Development and the Environment
Acid Rain
Seen Environmental Learning Information Sheet no 8
What is acid rain?
Acid rain is the term used to describe air
pollutants that cause water and soil to become
more acidic. In turn this can harm plants and
animals that depend upon these elements for
survival. Acid precipitation is a more accurate
name since it can occur in the form of acid snow,
acid dew, acid fog, acid frost, acid hail and acid
dust.
Rain water is never totally pure. It always
contains some impurities from dust particles or
from absorbing gases from the air. The acidity or
alkalinity of a liquid is usually measured by its
pH level. Normal rainwater has a pH of around
5.6 because it naturally absorbs carbon dioxide in
the air to form a mild carbonic acid. Water that is
polluted and becomes too acidic will usually
have a pH of below 5.
Acid rain on the PH scale
What causes acid rain?
Acid precipitation has both natural and human
causes. Every time there is a flash of lightening,
oxygen and nitrogen gases fuse with the moisture
in the air and cause some nitric acid to be
deposited in raindrops. In the soil the nitric acid
is absorbed and acts as a nitrogen fertilizer for
plants. Volcanoes and forest fires may emit
sulphur dioxide(SO2) which mixes with water
vapour in the air and sunlight to create small
amounts of sulphuric acid. Even carbon dioxide
can mix with the water vapour in the air to make
small amounts of weak carbonic acid.
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SO2+H2O+sunlightH2SO4
N02+H20+sunlightH2NO2
Acid precipitation has been occurring in
relatively small amounts for billions of years.
However as a result of the industrial revolution in
the last 200 years resulting in the burning of coal
and other fossil fuels, and the increase in motor
car emissions, acid precipitation has become a
problem. Increased industrial activities meant
more coal burning in steam engines for power.
The fossil fuels which were laid down hundreds
of millions of years ago contain chemical
elements including carbon, hydrocarbon, sulphur
and nitrogen which were once stored in the
bodies of living things. When these fossil fuels
are burnt these chemical elements are released
into the atmosphere. Sulphur from the coal
begins to mix with the oxygen in the air to create
sulphur dioxide. The sulphur dioxide mixes with
the moisture in the air and comes down as
sulphuric acid rain. The problem has been
increasing over the last 200 years but it has been
since the 1950s that it has been recognised as a
major environmental threat. These days only half
of the sulphur dioxide in the atmosphere, comes
from natural sources such as volcanoes and
rotting organic matter, the rest all comes from
human activity.(see diagram below)
Burning of fossil fuels like oil and coal in power
plants, various industrial processes and the
exhaust fumes from millions of vehicles
contribute to the large amount of sulphur dioxide,
nitrogen oxides and carbon dioxide in the
atmosphere. One big problem is the fact that the
gases are often carried hundreds of kilometres by
the wind and then fall as acid rain on woodland
areas, and surface water in un-expecting
communities. At present the worst affected
places in the world are in the USA, Canada and
Europe, especially central and northern Europe.
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Theme: Development and the Environment
Topic No 8: Acid Rain
Dry and wet deposition Source: Outreach
In southern Africa large-scale air pollution
causing acid rain is largely limited to the
Mpumalanga highveld and Soweto, which
experience extremely high levels of sulphur
dioxide pollution. About 64 % of South Africa’s
electricity is generated at coal power plants in
Mpumalanga. Gauteng generally ranks as one of
the world's worst polluted areas, putting it in the
same league as Bangkok, Mexico City, Los
Angeles, Beijing and Sao Paulo.
What are the effects of acid rain?
Some of the pollutants do not rise very far into
the air and instead they return to the earth within
a few hours. These pollutants may fall directly
onto the surface of plants, buildings, water or soil
and this is called dry deposition. These
deposits can become acidic if dew or rain comes
into contact with them and turns them into acidic
droplets. If the pollutants fall into water then they
will dissolve and form an acidic liquid.
The pollutants of dry deposition corrode building
materials. Monuments such as the Parthenon in
Greece and the Taj Mahal in India are all being
damaged by acids in the air and buildings in
every major city in India are under attack from
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industrial pollution. The pollutants are blown
onto nearby farmland and forests and cause
damage to the trees, stunt the growth of plants
and reduce crop yields.
The longer the sulphur dioxide and nitrogen
oxides remain in the air the more likely they are
to react with moisture in the air and create
sulphuric and nitric acids. These acid pollutants
can stay in the air for several days and can be
carried hundreds of kilometres by the wind. This
is called wet deposition.
Wet deposition can directly attack the leaves of
the trees that it lands on as well as being able to
soak into the soil and in turn damage plants. It
can also end up in rivers and water-courses
killing aquatic plants and animals and polluting
drinking water. 80% of Norway’s lakes are now
considered to be either ‘dead’ or in a critical state
as a result of acid pollution and 300 lakes in
Ontario, Canada are estimated to have a pH of
less than 5. When acid rain increases the acidity
of the soil or waterthese can in turn dissolve
toxic metals. If these are absorbed by plants they
can be passed on to humans or animals that eat
the plants or their fruit. Cubatao in Brazil, is
considered to be one of the most polluted
industrial centres in the world. 80% of it’s flora
is gone, fish die in its polluted waters, more
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Theme: Development and the Environment
Topic No 8: Acid Rain
babies are born deformed here than anywhere
else in Latin America.
There are also risks to humans associated with
acid precipitation. It is believed to be one of the
factors exacerbating asthma and other respiratory
problems and in Tokyo, Japan, the air is now
suspected of being a health risk.
Acid rain is not a major problem in southern
Africa. However in eastern Mpumulanga in
South Africa where most of the coal burning
power stations and large metal working
industries are found, acid rain regularly falls. It is
estimated that the region produces nearly two
million tones of sulphuric acid and one million
tones of nitric acid pollution annually.
Unfortunately the effects of this pollution have
not yet been properly investigated.
(tree death) had spread across 15 European
countries and has claimed more than 70,00
square kilometres of forest. The symptoms of
Waldsterben are leaf discolouration, needle loss,
thinning of crowns and reduced canopy cover
and eventually death. Although there are many
theories on what has caused the decline in the
number of trees, it seems certain that one of the
major culprits is acid precipitation. The effects of
acid precipitation maybe combined with other
factors such as extreme weather conditions and
forest management practices but it is indisputable
that acid precipitation plays a large role in these
deaths of trees. The United Kingdom was
reported in 1987 to have the highest percentage
of damaged trees in the whole of Europe. 67% of
Britain’s conifers showed severe damage whilst
28.9% were described as having moderate to
severe damage.
Acid rain on the leaves decreases the rate at
which photosynthesis occurs. The acidity of the
rain also disrupts the natural uptake of nutrients
in the soil by the trees. Some soil nutrients are
even washed away by the acid rain.
After the trees have been weakened by the
nutrient disruption they are more likely to be
affected by pests and diseases. The tree
eventually dies as a result of nutrient disruption
or from disease or a combination of factors. The
trees of Southern Africa seem to be less effected
by acid rain than the slow-growing trees which
are found in the northern hemisphere. In Eastern
Canada sugar maples are dying out over vast
areas due to the increased acidity of the soils.
Trees and acid rain
Although acid rain is though to be responsible for
the death of many trees in Europe and North
America the problem is not confined to these
places. In South Africa acid rain is suspected of
damaging buildings and vegetation. The Kruger
National Park as well as the Eastern Transvaal
highveld are thought to be the areas greatest at
risk. There is evidence of damage to vegetation
in Zambia where sulphur compounds are made
from copper and zinc smelting.
Diseased trees as a result of pollution were
discovered in Germany in the 1960s and by 1984
half of the country’s woodland was showing tree
damage. By the end of the 1980s Waldsterben
It is not just trees which are at risk but other
kinds of vegetation as well and in the
Chongquing district of Sichuan Province in
Dispersing the problem Source: Outreach 59
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Theme: Development and the Environment
Topic No 8: Acid Rain
China, large areas of paddy fields where rice is
grown, have turned yellow after acidic rain.
What are the solutions?
There are no simple solutions to the problem of
acid rain. In fact acid rain is just one more sign of
the increasing environmental degradation that is
going on around the world. Since the main
sources of sulphur dioxide are coal-fired power
stations and the main source of oxides of
nitrogen are vehicle emissions and the burning of
fuel, more research must be done into alternative
energy sources which do not produce the same
harmful acidic emissions. Removing the acidic
particles from the burning of coal is possible but
it is very expensive and is not a long-term
solution to the problem.
Other considerations
In much of southern Africa there are many
people who do not have constant access to
electricity or who cannot afford electricity or
their homes. What should the governments’
priority be? Should they strive to provide
electricity for all households or should they focus
on reducing the emissions of power stations
which cause acid rain? Is there another option?
What is the potential for alternative energy
sources?
Southern Africa relies on fossil fuels for industry
and in the increasing urbanisation there has been
more emphasis placed upon industrialization.
Many countries energy needs rely on the burning
of fossil fuel. Is it fair to expect developing
countries to have to invest money into research
for alternative energy sources when the richer
countries are not following their own advice?
BUT
Namibia has a huge potential for solar and wind
power. Maybe instead of following what other
industrialised nations have done and relying on
non-sustainable, environmentally unfriendly
sources of energy, Namibia and other countries
like it should take the lead. After the initial
expense, the energy from the sun can be
harnessed without expense. It is not only a clean
renewable energy sources but a reliable and
economically viable one.
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Theme: Development and the Environment
Topic No 8: Acid Rain
Important ideas to stress in your teaching and learning
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Acid rain is formed when gases in the air (sulphur dioxide and nitrogen dioxide) combine with
water vapour in the presence of sunlight.
The gases causing acid rain are man-made resulting from the burning of fossil fuels in industries
and power plants.
These gases may travel hundreds of kilometers in the wind before falling as acid rain. As a result
solving the problem requires international co-operation.
Acid rain damages buildings, kills plants (especially trees) and animals and pollutes lakes, soils
and groundwater supplies.
Acid rain is a big problem in northern Europe and the north east of the USA. With growing
industrialization based on burning coal and oil, it is a significant problem in parts of China and
India. In southern Africa acid rain is most severe in eastern Mpumulanga, RSA.
Acid rain can be controlled by reducing power station and vehicle emissions. This however is very
expensive and so is only slowly being introduced. In the long term a change to alternative
technologies that generate energy without polluting is required.
Glossary
Aquatic
Degradation
connected with, consisting of, or dependent upon water.
the decline in the quality and performance of the environment.
Fossil fuels
any carbon-containing fuel, for example coal, peat, petroleum, and natural gas,
derived form the decomposed remains of prehistoric plants and animals.
relating to, derived from, or characteristic of living things.
a process by which green plants and other organisms produce simple
carbohydrates from carbon dioxide and hydrogen, using energy that chlorophyll
absorbs from sunlight.
things that pollute or cause harm to an area of the natural environment, for
example, chemicals or waste products that contaminate the air, soil, or water.
rain, snow, or hail, all of which are formed by condensation of moisture in the
atmosphere and fall to the ground.
Organic
Photosynthesis
Pollutants
Precipitation
Urbanisation
the accustoming of somebody to living in a town or city rather than in the rural
areas, and the causing of people to migrate to a town or city from the rural
areas.
Sources/ Further Reading
OUTREACH 59 The changing atmosphere – Part 4 Acid rain pollution
Acid Earth: The global threat of acid pollution, John McCormack
Air Pollution, acid rain and the future of forests, Sandra Postel
Acid Rain and air pollution, Jacqueline Sawyer
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