CHAPTER 2: TOPIC: ACID RAIN
Learning Objectives
In this lecture, you will learn:
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Awareness about the global problems regarding the threats created by the man.
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What is Acid rain
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What are the causes of acid rain
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Mechanism of acid rain
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Effects of acid rain
We already studied one of the major global issues hampering the environment. Today we will be studying
rainfall with acid(s).
What goes up must come down... somewhere?
But it is true it is called acid rain which is also one of the global problems.
1. Why Do We Have Acid Rain?
Acid rain is strongly acidic rain which absorbs minute particles of nitric acid or sulfuric acid as it falls towards
the ground.
But why do we have acid rain?
The reason lies in atmospheric pollution. As human beings developed their economic behavior, they began to
use huge amounts of fossil fuels such as coal and petroleum. When fossil fuel is burned, sulfur and nitrogen
oxides are released into the atmosphere. These pollutants are affected by ultraviolet rays and combine with
atmospheric water vapor to form tiny particles of nitric acid and sulfuric acid that float in the atmosphere.
When these particles dissolve into rain, it falls to the ground as acid rain. This process is called “wet
deposition.” At other times these pollutants are absorbed into fog or mist in gas or particle form, are deposited
on forests, and eventually return to the soil; this is called “dry deposition.
Fig. 3.1: Chemical substances released from chemical plant chimneys are a factor in the contamination
of the atmosphere and lead to the formation of acid rain.
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Dr. Kamal E. M. Elkahlout
2. How Acidic Is Acid Rain?
When a substance’s pH (an index for the density of hydrogen ions) is lower than 7, it is acidic, and when its
pH is higher than 7 it is alkaline. Natural rain has a weak acidic level of about pH 5.6, because the carbon
dioxide in the atmosphere dissolves. Acid rain is defined as rain with an acidity level higher than pH 5.6. At
the present time, acid rain with a strong acidity level of pH 4 to 3 has been observed, and has caused serious
damage in many areas.
3. What does acid mean?
An acid is a substance with a sour taste that is characterized chemically by the ability to react with a base to
form a salt. Acids turn blue litmus paper (also called pH paper) red. Strong acids can burn your skin.
4. What is pH?
A pH scale (Fig. 3.2) is used to measure the amount of acid in liquid-like water. Because acids release
hydrogen ions, the acid content of a solution is based on the concentration of hydrogen ions and is expressed
as “pH.” This scale is used to measure the acidity of rain samples.
Fig. 3.2: pH Scale
The smaller the number on the pH scale, the more acidic the substance is. Rain measuring between 0 and 5 on
the pH scale is acidic and therefore called “acid rain.” Small number changes on the pH scale actually mean
large changes in acidity.
For example, a change in just one unit from pH 6.0 to pH 5.0 would indicate a tenfold increase in acidity.
Clean rain usually has a pH of 5.6. It is slightly acidic because of carbon dioxide which is naturally present in
the atmosphere. Vinegar, by comparison, is very acidic and has a pH of 3.
5. Where is acid rain a problem?
a. Atlantic region In western Canada, acid rain is not a problem now but could always become one if we are not
careful. Historically, lower levels of industrialization-relative to eastern Canada-combined with natural
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Dr. Kamal E. M. Elkahlout
factors such as eastwardly moving weather patterns and resistant soils (i.e., soils better able to neutralize
acidity), have preserved much of western Canada from the ravages of acid rain. If we monitor the
environment carefully and apply strict pollution controls when necessary, we should be able to prevent acid
rain from becoming an environmental concern in western Canada.
Fig. 3.3: Global view of Acid rain affected areas
b. Acid Rain: Downpour in Asia? Although there has been major progress in controlling acid-forming
emissions in some countries, the global threat from acid rain is far from over yet. In fact, the dimensions of
the acid rain problem are growing rapidly in Asia, with sulfur dioxide (SO2) emissions expected to as
much as triple from 1990 levels by 2010 if current trends continue. Curtailing the already substantial acid
rain damage in Asia and avoiding much heavier damages in the future will require investments in pollution
control on the order of those made in Europe and North America over the past 20 years. An estimated 34
million metric tons of SO2 were emitted in the Asia region in 1990, over 40 percent more than in North
America. Acid deposition levels were particularly high in areas such as southeast China, northeast India,
Thailand, and the Republic of Korea, which are near or downwind from major urban and industrial centers.
The effects are already being felt in the agriculture sector. Researchers in India found that wheat growing
near a power plant where SO2 deposition was almost five times greater than the critical load (the amount
the soil can safely absorb without harm) suffered a 49-percent reduction in yield compared with wheat
growing 22 kilometers away.
The damage created by acid rain in Europe and North America is truly grave. It is reported that in many
European countries about one half of the total area of forests has been damaged. In particular, in Germany’s
“Black Forest” or “Schwarzwald,” more than 50% of the forest has been severely damaged by acid rain. Many
trees have dried out and died already, while many others have discolored leaves. Even tree planting has failed
to halt this destruction, since the soil is already acidic.
Acid rain is a problem in eastern Canada because many of the water and soil systems in this region lack natural
alkalinity - such as a lime base - and therefore cannot neutralize acid naturally. Provinces that are part of the
Canadian Precambrian Shield, like Ontario, Quebec, New Brunswick and Nova Scotia, are hardest hit because
their water and soil systems cannot fight the damaging consequences of acid rain. In fact, more than half of
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Canada consists of susceptible hard rock (i.e., granite) areas that cannot neutralize the effects of acid rain. If the
water and soil systems were more alkaline-as in western Canada-they could neutralize or “buffer” against acid
rain naturally.
Table 3.1: Sources of the acids in clean and polluted air
6. Impact of Acid Rain
a. Human Health
Most importantly, acid rain can affect health of a human being. It can harm us through the atmosphere or
through the soil from which our food is grown and eaten from. Acid rain causes toxic metals to break loose
from their natural chemical compounds. Toxic metals themselves are dangerous, but if they are combined with
other elements, they are harmless. They release toxic metals that might be absorbed by the drinking water,
crops, or animals that human consume. These foods that are consumed could cause nerve damage to children or
severe brain damage or death. Scientists believe that one metal, aluminum, is suspected to relate to Alzheimer’s
disease.
One of the serious side effects of acid rain on human is respiratory problems. The sulfur dioxide and nitrogen
oxide emission gives risk to respiratory problems such as dry coughs, asthma, headaches, eye, nose, and throat
irritation. Polluted rainfall is especially harmful to those who suffer from asthma or those who have hard time
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breathing. But even healthy people can have their lungs damaged by acid air pollutants. Acid rain can
aggravate a person’s ability to breathe and may increase disease which could lead to death.
b. Destruction of Wildlife in Freshwater Habitats
• Acid rain can leach aluminium from the soil into ground water, lakes and rivers poisoning fish and plant roots
The damage caused by acid rain first appears in rivers, lakes and marshes. Normally, the pH of the water in
lakes and marshes is about 7 to 7.5. It raises the acidity of the water in rivers, lakes and marshes to a high level.
When the water of a lake or marsh declines to pH 5 or so, at first the plankton and aquatic vegetation disappear.
Needless to say, the fish that feed on these living things also decline in number.
When the water becomes more acidic than pH 4.5, this affects the hatching of fish eggs and the functioning of
the fishes’ gills, and eventually the fish die out. Then, as the acidity level of soil increases, aluminum and other
harmful metals contained in the soil begin to dissolve, which also contributes to the death aquatic life.
Eventually the lakes cannot support life at all. At the present time, the water of many rivers, lakes and marshes
has become highly acidic, leading to extensive eradication of aquatic life. The problem has become especially
serious in Northern Europe and Canada, and we are deluged with reports of fish disappearing from one lake
after another.
c. Destruction of Forests
• Acid rain disrupts the process of photosynthesis resulting in damage to plant life
• At low concentrations it retards the production of chlorophyll and at high concentrations it forms sulfuric acid
which kills the plant.
Acid rain makes soil highly acidic, which leads to the drying and death of forests. Soil naturally contains many
metallic ions such as calcium and magnesium, and these help to neutralize acid to a certain extent, even in the
case of acid rain. However, when this neutralization power is exhausted, the soil’s acidity increases, killing
microorganisms and worms.
The disappearance of microorganisms which dissolve decaying leaves and convert them into fertile soil has an
obvious effect on the growth of trees. When the soil’s acidity level exceeds pH 4.2, the metals in the soil begin
to dissolve. Aluminum is especially damaging, and damages tree roots.
There are also cases when acid rain is enters directly into the leaves, trunks, and branches of plants and
damages trees. This type of damage has been reported to be on a much larger scale for needle-leafed trees than
for deciduous trees. This is probably because the leaves of needle-leafed trees are present all year long, and are
thus more exposed to acid rain.
Fig 3.4: A forest killed by acid rain. In Europe and North America the death of forests as a result of acid
rain is now a serious problem.
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Dr. Kamal E. M. Elkahlout
d. Destruction of Historical Relic
• Acid rain is a form of environmental pollution that damages buildings and marble statues by reacting with the
calcium carbonate to form soluble calcium hydrogen carbonate (calcium bicarbonate, Ca(HCO3)2
CaCO3 + acid rain ——> Ca(HCO3)2(aq)
Historical sites and buildings are also damaged by acid rain, especially in Europe with its many historical
edifices made of marble and copper. The chemical reaction between marble and sulfuric acid changes the
marble into plaster. Therefore, ancient sculptural works made of marble often crumble after contact with acid
rain.
Copper items also break down in contact with acid rain. For example, the copper plates of the United States’
Statue of Liberty were eaten away to a dangerous degree by acid rain, and needed large-scale reconstruction.
Historical structures and remains, which have been handed down as cultural heritage for centuries, have been
in danger of being lost in the past several decades simply because of acid rain.
Fig. 3.5: A stone statue eaten by acid rain
7. Acid Rain Pollution is now widespread
At the present time, the problem of acid rain in Europe and North America is no longer a local or even a
regional problem; it has become a truly grave, wide ranging environmental problem. The polluting
substances that cause acid rain affect not only the areas near the pollutants’ source, but also move thousands
of kilometers on high-altitude air currents.
It has been clearly established that these pollutants cross national boundary lines and pollute very wide areas.
Until recently, tall smokestacks were built in large cities as an antipollution measure, but it was found that
tall smokestacks actually help to push smoke with pollutants to the upper atmosphere and contribute to the
spread of pollutants to locations far from the pollutants’ source. This border crossing, wide area pollution
created international problems, so that in Europe, the Long-Range Transboundary Air Pollution Treaty was
ratified in 1979.
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Dr. Kamal E. M. Elkahlout
Fig. 3.6: Deleterious effects of acid rain on historical buildings
8. What you can do to help reduce acid rain emissions?
Sulphur dioxide and nitrogen oxides are the main pollutants that cause acid rain. These pollutants are emitted
largely by the combustion of fossil fuels. Reducing the use of fossil fuels therefore, including the use of
electricity generated by coal- and oil-fired power plants, will help reduce acid rain-causing emissions.
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Dr. Kamal E. M. Elkahlout