During the August and September periods, ozone decreases quite rapidly over the Antarctic region (see Figure 1.05). It was recognized during the 1960s that ozone was naturally low over Antarctica as a result of the weaker poleward and downward circulation in the southern hemisphere. In 1985, Joesph Farman, Brian Gardiner, and Jonathan Shanklin of the British Antarctic Survey published a paper in Nature showing that ozone was disappearing over Antarctica during this southern hemisphere spring period. The amounts measured were much less than even the naturally occurring low amounts over Antarctica in the southern spring. This landmark paper set in motion a series of intensive field campaigns, analysis works, satellite investigations, and lab studies that have nearly fully characterized the processes that control Antarctic ozone.

Figure 1.09 shows the total amount of ozone above Earth's surface over Antarctica during October as measured by a series of satellite instruments such as TOMS (see Chapter 7 on ozone measurement techniques).

The top panels show ozone levels for earlier years (the 1970s) when chlorine and bromine levels were lower than at present, while the bottom panels show recent ozone levels (the 1990s). These recent years show ozone levels that are less than half of what was observed previously. These large Antarctic ozone losses are now popularly known as the Antarctic ozone hole. While there have been significant -- even severe -- losses of ozone recorded in the last several years over the Arctic, there is not a symmetrical "hole" of similar magnitude, extent, and duration ccntered over the North Pole. Figure 1.10 shows polar averaged Arctic and Antarctic ozone levels for the same local season, early spring (October in the southern hemisphere, March in the northern hemisphere). Because of the different climates of the Arctic and Antarctic, the Arctic ozone levels are naturally higher, yet both polar regions have shown declines over the last decade.

Polar ozone losses are directly caused by chlorine and bromine catalytic reactions. The production of chlorine and bromine reactive species is accelerated by chemical reactions that take place on the surfaces of cloud particles. Laboratory chemists realized long ago that some gases were affected by contact with the walls of reaction chambers. These "surface" reactions modified the results obtained for a pure gas reaction with another gas. In an identical fashion, it was realized that the surfaces of individual cloud particles could enable reactions that otherwise would not take place in the stratosphere. As the stratosphere cools to very cold temperatures over the Antarctic during the southern winter, polar stratospheric clouds (PSC's) form. Forms of chlorine (HCl and ClONO2) that do not affect ozone can react on the surfaces of these PSC's and produce chlorine products that can catalytically destroy ozone. These chlorine and bromine catalytic destruction reactions are so fast that all of the ozone over Antarctica between 12 and 20 km is destroyed within a few weeks during the September (Antarctic spring) period.

The key ingredients for polar ozone losses are: high chlorine and bromine levels, cold temperatures during the late winter, and relative isolation of the polar region from the midlatitudes. During winter, the polar regions of both the Arctic and Antarctica are surrounded by a very strong jet stream blowing from east-to-west. This jet acts as a barrier to air moving from the midlatitudes into the polar region, isolating the poles. Temperatures poleward of this jet are very cold, dropping to below -80°C. The increasing levels of CFCs and bromine compounds over the last few decades caused the Antarctic ozone hole, since Antarctic temperatures are always cold in late winter. Arctic losses appear to be related to these higher chlorine and bromine levels combined with colder temperatures which have begun to appear over the Arctic during the 1990s.