Environmental Engineering Reference
In-Depth Information
6
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
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
Table 6.1
Different forms of ultraviolet radiation
1 nm (nanometre)=1×10
-9
m=1×10
-3
µm
