Environmental Engineering Reference
In-Depth Information
closely linked to the height of emission. Pollution
introduced into the upper westerlies or jet streams
is taken further, and kept aloft longer, than that
emitted into the boundary layer circulation. For
example, a parcel of air tracked from Toronto,
at an altitude of 5,000 m, was found well out
over the Atlantic, some 950 km east of its source,
in less than 12 hours. In the same time span, a
parcel of air close to the surface covered less than
a quarter of that distance (Cho
et al.
1984). In
this way pollutants originating in the US Midwest
cause acid rain in Ontario, Quebec and the New
England states; emissions from the smelters and
power stations of the English Midlands and the
Ruhr contribute to the acidity of precipitation in
Scandinavia.
The ultimate example of LRTAP is provided
by acidic pollution in the Arctic, which has been
traced to sources some 8,000 km away in North
America and Eurasia (Park 1987). The winter
atmospheric circulation in high latitudes causes
polluted air to be drawn into the Arctic air mass
in sufficient quantity to reduce visibility—
through the creation of Arctic haze (see Chapter
5)—and increase environmental acidity. Sulphur
dioxide and sulphate particles are the main
constituents of the pollution which originates in
the industrial regions of Eurasia and North
America, with the former contributing as much
as 80 per cent of the total (see Figure 4.6). The
net result at locations in the Canadian Arctic is
an atmospheric sulphate content during the
winter months which is at least twenty to thirty
times that of the summer months (Barrie and
Hoff 1985). Slightly higher winter concentrations
have been recorded in the Norwegian and
Russian Arctic (Barrie 1986). This seasonal
variation is reflected in the acidity of the
precipitation and the snowpack, which varies
from a pH of about 5.6 in the summer to 4.9-
5.2 in the winter months (Barrie 1986).
The problem of acid rain obviously transcends
national boundaries, introducing political
overtones to the problem, and creating the need
for international co-operation, if a solution is to
be found. That co-operation was not easily
achieved. Since the ingredients of acid pollution
are invisible, and the distances they are carried
are so great, it is not possible to establish a visible
link between the sources of the rain and the areas
which suffer its effects. It was therefore easy for
polluters to deny fault in the past. The
introduction of airborne sampling systems, using
balloons (LaBastille 1981), or aircraft (Pearce
1982d) has helped to change that. Tracer
elements added to a polluted airstream or source-
specific chemicals allow emissions from a
particular area to be followed until they are
deposited in acid rain (Fowler and Barr 1984).
Mathematical models developed in Europe also
allow the final destination of specific acid
emissions to be identified (Cocks and Kallend
1988). With such developments in monitoring
techniques, denial of guilt becomes more and
more difficult.
ACID RAIN AND GEOLOGY
The impact of acid rain on the environment
depends not only on the level of acidity in the
rain, but also on the nature of the environment
itself. Areas underlain by granitic or quartzitic
bedrock, for example, are particularly susceptible
to damage, since the soils and water are already
acidic, and lack the ability to 'buffer' or neutralize
additional acidity from the precipitation. Acid
levels therefore rise, the environmental balance
is disturbed, and serious ecological damage is the
inevitable result. In contrast, areas which are
geologically basic—underlain by limestone or
chalk for example—are much less sensitive, and
may even benefit from the additional acidity. The
highly alkaline soils and water of these areas
ensure that the acid added to the environment
by the rain is very effectively neutralized. In areas
covered by glacial drift, or some other
unconsolidated deposit, the susceptibility of the
environment to damage by acid rain will be
determined by the nature of the superficial
material rather than by the composition of the
bedrock. In theory, it is important to establish
background levels of acidity or alkalinity, so that
the vulnerability of the environment to
acidification can be estimated; in reality, this is
