Environmental Engineering Reference
In-Depth Information
Particulate matter falls out of the atmosphere as
a result of gravity or is washed out by
precipitation. These processes have removed
extraneous gases and aerosols from the
atmosphere, usually quite effectively, for millions
of years.
Ongoing physical and biological activities—
such as volcanic eruptions, soil erosion and the
combustion or decay of vegetable matter—ensure
that the cleansing process is never complete, and
that, in itself, is a natural part of the system. There
are indications, for example, that a minimum
level of extraneous material is essential for the
working of such atmospheric processes as
condensation and precipitation. Thus, a
completely clean atmosphere may not be
desirable (Barry and Chorley 1992). Desirable
or not, it is unlikely to be achieved, given the
present rates of gaseous and particulate
emissions.
In the past, the main pollutants were natural
in origin, and sources such as the oceans,
volcanoes, plants and decaying organic material
continue to provide about 90 per cent of the total
global aerosol content (Bach 1979). Events, such
as the eruption of Mount Pinatubo, indicate the
continued capability of nature to provide massive
volumes of pollutants, but anthropogenic sources
are now paramount in many areas. Human
activities provide pollutants in such amounts, and
with such continuity, that the atmospheric
cleansing processes have been all but
overwhelmed, and a full recovery may not be
possible, even after large-scale attempts to reduce
emission levels.
Air pollution was one of the elements which
elicited a high level of concern during the heyday
of the environmental movement in the late 1960s
and early 1970s. It was mainly an urban problem
at that time, most common in large cities which
had high seasonal heating requirements, were
heavily industrialized, had large volumes of
vehicular traffic or experienced combinations of
all three. Even then, however, existing air
pollution control ordinances were beginning to
have an effect on the problem. In Pittsburgh, the
introduction of smokeless fuel, and the
establishment of emission controls on the iron
and steel industry, brought a steady reduction in
air pollution between 1945 and 1965 (Thackrey
1971). Similar improvements were achieved in
London, England, where sunshine levels in the
city centre increased significantly following the
Clean Air Act of 1956 (Jenkins 1969). The
replacement of coal by natural gas as the main
heating fuel on the Canadian Prairies, in the
1950s and 1960s, allowed urban sunshine totals
to increase there also (Catchpole and Milton
1976). Success was achieved mainly by reducing
the atmospheric aerosol content. Little was done
to reduce the gaseous component of pollution,
except in California, where, in 1952, gaseous
emissions from the state's millions of cars were
scientifically proven to be the main source of
photochemical smog (Leighton 1966). Prevention
of pollution was far from complete, but the
obvious improvements in visibility and sunshine
totals, coupled with the publicity which
accompanied the introduction of new air quality
and emission controls in the 1970s, caused the
level of concern over urban air pollution to
decline markedly by the end of the decade.
The relationship between pollution and
weather or climate is a complex one. Sometimes
climatic conditions will influence the nature and
extent of a pollution episode, while at other times
the linkages are reversed, allowing the pollutants
to instigate or magnify variations in climate. The
problem of acid rain illustrates quite well the
impact of atmospheric processes on the operation
and distribution of a particular group of
pollutants, whereas the issues of increased
atmospheric turbidity and the depletion of the
ozone layer illustrate the other relationship, in
which pollutants cause sufficient change in the
atmosphere to initiate climatic change.
The full complexity of the earth/atmosphere
system is only now beginning to be appreciated,
but in recent years, knowledge of the impact of
human social and technological development on
the atmospheric environment has grown quite
dramatically. Government sponsored studies on
such topics as the greenhouse effect and acid rain,
along with reports by respected scientists and
