Environmental Engineering Reference
In-Depth Information
surface, on occasion—usually as a product of air
pollution—but its main location is in the upper
atmosphere, where it effectively filters out short-
wave solar radiation at the ultraviolet end of the
spectrum. Any change in ozone levels, allowing
an increase or decrease in the transmission of
radiation, would therefore cause disruption of
the earth's energy budget, and lead to alterations
in temperature levels and distribution patterns.
It is also estimated that a reduction in ozone in
the upper atmosphere would allow an increase
in the incidence of skin cancer in humans, as well
as genetic mutation in lower level organisms as
a result of the increase in the proportion of
ultraviolet radiation reaching the earth's surface
(Dotto and Schiff 1978).
spectrum—with wavelengths between 1-30
µm—is captured, and the temperature of the
atmosphere rises. The capture of the outgoing
terrestrial radiation is effected largely by water
vapour and carbon dioxide, along with methane
and traces of about twenty other gases, which
together are called the greenhouse gases. The
whole process was labelled the greenhouse effect
since the gases, by trapping the heat, appeared
to work in much the same way as the glass in a
greenhouse. The name remains in common use,
although it is now generally accepted that the
processes involved are not exactly the same. For
example, the glass in the greenhouse acts as a
physical barrier to the transfer of energy. There
is no such barrier in the atmosphere. Whatever
the accuracy of the analogy, the selective nature
of the atmosphere in its response to radiation is
of supreme importance to the earth's energy
budget.
Since the greenhouse effect depends upon
carbon dioxide and the other gases in the
atmosphere, it follows that any change in these
gases, including their relative concentration, will
have an effect on the intensity of the greenhouse
effect. Changes in greenhouse gas levels in the
past were brought about by natural processes,
but, since the middle of the nineteenth century,
human activities have had a major role in
increasing the intensity of the greenhouse effect
through the production of higher volumes of
carbon dioxide, methane and a number of other
greenhouse gases. Concern over the impact of
such changes has promoted the intensification
of the greenhouse effect to its present position as
a significant environmental issue.
The minor gases and the greenhouse effect
Although gases other than oxygen or nitrogen
account for only about 1 per cent of the
atmospheric total, they have an influence quite
out of proportion to their volume. The most
abundant of these is argon at 0.93 per cent by
volume, but it is inert. Another of these minor
gases, carbon dioxide has a much more active
role in environmental processes. It comprises only
0.03 per cent by volume, yet it makes a significant
contribution to the heating of the atmosphere,
and is a major participant in the process of
photosynthesis by which sugars, starches and
other complex organic compounds are produced
in plants.
The atmosphere is quite selective in its
response to solar radiation. It is transparent to
high energy, short-wave radiation, such as that
from the sun, but partially opaque to the lower
energy, long-wave radiation emanating from the
earth's surface. For example, a major proportion
of the radiation in the visible range of the
spectrum, between 0.3 and 0.7 micrometres (µm),
is transmitted through to the surface without
losing its high energy content (see Figure 2.1).
Once it arrives it is absorbed, the surface heats
up, and begins to emit terrestrial long-wave
radiation back into the atmosphere. This
radiation, from the infrared end of the
Oxides of sulphur and nitrogen
There are many other gases which from time to
time become constituents of the atmosphere.
These include sulphur dioxide, oxides of nitrogen,
hydrogen sulphide and carbon monoxide, along
with a variety of more exotic hydrocarbons,
which even in small quantities can be harmful to
the environment. All of these gases are natural
constituents of the atmosphere, released as a
