Ozone depletion

Kategorie: Biológia (celkem: 966 referátů a seminárek)

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  • Přidal/a: anonymous
  • Datum přidání: 07. dubna 2007
  • Zobrazeno: 1618×

Příbuzná témata

Ozone depletion

Ozone gas (O3) forms a thin protective layer encircling the earth and screens out 99% of the sun’s harmful UV light. The ozone layer occupies the outer two-thirds of the stratosphere, 20 to 50 kilometers above the earth’s surface. The screening effect of the ozone layer protects all organisms from damage caused by UV light, which is known to be mutagenic and carcinogenic.
When UV light strikes ozone molecules, it causes them to split apart. The products, however, quickly reunite, reforming ozone and giving off heat. Thus, the ozone layer is a renewable layer that converts UV light into heat.
Three major human activities are believed that are destroying the ozone layer: (1) the use of spray cans and refrigerants that contain freon gas, (2) high flying supersonic and subsonic jets, and (3) the detonation of nuclear weapons.
Freons, as spray-can propellants, are in the use since 1951. Freons are also known as chlorofluorocarbons (CFCs). Two CFCs were commonly used: freon-11 (trichloromonofluoromethane), used as spray-can propellant, now banned in several countries, and freon-12 (dichlorodifluoromethane), used as coolant in refrigerators, freezers, and air conditioners, which is still used. Until the early 1970s chemists listed freons as inert (unreactive) chemicals.
The freons, or CFCs, are dissociated by UV light in the stratosphere. This produces a highly reactive chlorine free radical. The free radical can react with ozone layer, thus reducing the ozone concentration and eliminating the UV screen. A single molecule of freon gas can eliminate many thousands of molecules of ozone, because the chlorine free radical is regenerated. Chloride oxide (formed when the chloride free radical reacts with ozone) can also react with ozone.
The ozone layer may be vulnerable to other human activities as well. Supersonic and subsonic jets flying in the stratosphere may also destroy ozone through the release of nitric oxide produced by jet engines. The nitric oxide gas reacts with ozone to form nitrogen dioxide and oxygen.
The detonation of nuclear weapons in the atmosphere also produces nitric oxide. This suggests that nuclear war could cause a dangerous reduction.
Nitrogen fertilizers that farmers apply to their fields may be converted into nitric oxide gas.

It could gradually diffuse into the stratosphere, there reacting with ozone molecules.
Other chemical pollutants such as methyl chloride and carbon tetrachloride may also diffuse into the ozone layer and eliminate ozone molecules.
Natural pollutants such as nitrogen oxides from volcanoes and chloride ions from sea salt may also destroy the ozone layer, although these processes are presumably in balance with natural ozone replenishment.
Ozone depletion will increase the amount of UV light striking the earth. In reasonable amounts, UV light tans light skin and stimulates vitamin D production in the skin. However, excess UV exposure causes serious burns and may cause skin cancer. It would also be lethal to bacteria and plants.
In September, 1987, 24 nations signed a treaty, called the Montreal Protocol, which would cut CFC production in half by 1999.
Manufacturers now are pursuing several options: (1) production of less stable CFC compounds, (2) production of non-CFC substitutes, (3) conservation (using less to do the same job), and (4) recycling.
By adding a hydrogen atom to the stable CFC molecule, researchers can make less stable CFCs that break up in the lower atmosphere. The chlorine atoms released during this process, are less likely to reach the stratosphere.
One of these less stable CFCs, HCFC-22, is already on the market. It is 20 times less destructive than the CFC-12, and is used as a coolant in some home air conditioners. The main drawback is that it works at a higher pressure than CFC-12, therefore systems using this new chemical will require much heavier compressors and stronger tubes. That means more cost. There are some other substitutes...
International cooperation is badly needed to avert potential disaster.

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