Ozone Depletion

http://www.epa.gov/ozone/science/

Ozone is a gas that is naturally present in our atmosphere. Each ozone molecule contains three atoms of oxygen and is known chemically as O3. Ozone is found primarily in two regions of the atmosphere. About 10% of atmospheric ozone is in the troposphere, the region closest to Earth. The remaining ozone (90%) resides in the stratosphere.. The large amount of ozone in the stratosphere is often referred to as the ozone layer. It shields us from the harmful ultraviolet radiation from the Sun.
The most severe and most surprising ozone loss was discovered to be recurring in
springtime over Antarctica. The loss in this region is commonly called the “ozone hole” because the ozone depletion is so large.
Ozone in the stratosphere absorbs some of the ultraviolet radiation. Because of this beneficial role, stratospheric ozone is considered “good” ozone. In contrast, excess ozone at Earth’s surface that is formed from pollutants is considered “bad” ozone because it can be harmful to humans, plants, and animals.
The initial step in the depletion of stratospheric ozone by human activities is the emission, at Earth’s surface, of ozone-depleting gases containing chlorine and bromine. Most of these gases accumulate in the lower atmosphere because they are nonreactive and do not dissolve readily in rain or snow. Eventually, these emitted
source gases are transported to the stratosphere, where they are converted to more reactive gases containing chlorine and bromine. These more reactive gases then participate in reactions that destroy ozone. Finally, when air returns to the lower atmosphere, these reactive chlorine and bromine gases are removed from Earth’s
atmosphere by rain and snow. Once the gases react with others they become secondary pollutants compared to primary pollutants that are harmful even before combining with others.