Antarctic ozone hole expands in altitude.

Atmospheric pollution over Antarctica has pushed its assault on the ozone layer to new heights this year, causing a subtle worsening of the so-called ozone hole over the southern continent.

The zone of total ozone destruction, which had reached from 14 to 19 kilometers in altitude, crept up to 20.5 km this month, reports David Hofmann of the National Oceanic and Atmospheric Administration in Boulder, Colo. The shift may have resulted from increased chlorine- and bromine-containing pollution in the atmosphere, he says. Fluctuations in atmospheric temperatures may also have contributed to the rise.

The ozone hole develops in the extremely cold Antarctic stratosphere each August, when sunlight returns after the darkness of the polar winter. The light drives reactions in which chemical pollutants rapidly destroy ozone, usually reaching a peak during the first 2 weeks of October. During the next 2 months, the hole fills in over Antarctica, and the ozone-depleted air spreads throughout the Southern Hemisphere.

In the past, ozone at an altitude of 20 km has escaped total destruction. This year, however, balloons launched from the South Pole in early October could not detect any ozone at that altitude.

While the vertical extension of the hole was big enough to catch the attention of scientists, it had little effect on the total concentration of ozone in the sky above Antarctica, says Hofmann. The minimum value recorded this year, 112 Dobson units. matched the minimum of the last few years. The record low, 91 Dobson units, came in 1993, when volcanic acid particles from the 1991 eruption of Mount Pinatubo enhanced ozone loss in the Antarctic.

Satellite measurements confirm the balloon data. In terms of size and severity, the 1997 hole roughly equals last year's, according to Paul Newman of NASA's Goddard Space Flight Center in Greenbelt, Md. "This wasn't an outstanding ozone hole year," he says.

International limits on ozone-destroying chemicals have sharply slowed the release of additional chlorine and bromine into the atmosphere. Concentrations of such chemicals have already peaked in the lower atmosphere, but they are still increasing in the stratosphere: it takes 5 years for air to drift up that far.

Calculations suggest that the stratospheric concentrations should level off by the year 2000 and begin to decline sometime around 2010. Scientists will monitor the yearly ozone hole for signs of ozone's recovery.

Hofmann proposes that the uppermost altitude of total ozone destruction provides a sensitive measure of the atmosphere's health. "The extension at the top, which we have seen this year, is significant. This is where we can look for recovery in the next 10 to 15 years to see if the top starts coming back down."

Another indicator may be the average area of the hole, which expanded dramatically in the late 1980s and early 1990s but has remained constant recently, says Newman.