Antarctic ozone hole unexpectedly severe.

Antartic ozone hole unexpectedly severe

Only two years after the worst ozone hole on record, nature has repeated itslef. Ozone concentrations in the Antarctic stratosphere last week dropped to levels on a par with the severe depletions of 1987, contradicting some scientists' claims that the drastic ozone hole two years ago was a fluke. The ozone loss also confounds theories that predicted a relatively weak Antarctic hole for 1989.

"We weren't expecting it to be as severe as 1987. That was a surprise," says David J. Hofmann, a physicist from the University of Wyoming in Laramie who monitored the latest ozone los with balloon-borne instruments. "This year and 1987 were close to as bad as you can get," Hofmann told SCIENCE NEWS in a telephone interview from Christchurch, New Zealand.

The term "ozone hole" describes a loss of stratospheric ozone that has occurred over Antarctica each September and October since the late 1970s. Ozone in the stratosphere protects life by absorbing damaging ultraviolet radiation from the sun. In the last four years, atmosphereic scientists have accumulated significant evidence that industrially produced chlorofluorocarbons and related compounds cause the annual Antarctic ozone hole. These widely used compounds also eat away at the global ozone layer, through at a slower rate (SN: 9/2/89, p. 154).

Over the Antarctic, the region of ozone loss begins at an altitude of 12 kilometers and extends to 23 km. This year, ozone almost disappeared completely from certain stratospheric layers. "We're approaching total depletion in the 16-to-18-km range. In that slab we've gone from 50 Dobson units [of ozone] at the end of August to something like 5 Dobson units," Hofmann says.

In bad years such as 1987 and 1989, the region of ozone depletion covers almost twice the area of the Antarctic continent. In 1987, the average amount of ozone in this region fell from around 250 Dobson units in winter to 147 Dobson units on Oct. 5. In 1989, the average dropped to 150 Dobson units on Oct. 3 -- a level indistinguisable from the 1987 record, according to Arlin J. Krueger at the NASA Goddard Space Flight Center in Greenbelt, Md., who tracks the development of the ozone hole with instruments aboard a polar-orbiting satellite.

After 1987, some scientists suggested the conditions that year were abnormal and predicted the Antarctic would not experience such severe ozone loss for perhaps a decade or more, says Goddard's Mark R. Schoeberl. "But it looks like right now we're right back at 1987. It suggests the deep ozone hole phenomenon is more robust than has previously been though."

In both years, the stratosphere in the midlatitudes of the Southern Hemisphere remained extremely stable during winter and early spring. The quiet conditions allowed a strong vortex of stratospheric winds to encircle the polar region, essentially isolating the Antarctic stratosphere from the air in the midlatitudes. During the months of winter darkness, stratospheric temperatures inside the vortex dropped to around -85[degrees]C, at which point nitric acid and water condensed to form cloud particles. Scientists think the particles play a critical role inthe chemical reactions that destroy ozone (SN: 10/15/88, p. 249).

Earlier this year, atmospheric researchers thought high-pressure systems would develop during late winter in the midlatitude stratosphere and disrupt the polar vortex, leading to a relatively mild ozone hole. They based the prediction on an apparent correlation over the last few years between midlatitude activity and stratospheric winds in the tropics.

The tropical winds circle the globe, switching direction about every 13 months. In the past, the midlatitude conditions have remained quiet when the winds traveled from west to east. When wind moved in the opposite direction, pressure systems grew in the midlatitudes and weakened the ozone hole.

During the Antarctic winter of 1988, the tropical winds shifted and started flowing toward the west. In keeping with the pattern, the ozone hole was weak. Because the winds this winter still traveled toward the west, scientists expected another milk ozone hole. But the midlatitude stratosphere defied predictions and remained stable, permitting a strong vortext to form. Scientists will now have to look more closely at the processes that control midlatitude conditions, says Schoeberl.