Hole in the Ozone Layer

Ozone (O3) in the atmosphere has two very different roles. On the surface of the earth it is a pollutant and a health hazard. It is found in photochemical smog that builds up during atmospheric inversions. People with lung ailments are especially susceptible to its damaging effects. When automobiles in cities release large amounts of nitrogen compounds during the summer, sunlight breaks up nitrogen oxide (NO2) to nitrogen monoxide (NO), releasing an oxygen molecule that combines with oxygen (O2) to form ozone (O3). High in the atmosphere, however, ozone is beneficial, protecting life from ultraviolet radiation.

In the stratosphere, twelve to sixteen km above the surface, ultraviolet light from the sun converts the oxygen molecule to ozone. Here, the conversion process creates an ozone layer that absorbs ultraviolet radiation, protecting the earth from most of its damaging rays. Ultraviolet radiation causes skin cancer, and it is associated with cataracts in eyes, gene mutations, and immune system damage. Ozone also inhibits photosynthesis in plants.

Loss of ozone in the atmosphere was first noticed by scientists studying the atmosphere above Antarctica in the mid-1970s. This loss, which occurs during the summer, is described as the ozone hole, and scientific investigation since the loss was first noticed has shown that a number of factors are involved. During the winter polar night, sunlight does not reach the South Pole, and the air gets very cold. A strong whirlpool wind, the polar vortex, develops, causing the cold air to remain within the polar area, essentially cutting it off from the rest of the atmosphere. When the air temperature drops below -80 degrees centigrade, polar stratospheric clouds form that are composed of water ice droplets and nitric acid. Reactions take place on the surface of these ice crystals that convert benign chlorine compounds into chlorine and bromine compounds that are ozone destroyers. The source of these chemicals is CFCs (chlorofluorocarbons) and other industrial compounds containing bromine and other halogens. CFCs are used as a refrigerant in air-conditioning systems, in solvents, and in aerosols. Nitrogen compounds are produced by combustion, sources of which are jet aircraft emissions. These compounds are carried aloft by air currents and diffused into the stratosphere. Ozone loss does not occur until sunlight returns in the spring and starts a rapid cycle of ozone destruction, which usually begins in mid-September, widens in mid-October, and contracts again in December. The size and duration of the ozone hole also depends on the weather, and it exhibits the same range of variations as weather on the surface, making year-to-year variations difficult to forecast. In general, the colder the winter, the larger the number of clouds, the greater destruction of ozone, and the larger will be the hole. For example, it reached a record size in 2000, growing to three times larger than the United States and then disappearing completely by November 19.

Enough evidence has been gathered by scientists that the international community has become concerned about the hazards. The Montreal Protocol of 1987 was the result; it set provisions for the phasing out of the use of the chemicals determined to hasten ozone destruction. The agreement restricted the production of CFCs by 1995 and eventually many other halogen compounds. It was believed that these restrictions would lead to the recovery of the ozone layer by 2050. Although the ozone hole will not go away for a long time, just a few years after the phaseout program began, concentrations of CFCs have started to decline in the lower atmosphere and have leveled off in the stratosphere.

In 2000, a loss of ozone was noted over the North Pole. This is a great concern because of the substantial number of people living in the Arctic regions. Winter temperatures in the

Arctic are variable, and ozone loss occurs only during very cold winters, as in the Antarctic. Using information from the TOMS (Total Ozone Mapping Spectrometer) satellite, researchers believe that Arctic ozone loss is associated with volcanic activity, which emits sulfur compounds into the earth's atmosphere and forms sulfuric acid. Volcanic sulfuric acid clouds add to the ozone-destroying power of the polar stratospheric clouds. During cold years the combination of volcanic sulfuric acid clouds below and the polar stratospheric clouds above increases the potential for creating an ozone hole in the Arctic.

—Sidney Horenstein

See also: Atmosphere; Atmospheric Cycles; Climatology; Global Climate Change

Bibliography

Karl, Thomas R., and Kevin E. Trenfserth. 2001. "The Human Impact on Climate." Scientific American 281(6) 100-105; Laing, David. 1991. The Earth System: An Introduction to Earth Science. Dubuque, IA: Wm. C. Brown; Ledley, Tamara et al. 1999. "Climate Change and Greenhouse Gases." EOS 80:453-458; Lutgens, Frederick K., and Edward J. Tarbuck. 2001. The Atmosphere: An Introduction to Meteorology, 8th ed. Upper Saddle River, NJ: Prentice Hall.

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