There are two different types of ozone depletion, both are very similar. The first one has been a slow, but steady ozone depletion of 4% per decade of the Earth’s stratosphere(ozone layer). This has been happening constantly since the 1970’s. The other is a much larger, although seasonal loss of ozone over the polar regions. This yearly occurrence is called the ozone hole. There are many causes for ozone depletion, but the most important process in both trends is catalytic destruction of ozone by atomic chlorine and bromine. Both come from the breaking down of chloroflourocarbons(freons) by photons in the atmosphere.
Chloroflourocarbons(CFC) are the ”big dog” as far as causes of ozone depletion are concerned. CFC’s are man made chemicals that are very stable in the atmosphere. They take from 20 to 120 years to break down. All the while they are destroying ozone molecules. This is what happens: CFCs do not fall back to Earth with rain, nor are they destroyed by other chemicals. Because of their relative stability, CFCs rise into the stratosphere where they are eventually broken down by ultraviolet (UV) rays from the Sun. This causes them to release free chlorine. The chlorine reacts with oxygen which leads to the chemical process of destroying ozone molecules. The net result is that two molecules of ozone are replaced by three of molecular oxygen leaving. The chlorine then reacts again with the oxygen molecules to destroy the ozone and the process repeats 100,000 times per molecule. While naturally occurring chlorine has the same effect on the ozone layer, it has a shorter life span in the atmosphere.
Of all of the causes of ozone depletion, the release of CFCs is thought to have accounted for 80% of all stratospheric ozone depletion. With great forethought, the developed world has phased out the use of CFCs in response to international agreements, like the Montreal Protocol, to protect the ozone layer. On the downside though, because CFCs remain in the atmosphere so long, the ozone layer will not fully repair itself until at least the middle of the 21st century.
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The Montreal Protocol is an international agreement to address the causes of ozone depletion. While several substances were addressed, CHCs and HCFCs were the main ones that the international community agreed to phase out of production. The protocol also developed a fund to help underdeveloped countries to find other methods of production so that they could stop using CFCs and HCFCs, also.
There is a good article about the causes of ozone depletion and the Montreal Protocol at this link. Here on Universe Today we have a great article about what the ozone is and means to us. Astronomy Cast offers a good episode that describes what could happen if we lose enough of our ozone.