Ozone (O3) is a molecule that is composed of three oxygen atoms. Most atmospheric ozone resides in the stratosphere between 10 and 50 km.
1. An ozone molecule is formed when UV radiation at wavelengths less than 240 nm (in the UV-c) breaks an O2 molecule into two oxygen atoms. These oxygen atoms (O) then react with other oxygen molecules (O2) to form ozone (O + O2 --> O3).
2. Ozone is broken apart by the Sun's ultraviolet radiation into an oxygen molecule (O2) and an oxygen atom (O). The oxygen atom then reacts with another O2 molecule to reform ozone. This cycling of ozone to oxygen atoms consumes the UV radiation and converts it to heat.
3. UV radiation can break the chemical bonds in DNA molecules. Fortunately, ozone absorbs UV radiation at this DNA damaging part of the spectrum. Ozone is not effective in screening UV-a (320-400 nm), partially effective in screening UV-b (280-320 nm), and completely effective in screening the very energetic UV-c (200-280 nm)
4. Ozone is destroyed by catalytic processes, wherein the ozone molecule is lost, while the catalyst (chlorine, nitrogen, bromine or hydrogen) is reformed to destroy another ozone molecule.
5. Long-lived trace gases such as CFCs are lofted into the stratosphere from the tropical upper troposphere. After entering the stratosphere, the CFCs are typically transported into the midlatitudes and carried back out of the stratosphere. A small fraction of the CFCs are lofted into the upper stratosphere (above the ozone layer), where they are broken up by the Sun's extreme UV radiation, releasing the chlorine to cause ozone destruction.
6. The Antarctic ozone hole is directly a result of the human produced pollution of the stratosphere (principally CFCs). The hole appears each spring in the southern hemisphere (August-November) over Antarctica following the extremely cold Antarctic winter. The hole results from chemical reactions that occur on the surface of polar stratospheric cloud (PSC) particles. These special reactions release chlorine from benign forms (HCl and ClONO2) into a form that can quickly catalytically destroy ozone (ClO).
7. Compounds such as CFCs that can cause ozone loss have now been regulated by international agreement. Chlorine compounds that can destroy ozone have begun to decrease in the lower atmosphere (as of 1996), and have begun to decrease in the stratosphere. Future impacts on the stratosphere from aircraft and greenhouse gas effects are currently under investigation.
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