Research on the effects of acid rain has shown
that it is damaging to aquatic life in lakes and streams,
diminishes the productivity of forests and crops, accelerates the
deterioration of buildings and other exposed structures, and
contributes to human health hazards. It is the result of acid
precipitation in the atmosphere through chemical transformation of
sulphur dioxide (SO2) and several nitrogen oxides
(NOX). In the air they become oxidized to form acid
sulphate (SO4) and acid nitrate (NO3),
respectively. These acids return to the earth in rain, snow, and
hail, and as dry microscopic particles, sometimes after travelling
hundreds or even thousands of miles.
Man-made sources of these air pollutants include
electric generating plants, smelting of ores, industrial
installations, motor vehicles, and residential and commercial
establishments using fossil fuels. Globally the natural and
man-made emissions are roughly equal. In North America and Europe,
however, man-made sources contribute more than 90 per cent of the
total. Man-made emissions of sulphur dioxide rose from about 7
million tons per year in 1860 to about 155 million tons per year in
1985, although the emissions during the period 1970- 1985 were
reduced substantially in several countries as strict pollution
standards were imposed.
In industrial economies, generation of
electricity in power plants using fossil fuels is the largest
single contributor to the emissions of sulphur dioxide
(SO2), and the transportation sector is the largest
source of nitrogen oxides. Although emissions of nitrogen oxides
were slightly reduced or stabilized during the period 1970-1985 in
some of the developed market economies, the total amount of
nitrogen released annually is still rising. Motor vehicles are the
major source and are more difficult to identify and regulate than
the major sources of SO2emissions.
Acid deposition far downwind from big pollution
sources greatly exceeds emissions from local sources in many
regions. In Austria, eastern Canada, the Netherlands, Sweden, and
Switzerland more than 60 per cent and in Norway about 75 per cent
of the sulphur deposition comes from other areas. Acid rain falling
over the Atlantic as far out as Bermuda and the acidity of snow in
the Arctic are other examples of long-range transport of acids.
Because relatively little of the harm from acid rain is suffered by
the communities where it is produced, many countries have been
unwilling to bear the costs of preventing it. Nevertheless, the ECE
protocol and recent consultations between Canada and the United
States indicate significant progress in international co-operation
on preventive measures.
A variety of approaches and technologies aimed at
the reduction of acid rain are available:
(a) Selection of fuels
low in sulphur and nitrogen content;
(b) Combustion
modification using limestone injection multi-stage burners,
fluidized bed combustion, and post-combustion emission controls
using scrubbers, which allow removal of 50 to 90 per cent of the
SO2 in coal-fired electric power plants;
(c) Use of the catalytic
converter to reduce emissions of nitrogen oxides in new automobiles
that burn unleaded gasoline;
(d) Increased fuel
efficiency;
(e) Vehicle sharing and
limitation of private automobile use.