Environmental Engineering Reference
In-Depth Information
result of biological activity, created during
volcanic eruptions or produced by natural wood
and grass fires. Increasingly, however, their
presence is associated with pollution from
industrial or vehicular sources. In recent years,
concern has centred on the widespread
dissemination and detrimental environmental
impact of some of these gases. Increasing
industrial activity, and the continued reliance on
fossil fuels as energy sources, has caused a
gradual, but steady, growth in the proportion of
sulphur and nitrogen oxides in the atmosphere
over the past 2-3 decades. In combination with
atmospheric water, these gases—whether natural
or anthropogenic in origin—are the main
ingredients of acid rain. Anthropogenically
produced acid rain is commonly many times more
acid than its natural conterpart, however, and
has been identified as a major cause of damage
to aquatic and terrestrial ecosystems in North
America and Europe. Although it has received
less attention in recent years, it remains a
significant environmental problem in the
industrialized nations of the world, and there is
growing evidence that areas currently little
affected—the southern hemisphere, for
example—may not always be immune.
WATER IN THE ATMOSPHERE
The creation of acid rain would not be possible
without water, another of the major natural
constituents of the atmosphere. Lists of the
principal gases in the atmosphere—such as Table
2.1—commonly refer to dry air, but the
atmosphere is never completely dry. The
proportion of water vapour in the atmosphere,
in the humid tropics, may be as much as 4 per
cent by volume, and even above the world's driest
deserts there is water present, if only in fractional
amounts. At any one time, the total volume of
water in the atmosphere is relatively small, and,
if precipitated completely and evenly across the
earth's surface, would produce the equivalent of
no more than 25 mm of rainfall (Barry and
Chorley 1992). In reality, the distribution is very
Figure 2.2
The hydrologic cycle
