THE OZONE SHIELD

DEFINITION

• Ozone layer- an area in the stratosphere where ozone is highly concentrated.
• chlorofluorocarbons (CFCs)- human made chemicals
• ozone hole- a thinning of stratospheric ozone that occurs over the poles during the spring.
• polar stratospheric clouds- When temperatures fall below about -80°C, high-altitude clouds made of water and nitric acid

OZONE

• At the Earth’s surface, CFCs are chemically stable.they do not combine with other chemicals or break down into other substances. But CFC molecules break apart high in the stratosphere, where UV radiation, a powerful energy source, is absorbed. Once CFC molecules break apart, parts of the CFC molecules destroy protective ozone.

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• Each CFC molecule contains from one to four chlorine atoms, and scientists have estimated that a single chlorine atom in the CFC structure can destroy 100,000 ozone molecules.
• Over a period of 10 to 20 years, CFC molecules released at the Earth’s surface make their way into the stratosphere.

OZONE HOLE

• After the results from the studies from Hailey Bay were pub lished, NASA scientists reviewed data that had been sent to Earth by the Nimbus 7 weather satellite since the satellite’s launch in 1978. They were able to see the first signs of ozone thinning in the data from 1979. Although the concentration of ozone fluctu ates during the year, the data showed a growing ozone hole

HOW ITS FORMED

• On the surfaces of polar stratospheric clouds, the products of CFCs are converted to molecular chlorine. When sunlight returns to the South Pole in spring, molecular chlorine is split into two chlorine atoms by ultraviolet radiation. The chlorine atoms rap idly destroy ozone. The destruction of ozone causes a thin spot, or ozone hole, which lasts for several months. Some scientists esti mate that as much as 70 percent of the ozone layer can be destroyed during this period.

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• ozone is also being produced as air pollution, you may wonder why this ozone does not repair the ozone hole in the stratosphere. The answer is that ozone is very chemically reactive. Ozone produced by pollution breaks down or combines with other substances in the troposphere long before it can reach the stratosphere to replace ozone that is being destroyed.

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• UV light is dangerous to living things because it damages DNA. DNA is the genetic material that contains the information that determines inherited characteristics. Exposure to UV light makes the body more susceptible to skin cancer, and may cause cer tain other damaging cffects to the human body. High levels of UV light can kill single-celled organisms called phytoplankton that live near the surface of the ocean.

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• The loss of phytoplankton could disrupt ocean food chains and reduce fish harvests. In addi tion, a reduction in the number of phytoplankton would cause an increase in the amount of carbon dioxide in the atmosphere.Some scientists believe that increased UV light could be espe cially damaging for amphibians, such as toads and salamanders. Amphibians lay eggs that lack shells in the shallow water of ponds and streams.

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• UV light at natural levels kills many eggs of some species by damaging unprotected DNA. Higher UV levels might kill more eggs and put amphibian populations at risk.

PROTECTING

• 1987, a group of nations met in Canada and agreed to take action against ozone depletion. Under an agreement called the Montreal Protocol, these nations agreed to sharply limit their production of CFCs. A second conference on the problem was held in Copenhagen, Denmark, in 1992. Developed countries agreed to eliminate most CFCs by 1995. The United States pledged to ban all substances that pose a significant danger to the ozone layer by 2000.

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• After developed countries banned most uses of CFCs, chemical companies developed CFC replacements. Aerosol cans no longer use CFCs as propellants, and air conditioners are becoming CFC free. Because many countries were involved and decided to control CFCs, many people consider ozone protection an international environmental success story. he battle to protect the ozone layer is not over.

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• CFC mol ecules remain active in the stratosphere for 60 to 120 years. CFCs released 30 years ago are still destroying ozone today, so it will be many years before the ozone layer completely recovers.