The Antarctic Ozone hole
What is it?The antarctic ozone hole is an area of the antarctic stratosphere in which the recent (since about 1975) ozone levels have dropped to as low as 33% of their pre-1975 values. The ozone hole occurs during the antarctic spring, from September to early December, as strong westerly wind start to circulate around the continent and create an atmospheric container. In this container over 50% of the lower stratospheric ozone is destroyed.
Why is it important?While the effective of the antarctic hole in decreasing the global ozone is relatively small, estimated at about 4% per decade, the hole has generated a great deal of interest because:
What is so special about Antarctic conditions?Polar regions get a much larger variation in sunlight than anywhere else, and during the 3 months of winter spend most of time in the dark without solar radiation. Temperatures hover around or below -80'C for much of the winter and the extremely low antarctic temperatures cause cloud formation in the relatively ''dry''stratosphere. These Polar Stratospheric Clouds (PSC's) are composed of ice crystals that provide the surface for a multitude of reactions, many of which speed the degredation of ozone molecules.
Summary of what happens in the Anarctic holeFor details of the chemistry of ozone depletion see the Ozone chemistry tutorial . Most of the following infornation on the Anarctic hole comes from Robert Parson's FAQ on the subject.
1. As mid-May brings on the onset of winter, the antarctic stratosphere cools and descends closer to the surface. The Coriolis effect (caused by the earths rotation) sets up a strong westerly circulation around the south pole, forming an oblong vortex which varies in size from year to year. Current theory [Tuck 1989] holds that the vortex is like a semi-sealed reaction vessel with most of the antarctic air staying trapped inside the vortex. As temperatures in the lower stratosphere cools below -80'C, Polar Stratospheric Clouds (PSC's) start to form.
2. Most of the anarctic stratospheric chlorine ends up in resevoir compounds such as ClONO2 or HCl. Resevoir compounds are so named because they hold the atmospheric chlorine in an inactive form but can react later, usually after a hit by ultraviolet radiation, and release reactive chlorine molecules. On the surface of the PSC crystals, nitrogen compounds are readily absorbed and chlorine resevoir compounds are converted to far more reactive compounds such as Cl2 and HOCl.
3. The small amounts of visible light during the antarctic winter
are sufficient to convert much of the atmospheric Cl2 to ClO:
4. Spring brings an increase of ultraviolet light to the lower antarctic stratosphere, providing the energy needed needed for the rapid catalytic break-down of ozone by ClO and its dimer ClOOCl. Another mechanism involving Bromine adds another 33% to the depletion total. Over 50% of the stratospheric ozone is destroyed by these two mechanims, most of the damage occuring in the lower stratosphere.
5. Towards the end of spring (mid-December) the warming temperatures cause the vortex to break up; ozone-rich air from the surrounding area comes flooding in and masses of ozone-depleated air go wandering off, temporarly lowering the ozone in areas of South America and New Zealand by up to 10%.
Factors influencing the magnitude of the hole are essentially the same as those factors affecting global ozone levels but an area of great uncertainty are the surface reactions that happen in the polar stratospheric clouds. Reactions have been proposed and tested in labs in which chlorine, bromine and nitogen-oxides can cooperate to break down ozone many times faster than they could alone, and the presence of the polar clouds is important in this cooperation.
Author: Brien Sparling