Geography Reference
In-Depth Information
3
Acid precipitation
A.M.Mannion
INTRODUCTION
Precipitation comprises all solid and liquid forms
of water that are deposited on the Earth's surface
from the atmosphere. It includes rain, snow, hail,
dew and sleet. All forms of precipitation are acid in
so far as they have a pH of less than 7; in general,
precipitation unaffected by human activity has a pH
of 5.6. This naturally acidic state of precipitation is
caused by the combination of water and carbon
dioxide in the atmosphere to produce carbonic
acid. However, the term acid precipitation, or acid
rain, is usually applied to precipitation characterised
by a pH of less than 5.1 (Elsworth 1984) and that
contains sulphurous and nitrous acids. The latter are
derived from various sources, among which fossil
fuels are the most important.
The phenomenon of acid precipitation was
first recognised by Robert Angus Smith, a Scottish
chemist, in 1852 following a survey of air
pollution in Manchester. Smith coined the term
'acid rain', which he associated with sulphur
dioxide emissions from fossil fuels burned in local
factories.Various observers subsequently noted the
impact of acid precipitation on aquatic and
terrestrial ecosystems. For example, Gorham
(1958) noted that the chemistry of upland lakes in
the English Lake District was affected by acid
precipitation from air masses that had passed over
Britain's industrial heartland. Despite this
recognition of its impact, acid precipitation did
not emerge as a major environmental issue until
the late 1960s. By this time, Scandinavian
ecologists were becoming concerned about
declining fish stocks; they were also beginning to
recognise transboundary transportation of acid
precipitation, i.e. the export of acid precipitation
from source areas such as the industrial regions of
Europe and the UK and its transport to and
deposition in far distant areas such as Scandinavia.
In this context, acid precipitation became a
political as well as an ecological issue. The polluters
were unwilling to recognise this, and the polluted
demanded mitigation measures. The impact of
acid precipitation manifests in many ways. Both
terrestrial and aquatic ecosystems may be adversely
affected through reductions in pH, which have
repercussions for the biota and water quality;
human health may be impaired and building
materials may be corroded.
Internationally agreed measures to curb acid
precipitation are now in operation in Europe and
North America, where the problem is most acute.
The first of these was established in 1979. This was
the Convention on Long Range Transboundary
Air Pollution (CLRTAP), a protocol that was
adopted in 1985 and that became known as the
'30 percent club' because of the agreement
between its thirty-five members to reduce sulphur
emissions by 30 per cent of 1980 levels by 1993.
Britain, Poland, Spain and the USA declined to
subscribe to the convention, although eventually
all succeeded in reducing sulphurous emissions to
a degree. Another protocol was signed in 1994 in
Oslo to tailor targets to polluters rather than to
reassert overall objectives.
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