Ozone Layer Depletion

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Ozone Layer Depletion

The ozone layer is an atmospheric layer that helps shield the surface of the globe from excessive ultra-violet radiation, which helps minimize the ultraviolet-mediated breakage of the double helix of deoxyribonucleic acid, DNA. As the result of pollution, the ozone layer is being depleted. The thinning of the ozone layer, and its complete absence over the antarctic, is allowing increased amounts of ultraviolet light to reach Earth.

Ozone occurs naturally in relatively large concentrations in the upper-atmospheric layer known as the stratosphere, which is located 5–10.6 miles (8–17 kilometers) to about 31 miles (50 kilometers) above Earth’s surface.

Stratospheric ozone is formed and consumed naturally by photochemical reactions involving ultraviolet radiation. At any time, the formation and consumption of ozone proceed simultaneously. The concentration of ozone in the stratosphere naturally varies with latitude and with time. Rates of ozone formation are largest over the equatorial regions of Earth because solar radiation is most intense over those latitudes. However, stratospheric winds carry tropical ozone to polar latitudes, where it tends to accumulate.

The depletion of the ozone layer was first observed in the mid-1980s. The depletion is caused by complex photochemical reactions involving chlorofluorocarbons (CFCs). CFCs are compounds that contain atoms of carbon, chlorine, and fluorine. They are very stable chemicals, which have had many industrial uses, especially in refrigeration, as propellants in aerosol sprays, as blowing agents used to manufacture synthetic foams and insulation, as cleaning agents for electronic components, as carrier gases for sterilizers of medical instruments, and as dry-cleaning fluids. After most of these uses, CFCs are emitted to the lower atmosphere, where they can persist for up to 50 years. The CFCs slowly penetrate into the stratosphere, where exposures to highly energetic, short wave solar radiation are intense, causing the CFCs to degrade. This releases chlorine and fluorine atoms, which are then available to consume ozone molecules in secondary reactions. A chlorine atom is capable of destroying as many as 100,000 ozone molecules before it is removed from the upper atmosphere.

With the persistence of the chloroflurocarbons in the atmosphere, there is ample opportunity for ozone destruction. It has been estimated that CFCs account for at least 80% of the depletion of stratospheric ozone worldwide. It has been estimated that the depletion of

the ozone layer most evident over Antarctica, where it is periodically completely absent, is almost 100% the result of CFCs.

The first concerns about depletion of stratospheric ozone were raised in the 1960s. At that time, a number of scientists suggested that emission of water vapor and various other chemicals from high-flying military jets and rockets might cause a consumption of stratospheric ozone. These discussions intensified during the early 1970s, when there were proposals to develop fleets of supersonic aircraft flying in the stratosphere. This idea was scrapped due to the huge price tag. Some scientists additionally suggested that emissions of oxides of nitrogen from vehicles and agricultural practices might also have some effect on the ozone layer, as could emissions associated with launchings of space shuttles and other spacecraft.

Since the 1970s, there has been evidence of large decreases in the concentrations of stratospheric ozone at polar latitudes—the ozone hole—during the late winter to early springtime. The Antarctic hole tends to develop between September and November, when the stratosphere is intensely cold but sunlight is intense, at altitudes of 7.4–16 miles (12–25 kilometers). The average decreases in springtime stratospheric ozone concentrations over Antarctica have been 30–40%. However, in some years the decrease in ozone has been over 60%.

The Antarctic ozone hole has been increasing in diameter. Measurements obtained in October, 2006 have revealed that the ozone hole is almost as large as has ever been recorded, in 2005. Then, the hole was almost 25 million square kilometers big—over three times the size of Australia.

Concentrations of stratospheric ozone can also be affected at non-polar latitudes, although the ozone depletion is relatively small. This happens during the late springtime, when the normal lower-latitude ozone concentrations are diluted by ozone-depleted polar air that becomes widely dispersed as the ozone holes break up and dissipate. Ozone layer depletions have been observed over North America, Europe, Asia, most of Africa, Australia, and South America. Ozone levels over the United States have fallen 5–10%, depending on the season.

The reason that ozone depletion is of concern is because of ozone’s ability to absorb the genetically-destructive wavelengths of ultraviolet radiation. As such, stratospheric ozone helps to protect humans and other organisms on Earth’s surface from some of the harmful effects of exposure to this high-energy electromagnetic radiation. In fact, without the protective action of the stratospheric ozone layer, it is likely that life would not be possible on Earth’s surface, and that life in the ocean would be restricted to the depths where sunlight does not penetrate.

The most common effect of ultraviolet overexposure is a sunburn. While this is seldom serious, other threats are more serious. Genetic damage increases the incidence of skin cancers. Basal carcinomas account for about 75% of human skin cancers, and squamous cell carcinomas about 20%. These are both serious diseases, but they can usually be successfully treated if detected early enough. The other skin cancer is malignant melanoma, a deadly disease that accounts for about 5% of total skin carcinomas, and which is often fatal soon after it is diagnosed.

Other human-health effects of ultraviolet exposure include increased risks of developing cataracts and other damage to the cornea, damage to the retina, a suppressed immune system, sunburns of exposed skin, skin allergies, and an accelerated aging of the skin.

As a result of widespread awareness and concerns about the role of CFCs in the depletion of stratospheric ozone, the uses and emissions of these chemicals were curtailed. For example, the use of CFCs as propellants in aerosol spray cans was banned in the 1980s. A conference sponsored by the United Nations Environment Programme in 1987 resulted in the so-called Montreal Protocol, which was subsequently revised and made more stringent in 1990, when it called for a complete phase out of global CFC use by the year 2000. The United States, Canada, Australia, and other developed countries have completely phased out the production of CFCs. As of 2006, the complete phase-out has not been achieved; according to the protocol developing countries have until the year 2010 to complete their phase out.

Other substances that can find their way into the stratosphere can increase the rate of ozone depletion as well. Halons, which are compounds consisting of bromine, fluorine, and carbon, can end up in the upper atmosphere where the halogens found in the compounds catalyze the ozone consuming reactions. Methyl bromide provides the catalyst bromine. Hydrochlorofluorocarbons (compounds consisting of hydrogen, chlorine, fluorine, and carbon) and carbon tetrachloride release chlorine and fluorine into the upper atmosphere.

Data from both ground-based and satellite measurements indicate that after decades of continual increase, concentrations of chlorine in the stratosphere are starting to plateau. The concentrations of bromide, however are continuing to increase and the depletion of ozone as a result of halides has been as great as 30% over the last decade. However, if halide and CFC concentrations continue to fall as directed by the Montreal Protocol, computer models predict that Antartic ozone concentrations should begin to increase in 2010.