Environmental Engineering Reference
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diminish the impact of other elements such as
the greenhouse effect or ozone depletion which,
in whole or in part, make their presence felt
through that budget.
background level is rising, as a result of the
increasing volume of aerosols of anthropogenic
origin, although the evidence is sometimes
contradictory (Bach 1979).
Measurements since the 1930s—in locations
as far apart as Mauna Loa in Hawaii, Davos in
Switzerland and the Russian Caucasus—show a
sharp rise in the atmosphere's aerosol content,
or turbidity as it is called. Results from such
stations, located at high altitudes, and relatively
remote from the world's main industrial areas,
are considered representative of global
background aerosol levels. Recent observations
of increasing cold season atmospheric pollution
in high latitudes—the so-called 'Arctic haze'—
are also considered indicative of rising global
levels (Environment Canada 1987). Volcanic
activity may also have provided some natural
enhancement in recent years, but the close
correspondence between elevated turbidity levels
and such indicators of human development as
industrialization and energy use suggests that
anthropogenic sources are major contributors.
Some studies claim, however, that the
observations are insufficient to allow the human
contribution to increased turbidity to be
identified (Bach 1979).
Any increase in the turbidity of the atmosphere
should cause global temperatures to decline, as
the proportion of solar radiation reaching the
earth's surface is reduced by scattering and
absorption. In addition, the condensation of
water vapour around atmospheric aerosols
would lead to increased cloudiness and a further
reduction in the transmission of incoming
radiation. This approach has been used to explain
the decline in global average temperatures which
occurred between 1940 and 1960, and in the
1970s it was seen by some as the mechanism by
which a new ice age would be initiated (Ponte
1976). Such thinking is also central to the concept
of nuclear winter which would be caused by a
rapid temperature decline following the injection
of large volumes of aerosols into the atmosphere
(Bach 1986).
Providing a dissenting opinion are those who
claim that an increase in atmospheric aerosols
ATMOSPHERIC AEROSOLS
In addition to the gaseous components of the
atmosphere and the water in its various forms,
there are also solid or liquid particles dispersed
in the air. These are called aerosols, and include
dust, soot, salt crystals, spores, bacteria, viruses
and a variety of other microscopic particles.
Collectively, they are often regarded as
equivalent to air pollution, although many of
the materials involved are produced naturally
by volcanic activity, forest and grass fires,
evaporation, local atmospheric turbulence, and
biological processes. The proportion of
particulate matter in the atmosphere has
increased from time to time in the past,
sometimes dramatically, but in most cases the
atmosphere's built-in cleansing mechanisms
were able to react to the changes, and the
overall impact was limited in extent and
duration. When the island of Krakatoa
exploded in 1883, for example, it threw several
cubic kilometres of volcanic dust into the
atmosphere. Almost all of it is thought to have
returned to the earth's surface in less than five
years, as a result of particle coagulation, dry
sedimentation and wash-out by precipitation
(Ponte 1976). The 'red-rain' which occasionally
falls in northern Europe is a manifestation of
this cleansing process, being caused when dust
from the Sahara is carried up into the
atmosphere by turbulence over the desert, and
washed out by precipitation in more northerly
latitudes (Tullett 1984). Thus, the atmosphere
can normally cope with the introduction of
aerosols by natural processes. Cleansing is never
complete, however. There is always a global
background level of atmospheric aerosols which
reflects a dynamic balance between the output
from natural processes and the efficiency of the
cleansing mechanisms. Data collected over the
past several decades suggest that the
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