Why the Ozone Hole?
Why did the ozone hole develop over Antarctica, and not over Detroit or some other manufacturing center where chlorofluorocarbons, or CFCs, are released prodigiously? The reasons are explained by Rebecca L. Johnson, who participated in NSF's Antarctic Artists and Writers Program in 1991, 1994, and 1997.
In winter, the stratosphere above the Antarctic continent gets colder than it does anywhere else on Earth. Temperatures frequently drop below -112ºF. Antarctica is also one of the windiest places on Earth. In May and June, strong winds in the stratosphere begin to blow clockwise around the continent. These howling stratospheric winds gradually form an enormous ring of moving air, called the Antarctic polar vortex, that swirls around and around, far above the frozen land
During the winter, temperatures inside the Antarctic polar vortex fall so low that water vapor and several other types of molecules in the stratosphere condense into extremely small icy particles. These icy particles, in turn, make up polar stratospheric clouds (PSCs). When the sun sets in the Antarctic around the end of March each year, its disappearance marks the beginning of a long, dark winter. Once the last rays of sunlight have faded away, temperatures on land and in the air fall very quickly.
In the stratosphere, high-altitude winds that create the polar vortex begin to blow around the continent. Isolated from warmer air outside the vortex, the air inside gets colder and colder. Eventually, it is cold enough for PSCs to form. And that is when the trouble really begins.
Drifting around inside the polar vortex are reservoir molecules that have bonded with chlorine atoms and in so doing prevented themso farfrom attacking ozone. When PSCs form above Antarctica, chlorine reservoir molecules bind to the icy particles that make up the clouds. Once this happens, complex chemical reactions begin to take place that result in molecules of chlorine gas (Cl) being released from the reservoirs. In this form, however, chlorine doesn't attack ozone. It just collects inside the vortex. All through the long, dark winter, especially during July and August, the chemical reactions taking place on the surfaces of the PSC particles continue, and more and more Cl builds up inside the vortex. At this point, the stage is set for ozone destruction. All that is needed is a trigger to get the process going.
That trigger comes in late August, when the sun begins to rise. As the first rays of spring sunlight strike the stratosphere high over the frozen continent, conditions change very rapidly. The UV rays coming from the sun strike the Cl molecules inside the vortex. The molecules break apart, releasing billions of chlorine atoms that begin an attack on ozone molecules. The result is massive ozone destruction. Before long, so much ozone is destroyed inside the vortex that an ozone hole is formed.
Ozone destruction continuesand the hole remainsuntil conditions in the stratosphere above Antarctica change. This change usually begins in early October, when the continent and the air above it finally begin to warm up. Warmer temperatures in the stratosphere melt the icy particles that make up PSCs. The PSCs disappear, and the reservoir molecules that were bound to the icy particles are released. Free at last, the reservoir molecules bind Cl atoms once again, and ozone destruction stops.
By early November, the strong stratospheric winds circling Antarctica die down, and the polar vortex breaks up. As it does, ozone-rich air from outside the vortex flows in, and much of the ozone that was destroyed is replaced. In a sense, the hole in the ozone layer fills in. Usually by the end of November, the amount of ozone in the stratosphere over Antarctica has almost returned to normal. The next winter, however, the cycle will begin again.
From Investigating the Ozone Hole by Rebecca L. Johnson © 1993 by Lerner Publications Company. Used by permission of the publisher. All rights reserved. Ms. Johnson is also the author of Science on the Ice: An Antarctic Journal (1995) and Braving the Frozen Frontier: Women Working in Antarctica (1997).