Winter ozone gap detected over the Arctic.

Winter ozone gap detected over the Arctic

Balloon-borne instruments revealed a thin region in the Arctic ozone layer last January--a pattern suggesting the possible birth of a small, transient ozone hole there, according to a new report. Scientists maintain, though, that the Arctic will not suffer the same sweeping ozone loss that afflicts the Antarctic stratosphere each year.

Scores of atmospheric scientists traveled to Norway last winter to assess the threat to Arctic ozone (SN: 2/25/89, p.116). On airplanes flying through dark northern skies, they detected high stratospheric levels of the same chlorine compounds that eat away at ozone in the Antarctic. They described the Arctic atmosphere as primed to destroy ozone and waiting for spring sunlight to combine with chlorine compounds to fuel a catalytic cycle. But they left open the question of how much ozone has been destroyed by the chlorine, which comes largely from human-made chlorofuorocarbons.

Filling in some of the details, a separate team now reports the results of concurrent balloon investigations from Sweden. The balloons carried sensors that measured ozone levels and counted cloud particles. According to the leading theory for ozone destruction, stratospheric cloud particles play a crucial role by activating chlorine compounds to break apart ozone molecules.

The measurements revealed a peculiar gap in the Arctic ozone layer, say David J. Hofmann of the University of Wyoming in Laramie and his colleagues in the July 13 NATURE. On Jan. 23, they observed that the ozone concentration rose relatively steadily with altitude from 10 to 22 kilo-meters. But the concentration dropped as the balloon ascended beyond about 22 km. It started to rise again after 24 km, resuming a more normal pattern of increase at about 26 km. The particle instrument detected far more cloud particles in the low-ozone region than in adjacent areas.

"This looks very familiar," says Hofmann, who has observed September ozone losses in the Antarctic for several years. The apparent thinning in Arctic ozone between about 22 and 26 km closely resembles a pattern signaling the onset of the Antarctic ozone hole, he says.

Early each September, ozone over Antarctica starts disappearing at about 22 km as the spring sun energizes the destructive chemical reactions. The reactions migrate downward as days get longer and the sunlight reaches farther into the polar vortex -- a wind pattern circling the polar regions during winter.

In the Antarctic, the vortex persists into springtime and the destruction process removes a substantial fraction of the ozone from the stratosphere. In 1987, half the Antarctic ozone layer disappeared. In the Arctic, though, the stratosphere is less stable and invading winds can rip apart the vortex several times during winter. With the vortex breached, temperatures remain too high to form stratospheric cloud particles, and the chlorine chemicals get dispersed.

The similarity in ozone profiles from both poles leads Hofmann to suggest the Arctic finding represents the beginning of an ozone hole that never fully developed. However, he says, there's a slim chance the gap is a normal feature of the Arctic stratosphere.

Atmospheric chemist Susan Solomon of the National Oceanic and Atmospheric Administration in Boulder, Colo., agrees the case is not closed. "I think Hofmann's probably right, but it's just not tight enough yet to say," she comments.

If chemicals are thinning Arctic ozone, the vortex instability should keep levels there from dropping as severely as in the Antarctic. Hofmann calculates that last winter's ozone gap represented a 25 percent drop from expected levels at altitudes of 22 to 26 km. This may seem a big drop, but that portion of the stratosphere does not normally hold much ozone. Overall, stratospheric ozone fell short of expected levels by only 3 percent, he says.