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Ozone Depletion


Why is the ozone hole over Antarctica? That is one of the first questions that comes to mind when people think about the ozone hole. Every winter and spring since the late 1970s, an ozone hole has formed in the stratosphere above the Antarctic continent. In recent years this hole has become both larger and deeper, in the sense that more and more ozone is being destroyed. As summer approaches, the hole repairs itself, only to reform during the following spring.

Man-made emissions of ozone-depleting chemicals (ODCs) occur mainly in the Northern Hemisphere, with about 90% released in Europe, Russia, Japan, and North America. The global system of winds helps to distribute these ODCs, moving them towards the North and South Poles. Normally, the chlorine and bromine which destroys the ozone when free, is locked up within the fairly unreactive and stable ODCs. However, during the Antarctic winter months (June to August) when the region receives no sunlight, the stratosphere becomes cold enough (-80C) for high level [ice] clouds to form, called Polar Stratospheric Clouds (PSCs). These PSCs provide an ideal catalytic surface on which chlorine and bromine can react with the ozone, thus destroying the ozone layer. This reaction requires sunlight, and therefore only begins when the Sun returns to Antarctica in spring (September to October), before the PSCs have had a chance to melt. The ozone hole disappears again when the Antarctic air warms up enough during late spring and summer

During the Southern Hemisphere winter Antarctica is isolated from the rest of the world by a natural circulation of wind called the polar vortex. This prevents atmospheric mixing of stratospheric ozone, thus contributing to the depletion of ozone above Antarctica. Although some ozone depletion occurs over the Arctic, meteorological conditions there are very different to Antarctica and so far have prevented the formation of ozone holes as large as in the Southern Hemisphere.