Arctic ozone succumbs to chemical assault.

Arctic ozone succumbs to chemical assault

When a huge research expedition returned a year ago from studying the Arctic stratosphere, participants reported spotting an atmospheric murder weapon but no clear sign of a murder. They had measured very high levels of ozone-destroying chlorine chemicals, but without further analysis they couldn't say whether the chlorine had actually depleted the Arctic's ozone levels (SN: 2/25/89, p. 116).

Since then, however, scrutiny of the expedition data has revealed that chemicals did indeed destroy stratospheric ozone in some Arctic regions, according to a series of reports in a special issue of GEOPHYSICAL RESEARCH LETTERS released last week. And all the evidence points to chlorine as the culprit.

For six weeks in January and February of 1989, scientists probed the dark Arctic stratosphere with instrument-laden aircraft. The expedition followed an Antarctic project two years earlier that helped scientists identify how chlorine -- which comes largely from manufactured compounds called chlorofluorocarbons -- creates the dramatic ozone hole there each September.

When the Antarctic hole develops, ozone can disappear almost completely from some altitudes, so it's relatively easy to spot the loss. But Arctic depletions are more subtle. Mark R. Schoeberl of the NASA Goddard Space Flight Center in Greenbelt, Md., and his colleagues report that altitudes near 20 kilometers suffered an average loss of about 15 percent over a 35-day period during the Arctic mission. Using a different detection technique, Edward V. Browell from the NASA Langley Research Center in Hampton, Va., and his co-workers identified regions with losses of up to 17 percent.

Preliminary analyses suggested the Arctic stratosphere held the potential to destroy significant amounts of ozone. In the new reports, researchers describe finding chlorine monoxide at levels 100 times higher than those over the United States. Moreover, they say, the Arctic stratosphere lost much of its active nitrogen; at sufficient levels, active nitrogen prevents ozone destruction.

But several factors in the Arctic inhibit the development of a real ozone hole. Wind patterns isolate the Antarctic from the rest of the globe's stratosphere into mid-spring, giving activated chlorine plenty of time to eat up ozone with the help of sunlight. The Arctic stratosphere, however, gets invaded by warmer air during late winter, before significant sunlight reappears and energizes the destructive chemical reactions. When the fresh air breaks into the Arctic stratosphere during late February, it deactivates the chlorine molecules and shuts down the ozone depletion cycle. In addition, the Arctic stratosphere does not get cold enough to allow sufficiently widespread formation of the cloud particles that help activate chlorine molecules. Such factors currently prohibit massive ozone loss in the Arctic, but researchers warn that conditions could change.

While scientists came away from the expedition with a better understanding of the ozone destruction process, the mission also highlighted some important knowledge gaps. In particular, investigators still need to identify the process that pulls active nitrogen out the polar stratosphere.

In the Antarctic, researchers had found the atmosphere lacked both active nitrogen and water vapor. They reasoned that frozen water in polar regions coats small cloud particles made of frozen nitric acid, forming ice particles that fall out of the stratosphere after a few days. But the Arctic measurements refute this idea, because intense nitrogen loss occurred there without intense dehydration, reports David W. Fahey of the National Oceanic and Atmospheric Administration in Boulder, Colo., in the March 22 NATURE. This suggests that polar nitrogen gets bound into a large frozen particle through some other process that does not require much of the water to fall out of the stratosphere.