54 / Basics of Environmental Science
thousands of ozone mol-ecules (Cl + O 3 → ClO + O 2 ; ClO + O → Cl + O 2 ) (HIDORE AND
OLIVER, 1993, pp. 74–77). CFCs are believed to be the principal source of stratospheric
chlorine. Although very stable, they are decomposed by ultraviolet radiation at wavelengths
below 0.23 μm, releasing free chlorine. As spring advances, the vortex disappears, ozone
moves poleward from lower latitudes, and the ozone layer recovers. Seasonal depletion over
the Arctic has also been reported, but it is less severe and of shorter duration, because Arctic
winter stratospheric temperatures are higher than those of the Antarctic and a polar vortex
rarely forms.
Ozone depletion may lead to increased exposure to ultraviolet radiation at the surface, the biological
significance of which is uncertain. Ultraviolet radiation causes cataracts and non-melanoma skin
cancer in fair-skinned humans (recent increases being due to the popularity of sunbathing in hot
climates to which people are not acclimatized, and not to ozone depletion). It might have an adverse
effect on land plants especially susceptible to it and may also affect organisms living in the uppermost
few millimetres of the ocean surface; below that depth ultraviolet radiation is absorbed by sea water.
Ozone is a very minor constituent of the atmosphere. In the troposphere it occurs locally, some
naturally but more commonly as a pollutant which causes respiratory irritation in humans and can
damage plants, produced by photochemical reactions involving vehicle exhaust fumes. It is a
constituent of photochemical smog and responsible for some of the damage attributed to acid rain.
The principal ingredients of the atmosphere are listed in Table 2.3. Water vapour comprises up to 4
per cent in the lower atmosphere, but above
about 12 km it is virtually absent. The source
of the water vapour from which PSCs form
is unknown; it may have entered the
stratosphere as water vapour and
accumulated in the polar vortex or may result
from the oxidation of methane (CH 4 + 2O 2
→ CO 2 + 2H 2 O ). Water vapour apart, the
composition of the atmosphere remains
constant to a considerable height, because
of mixing caused by turbulence. Beyond the
mesosphere, however, the proportions of its
ingredients change. Figure 2.16 illustrates
how the chemical composition changes with
height.
15. General circulation of the atmosphere
If the Earth faced it directly, the Sun would be overhead at the equator at noon every day of the year.
This would have a profound effect on climates, because there would be no seasons. In fact, of course,
the Earth is tilted on its axis, so we do not face the Sun directly. Our orbit traces the circumference of
a plane, called the ‘ecliptic’, and our rotational axis is tilted to the plane of the ecliptic by 23.5° (though
the angle of tilt varies between 21.8° and 24.4° over an approximately 41000-year cycle). This means
that from March to September the northern hemisphere is inclined toward the Sun and from September
to March the southern hemisphere is inclined inwards, bringing summer to the two hemispheres in
turn. Figure 2.17 shows how the tilted axis produces our seasons; these are labelled for the northern
hemisphere and the names should be reversed for the southern hemisphere (winter becomes summer,
Table 2.3 Average composition of the
troposphere and lower stratosphere