global ozone layer has thinned since the late 1970s, but if the present instrument fails before a replacement arrives, the calibration opportunity will be lost.