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UV Light and the Ozone Layer

UV Light and the Ozone Layer

If all of the Sun’s ultraviolet radiation reached the Earth’s surface, there would be extremely grave effects on the biosphere from the severe cell damage it causes. However, the layer of ozone (\({\text{O}}_{3}\)) in our upper atmosphere (10 to 50 km above the Earth) protects life by absorbing most of the dangerous UV radiation.

Unfortunately, today we are observing a depletion in ozone concentrations in the upper atmosphere. This depletion has led to the formation of an “ozone hole” in the upper atmosphere. The hole is more centered over the southern hemisphere, and changes with the seasons, being largest in the spring. This depletion is attributed to the breakdown of ozone molecules by refrigerant gases called chlorofluorocarbons (CFCs).

The UV radiation helps dissociate the CFC’s, releasing highly reactive chlorine (Cl) atoms, which catalyze the destruction of the ozone layer. For example, the reaction of \({\text{CFCl}}_{3}\) with a photon of light \((\text{hv})\) can be written as:

\({\text{CFCl}}_{3}+\text{h}v\text{}\to {\text{CFCl}}_{2}+\text{Cl.}\)

The Cl atom then catalyzes the breakdown of ozone as follows:

\(\text{Cl}+{\text{O}}_{3}\text{}\to \text{ClO}+{\text{O}}_{2}\phantom{\rule{0.25em}{0ex}}\text{and ClO}+{\text{O}}_{3}\text{}\to \text{Cl}+{\text{2O}}_{2}.\)

A single chlorine atom could destroy ozone molecules for up to two years before being transported down to the surface. The CFCs are relatively stable and will contribute to ozone depletion for years to come. CFCs are found in refrigerants, air conditioning systems, foams, and aerosols.

International concern over this problem led to the establishment of the “Montreal Protocol” agreement (1987) to phase out CFC production in most countries. However, developing-country participation is needed if worldwide production and elimination of CFCs is to be achieved. Probably the largest contributor to CFC emissions today is India. But the protocol seems to be working, as there are signs of an ozone recovery. (See this figure.)


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