U.N. scientists say they expect a "normal" ozone hole this year.
Geir Braathen of the World Meteorological Organization says it's still too early to say for sure how big the ozone hole will be over Antarctica.
He told journalists in Geneva on Friday that it will likely be smaller than the very large hole of 2006, but more pronounced than last year's relatively small hole.
The hole has been forming since the mid-1980s in the extremely low temperatures that mark the end of Antarctic winter. Generally, the hole is biggest around late September.
The hole is caused by thinning in the ozone layer largely due to chemical compounds leaked from refrigerators, air conditioners and other devices. It exposes the Earth to harmful solar rays.
Ozone depletion describes two distinct, but related observations: a slow, steady decline of about 4 percent per decade in the total amount of ozone in Earth's stratosphere since the late 1970s; and a much larger, but seasonal, decrease in stratospheric ozone over Earth's polar regions during the same period. The latter phenomenon is commonly referred to as the ozone hole. In addition to this well-known stratospheric ozone depletion, there are also tropospheric ozone depletion events, which occur near the surface in polar regions during spring.
The detailed mechanism by which the polar ozone holes form is different from that for the mid-latitude thinning, but the most important process in both trends is catalytic destruction of ozone by atomic chlorine and bromine. The main source of these halogen atoms in the stratosphere is photodissociation of chlorofluorocarbon (CFC) compounds, commonly called freons, and of bromofluorocarbon compounds known as halons. These compounds are transported into the stratosphere after being emitted at the surface. Both ozone depletion mechanisms strengthened as emissions of CFCs and halons increased.
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