Ozone Layer Depletion
False color images showing ozone depletion from October 1979 to October 1990. U.S. National Aeronautics and Space Administration (NASA).The ozone layer is a part of the atmosphere between 18.6 mi and 55.8 mi (30 and 90 km) above the ground. The ozone present is responsible for blocking potentially harmful ultraviolet radiation reaching the surface of the earth. During the last twenty years, evidence has accumulated that human activity may be the cause of a generalized depletion of the ozone layer. This phenomena is global and distinct from the natural factors that induce annual ozone layer hole formation over Antarctica.
Ozone is constantly created and destroyed in natural processes (manufactured by the action of lightning on oxygen and destroyed by the action of ultraviolet radiation), however the amounts normally balance each other out so there is no net increase or decrease due to natural processes. In 1970, Paul Crutzen showed that naturally occurring oxides of nitrogen can catalytically destroy ozone. In 1974, F. Sherwood Rowland and Mario Molina demonstrated that chlorofluorcarbons (CFCs) could also destroy ozone. In 1995, all three were jointly awarded the Nobel Prize for chemistry.
The CFCs that were observed as being damaging included Freon 11 (CFCl3) and Freon 12 (CF2Cl2). These chemicals are widely used in industry and the home. They have uses as propellants in aerosol spray cans, refrigerant gases, and foaming agents for blown plastics. One problem associated with these gases is their relative lack of reactivity. When released there is very little that will break them down and, as they are not soluble in water, they are not removed from the atmosphere by rain. As a consequence, once released they tend to concentrate in the upper regions of the atmosphere. It is estimated that some several million tons of CFCs are present in the atmosphere.
Once in the upper atmosphere the CFCs are exposed to high energy radiation that can cause disassociation of the molecule, producing free chlorine atoms. This atomic chlorine reacts readily with ozone to produce chlorine monoxide and molecular oxygen. The chlorine monoxide can further react to produce molecular oxygen and more atomic chlorine. This all accelerates the destruction of ozone beyond its natural ability to regenerate. Overall, there is a net reduction in the amount of ozone present in the upper atmosphere. This has led to a thinning of the ozone layer. The majority of this loss is at an altitude between 7.44 mi and 18.6 mi (12 and 30 km) and in the late 1990s evidence was seen that suggested losses were also occurring at other altitudes. In addition to the annual holes in the ozone layer now detected over Antarctica, in the late 1990s, holes were detected over Australia and atmospheric sampling indicated a dramatic thinning of the ozone layer in the Northern Hemisphere during the winter months. In the Northern Hemisphere losses of some 30% have been recorded at an altitude of 12.4 mi (20 km).
In 1987, the Montreal Protocol was signed with the appropriate countries agreeing to reduce CFC production. By 1996, more than 100 countries agreed to cease widespread commercial use of CFCs and to stop or curtail production of CFCs.
In the absence of the ozone layer, harmful ultraviolet radiation is able to reach the surface of the earth in higher doses. This can lead to increases in skin cancers.
Atmospheric Chemistry; Greenhouse Gases and Greenhouse Effect; Ozone Layer and Hole Dynamics; Ultraviolet Rays and Radiation
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