Ozone Destruction Closer to Home

The finger of suspicion pointed to the CFCs a year ago when an airborne expedition probing the Arctic stratosphere found an abundance of ozone-destroying chlorine from CFCs (Science, 24 February 1989, p. 1007). But expedition researchers had their hands full showing that the chlorine was actually destroying the Arctic ozone. Now it may have finally been caught in the act.

The first direct evidence against CFCs as the culprit comes in a series of papers published in this month's Geophysical Research Letters, a special issue devoted to results from last year's expedition. As summarized in a prologue by atmospheric physicist Richard Turco of the University of California, Los Angeles, and others, the papers show that "the initial phases of a widespread ozone depletion apparently were observed."

The Arctic losses are a far cry trom those seen every October in the Antarctic ozone hole, however. In the Antarctic, as much as half of all the stratosphere's ozone has been destroyed in some years, with the losses reaching more than 95% at some altitudes. Over the Arctic, total ozone destruction probably did not exceed a tew percent and the hardest hit layers, those at and just above 20 kilometers, might have lost only 15 to 20% of their ozone.

Pinning down such small losses was not easy. For example, during the 39 days when the 1989 Airborne Arctic Stratospheric Expedition, as it is officially known, was collecting data, ozone concentrations within the 3000-kilometer-wide vortex of winds that swirl around the Arctic stratosphere actually increased below 20 kilometers as high-altitude, ozone-rich air sank into the vortex. So the challenge was to see whether the ozone increase was less than it should have been.

Participants in the Arctic expedition approached this problem by using the concentration of nitrous oxide, a relatively stable gas, as a benchmark against which to measure any loss of ozone. By determining the relative ozone and nitrous oxide concentrations, a multi-institutional group headed by atmospheric physicist Mark Schoeberl of NASA's Goddard Space Flight Center in Greenbelt, Maryland, found an ozone loss at around 20 kilometers of 15 + 10% (95% confidence limits) during the expedition.

There was another detection of apparent ozone destruction, this one by a group headed by Edward Browell of NASA's Langley Research Center in Hampton, Virginia. Browell's group detected two patches of air that had up to 17% less ozone than the surrounding air. The nitrous oxide data again seemed to require chemical destruction of ozone. In addition, the altitude range of the patches, 17 to 23 kilometers, coincided with that of the polar stratospheric clouds. In the Antarctic ozone hole, these icy clouds catalyze the production of ozone-destroying chlorine. The coincidence of ozone loss and clouds in the Arctic implies the same may be happening there.

Despite this evidence, there are still doubters, but the link between CFCs and ozone loss was buttressed by calculations by three different groups of the amount of ozone that should have disappeared during the expedition, given the chemical state of the atmosphere at its start. Although the groups all used different modeling approaches, the results agreed "quite well with the limited observations of actual Arctic ozone variations," note the authors of the special issue's prologue.

Now that new evidence has been found that CFC-derived chlorine is destroying ozone within the Arctic vortex, the next step is to find out whether the Arctic vortex is exporting ozone-depleted air and ozone-destroying chlorine to the mid-latitudes. Such atmospheric transport could be behind the decreased wintertime ozone there. Making this connection will be particularly difficult, however, and politicians considering whether to decrease ozone destruction by further reductions of CFC emissions will probably have to settle for less than a perfect chain of cause and effect.

Kerr, Richard A., 1990: Ozone destruction closer to home. Research News, March 1990, pg 1297.