Environmental Engineering Reference
In-Depth Information
4
Acid rain
The unrelenting pollution of the atmosphere by
modern society is at the root of several global
environmental issues. The emission of pollutants
into the atmosphere is one of the oldest human
activities, and the present problems are only the
most recent in a lengthy continuum. Issues such
as acid rain, increased atmospheric turbidity and
the depletion of the ozone layer, include
essentially the same processes which led to the
urban air pollution episodes of a decade or two
ago. The main difference is one of scale. The
previous problems had a local, or at most, a
regional impact. The current issues are global in
scope, and therefore potentially more
threatening.
which is oxidized into SO
2
and methane
sulphonic acid (MSA). The MSA is ultimately
converted into sulphate (Cocks and Kallend
1988). Acids formed in this way fall out of the
atmosphere in rain to become involved in a
variety of physical and biological processes once
they reach the earth's surface. The return of
nitrogen and sulphur to the soil in naturally acid
rain helps to maintain nutrient levels, for
example. The peculiar landscapes of limestone
areas—characterized by highly weathered
bedrock, rivers flowing in steep-sided gorges or
through inter-connected systems of under-
ground stream channels and caves—provide
excellent examples of what even moderately
acid rain can do.
In reality, since 'acid rain' includes snow, hail
and fog as well as rain, it would be more
appropriate to describe it as 'acid precipitation'.
The term 'acid rain' is most commonly used for
all types of 'wet deposition', however. A related
process is 'dry deposition', which involves the
fallout of the oxides of sulphur and nitrogen
from the atmosphere, either as dry gases or
adsorbed on other aerosols such as soot or fly
ash (Park 1987). As much as two-thirds of the
acid precipitation over Britain falls as dry
deposition in the form of gases and small
particles (Mason 1990). On contact with
moisture in the form of fog, dew or surface
water they produce the same effects as the
constituents of wet deposition. At present, both
wet and dry deposition are normally included in
the term 'acid rain' and, to maintain continuity,
that convention will be followed here.
THE NATURE AND DEVELOPMENT OF
ACID RAIN
Acid rain is normally considered to be a by-
product of modern atmospheric pollution. Even
in a pure, uncontaminated world, however, it is
likely that the rainfall would be acidic. The
absorption of carbon dioxide by atmospheric
water produces weak carbonic acid, and nitric
acid may be created during thunderstorms,
which provide sufficient energy for the synthesis
of oxides of nitrogen (NO
X
) from atmospheric
oxygen and nitrogen. During volcanic eruptions
or forest fires, sulphur dioxide (SO
2
) is released
into the atmosphere to provide the essential
component for the creation of sulphuric acid.
Phytoplankton in the oceans also emit sulphur
during their seasonal bloom period. The sulphur
takes the form of dimethyl sulphide (DMS)
