Ques : Why is the hole in the Antarctic?
This was a mystery when the hole was first observed, but it is now well understood. brief survey of the present theory, and refer the reader to two excellent nontechnical articles [Toon and Turco] [Hamill and Toon] for a more comprehensive discussion. Briefly, the unusual physics and chemistry of the Antarctic stratosphere allows the inactive chlorine "reservoir" compounds to be converted into ozone- estroying chlorine radicals. While there is no more chlorine over antarctica than anywhere else, in the antarctic spring most of the chlorine is in a form that can destroy ozone.
The story takes place in six acts, some of them occurring simultaneously on parallel stages:
a.) The Polar Vortex
As the air in the antarctic stratosphere cools and descends during the winter, the Coriolis effect sets up a strong westerly
circulation around the pole. When the sun returns in the spring the winds weaken, but the vortex remains stable until November. The air over antarctica is largely isolated from the rest of the atmosphere, forming a gigantic reaction vessel. The vortex is not circular, it has an oblong shape with the long axis extending out over Patagonia.
There is some controversy about just how isolated the air in the vortex is. Some believe that the vortex is better
thought of as a flow reactor than as a containment vessel; ozone-rich air enters the vortex from above while ozone-poor and ClO-rich air is stripped off the sides.
b.) Polar Stratospheric Clouds ("PSC")
The Polar vortex is extremely cold; temperatures in the lower stratosphere drop below -80 C. Under these conditions large numbers of clouds appear in the stratosphere. These clouds are composed largely of nitric acid and water, probably in the form of crystals of nitric acid trihydrate ("NAT"), HNO3.3(H2O). Stratospheric clouds also form from ordinary water ice (so-called "Type II PSC"), but these are much less common; the stratosphere is...