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Rowland and others figured it was a combination of factors that made the ozone over Antarctica particularly vulnerable. First, the polar vortex collects CFCs that waft in from the industrialized world. Second, the superfrigid air of the Antarctic night causes clouds of tiny ice crystals to form high up in the stratosphere. When the CFCs break down, the resulting chemicals cling to the crystals, where they can decompose further into ClO, among other substances. And finally, when the sun rises after the long winter night, its light triggers a wholesale demolition of ozone by chlorine monoxide.
In Antarctica winds circulate unimpeded over the frozen landmass. In the north, though, the polar vortex is less well defined. Winds travel alternately over land and water, whose differing temperatures disrupt the smooth flow of air. The vortex wobbles and sometimes breaks up entirely. Moreover, the Arctic stratosphere is not as cold as that over the Antarctic, and ice clouds are less likely to form. So while scientists knew that some ozone destruction should take place, they presumed it would not be nearly as severe as the southern hole. A reanalysis of 10 years' worth of ground-based and satellite data, completed last year, revealed a relatively mild but widespread depletion over the northern hemisphere, with losses of 4% to 8% over much of the continental U.S.
When NASA's ex-spy plane, the ER-2, began a series of flights out of Bangor, Maine, in October, it quickly became clear that something strange was ( happening. For one thing, volcanic ash, lofted into the stratosphere from last year's Mount Pinatubo eruption, was evidently taking the place of ice crystals, giving CFC byproducts the platform they needed for their chemical reactions. Moreover, the scientists found that naturally occurring nitrogen oxides, compounds that tend to interfere with and slow down these reactions, were virtually gone from the atmosphere. Why? Besides enhancing the reactions that create ozone-destroying forms of chlorine, explains Susan Solomon, a chemist with the National Oceanic and Atmospheric Administration, "the volcanic aerosols provide a surface for chemical reactions that suppress nitrogen oxides."
Another flight that took off from Maine on Jan. 20 provided the clincher. The polar vortex had temporarily dipped as far south as Bangor -- "It was almost as if we were deployed over the North Pole," says geophysicist Darin Toohey of U.C.-Irvine -- just in time for the sensitive instruments on board to detect ClO in a world-record concentration of 1.5 parts per billion. Data from the Upper Atmosphere Research Satellite had already found comparable levels of ClO over Northern Europe, and the evidence pointed to a potential ozone loss of 1% to 2% a day.
Even with all these factors in place, there is still one element necessary before a certified ozone hole can form: the sun. If the polar vortex breaks up before the sun rises after months of darkness to trigger the reaction, there will be no hole this year. If the vortex holds together until late February or early March, keeping its brew of dust particles and chemicals intact, ozone levels will almost certainly drop. Says Harvard chemist James Anderson: "We are now protected only by the hope of a rapid breakup of this vortex." But even if the hole does not appear this spring, says Anderson, it will almost certainly appear within the next few years.