The Ozone Balance

Polar regions reflect the greatest changes in ozone concentrations, especially the South Pole. The topography of Antarctica is such that a stagnant whirpool of extremely cold stratospheric air forms over the region during the long polar night. The air stays within this polar vortex all winter, becoming cold enough to allow the formation of polar stratospheric clouds.

Polar stratospheric clouds speed up the natural process of ozone destruction by providing ice crystal surfaces on which the destructive reactions take place. After the long polar winter, ozone within this extremely cold vortex is very vulnerable to the arrival of sunlight. As spring arrives, major ozone losses occur. In the southern hemisphere, the area of most severe ozone depletion is localized above Antarctica and is generally referred to as the ozone hole. The hole appears in the southern spring, following the continent's coldest season and polar night.

Ozone depletion over the Arctic is not as well defined as in Antarctica. The rugged topography results in an air circulation pattern that is quite different from that of the South Pole, but expeditions have shown that the atmospheric chemistry of the two polar regions is very similar. In the Northern Hemisphere, the polar vortex is not as strong. It can break up and reform several times during the course of winter. One air mass after another enters the polar darkness and soon emerges back into low sunshine. Thus, a bit of ozone is lost from each parcel of air, rather than a large amount from one parcel as in the southern hemisphere.

The end result is that we are losing ozone in both hemispheres. (26k jpeg) Ozone levels in the atmosphere have been monitored from the ground since the 1950s and by satellite since the 1970s. Regional total ozone levels measured from satellites over Antarctica have decreased 30-50% since their monitoring began.

Since ozone is created and destroyed by solar UV radiation, there is some correlation of ozone concentration with 11-year sunspot cycles. Sunspots emit high levels of electromagnetic radiation. The increased UV radiation contributes to ozone production. Sunspot variations only account for 2 to 4 % of the total variation in ozone concentrations. Natural cycles in ozone variation are also associated with the quasi-biennial oscillation in which tropical winds switch from easterly to westerly every 26 months. This cyclic change in wind direction accounts for approximately 3 % of the natural variation in ozone concentration.

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