Ozone Layer Depletion
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. The ozone layer is gradually being destroyed by human activity.
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) but the amounts 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 of these two characteristics once released they tend to concentrate in the upper regions of the atmosphere. It is estimated that some several million tonnes of CFCs are present in the atmosphere.
Once in the upper atmosphere the CFCs are exposed to high energy radiation which 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 that in extreme cases has led to the production of a hole where there is an absence of ozone completely. Ozone holes have so far only been detected in the Southern Hemisphere principally over Antarctica between September and October. 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 total over 70% of the ozone above Antarctica is lost during these periods. These holes are increasing in size and in the late 1990s holes were detected over Australia and dramatic thinning of the ozone layer in the Northern Hemisphere was also recorded 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. A more stringent set of guidelines was introduced in 1992 whereby 100 countries agreed to stop CFC production by 1996. There will be a lag effect from these treaties because of the long life that CFCs experience in the atmosphere, so any reduction in depletion from these protocols will not be seen for several years to come, the exact length of time is not known.
Refrigerants and propellants are not the only damaging items to the ozone layer. High flying aircraft, ranging from commercial craft such as Concorde to military spy planes and the space shuttle, contribute to ozone depletion. The engines of these vehicles, particularly those traveling at supersonic speeds, produce oxides of nitrogen which will also attack the ozone layer.
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.
The ozone layer is being depleted by human activity at a rate faster than it can replenish itself. This has lead to holes appearing in the ozone layer. Despite agreements to limit production and use of damaging CFCs, the damage seen will not peak for several years due to the long-lived nature of CFCs.
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