Ozone in the stratosphere is controlled by the balance between production and destruction rates. It is formed by dissociation of oxygen (O2) caused by sunlight of very short wavelength: O2 --> O + O. Each of the oxygen atoms then recombines with O2 to form ozone, O3: O + O2 --> O3. This mechanism of production is balanced by destruction of ozone, which in the normal stratosphere is caused by catalytic cycles involving naturally occurring substances, OH and NO. However, extensive ozone depletion is caused when man-made chlorofluorocarbons (CFCs) reach the stratosphere and are dissociated by sunlight into free chlorine atoms. The atoms participate in another catalytic cycle that converts ozone (which has three oxygen atoms) into oxygen (which has two). The reason this is a problem is that the ozone in the stratosphere absorbs ultraviolet radiation and prevents it from reaching the surface of the earth. It turns out that the peak of the absorption spectrum in ozone (at around 250 nm) [12] and the peak of the absorption spectrum of DNA occurs at the same wavelength. Thus, ozone in the stratosphere prevents sunlight from breaking apart our DNA. It has been estimated that a 1% decrease in the concentration of stratospheric ozone would be responsible fro 100,000 cases of cataracts per year and 50,000 skin melanomas.
There is ample evidence that the level of ozone in the stratosphere is decreasing. Even globally averaged, there is a significant decrease in the level in recent times compared to former periods. There is also the problem of the "ozone hole" [12]: the concentration of ozone in the stratosphere above antarctica falls dramatically during the spring. The size of the hole [12] has been increasing. There is also little doubt as to the cause [14]: planes flying toward the south pole see that ozone concentration goes down when the concentration of ClO, the dominant chlorine containing species in this region, goes up.
The seriousness of this situation was realized by scientists some years ago (three of whom won the Nobel Prize in 1995 for their early work in this area). As in the case with global warming, there are serious economic issues surrounding the ozone depletion problem. Chlorofluorocarbon use is now limited by an international agreement known as the Montreal Protocol. Even so, these compounds will be around for a long time. The current "solution" to the problem is to use CFC substitutes. Other solutions, like putting ozone back into the stratosphere, are either not practical or not well enough understood to be without substantial risk.
