The threat to the ozone layer

One of the most important functions of the atmosphere is to provide the surface of the earth with protection from solar radiation. This may seem contradictory at first sight, since solar radiation provides the energy which allows the entire earth/atmosphere system to function. As with most essentials, however, there are optimum levels beyond which a normally beneficial input becomes harmful. This is particularly so with the radiation at the ultraviolet end of the spectrum (see Table 6.1). At normal levels, for example, it is an important germicide, and is essential for the synthesis of Vitamin D in humans. At elevated levels it can cause skin cancer, and produce changes in the genetic make-up of organisms. In addition, since ultraviolet radiation is an integral part of the earth's energy budget, changes in ultraviolet levels have the potential to contribute to climatic change.

Under normal circumstances, a layer of ozone gas in the upper atmosphere keeps the ultraviolet rays within manageable limits. Close to the equator, ozone allows only 30 per cent of the ultraviolet-B (UV-B) radiation to

Table 6.1 Different forms of ultraviolet radiation



ultraviolet-A (UV-A)

320-400 nm

ultraviolet-B (UV-B)

280-320 nm

ultraviolet-C (UV-C)

200-280 nm

1 nm (nanometre)=1x1C-9m=1x1C-Spm

1 nm (nanometre)=1x1C-9m=1x1C-Spm reach the surface. A comparable value for higher latitudes is about 10 per cent, although during the summer months radiation receipts may approach equatorial levels (Gadd 1992). Specific amounts vary in the short-term with changes in such factors as cloudiness, air pollution and natural fluctuations in ozone concentrations. Ozone is a relatively minor constituent of the atmosphere. It is diffused through the stratosphere between 10 and 50 km above the surface, reaching its maximum concentration at an altitude of 20 to 25 km. If brought to normal pressure at sea-level, all of the existing atmospheric ozone would form a band no more than 3 mm thick (Dotto and Schiff 1978). This small amount of a minor gas, with an ability to filter out a very high proportion of the incoming ultraviolet radiation, is essential for the survival of life on earth. It removes most of the extremely hazardous UV-C wavelengths and between 70 and 90 per cent of the UV-B rays (Gadd 1992). The amount of ozone in the upper atmosphere is not fixed; it may fluctuate by as much as 30 per cent from day to day and by 10 per cent over several years (Hammond and Maugh 1974). Such fluctuations are to be expected in a dynamic system, and are kept under control by built-in checks and balances. By the early 1970s, however, there were indications that the checks and balances were failing to prevent a gradual decline of ozone levels. Inadvertent human interference in the chemistry of the ozone layer was identified as the cause of the decline, and there was growing concern over the potentially disastrous consequences of elevated levels of ultraviolet radiation at the earth's surface. The depletion of the ozone layer became a major environmental controversy by the middle of the decade. Its technological complexity caused dissension in scientific and political arenas, and—with more than a hint of science fiction in its make-up—it garnered lots of popular attention. In common with many environmental concerns of that era, however, interest waned in the late 1970s and early 1980s, only to be revived again with the discovery in 1985 of what has come to be called the Antarctic ozone hole.

The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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