Acid Rain
What is acid rain? Acid rain is the term for pollution caused when sulfur and
nitrogen dioxides combine with atmospheric moisture. The term ‘acid rain’ is
slightly misleading, and would be more accurate if deemed ‘enhanced acid rain’,
as rain occurs acidic naturally. Acidity is measured on what is know as the pH
scale. Fourteen is the most basic, seven is the most neutral, and zero is the
most acidic. Pure rain has a pH level of 7, which is exactly neutral. The
acidity of rain is determined by the pH of pure water in reaction with
atmospheric concentrations of carbon dioxide, resulting in carbonic acid. These
particles partly dissociate to produce hydrogen ions and bicarbonate ions. A
bicarbonate atom is an ion formed by one hydrogen atom, one carbon at atom, and
three oxygen atoms, and is very effective in natural waters at neutralizing
hydrogen ions and reducing acidity. The dissociation results in the natural
acidity of pure rain, which is moderately acidic at a pH of 5.7. Rain less than
5.7 is considered ‘acid rain’, meaning it has reacted with acidic atmospheric
gases other than carbon dioxide, such as sulfur dioxide and nitrogen dioxide.
Sulfur dioxide is produced by electric utilities, industrial, commercial and
residential heating, smelters, diesel engines and marine and rail transport,
which creates sulfuric acid in rain. Nitrogen dioxide will also react with the
rain, caused largely by transportation (cars, trucks, planes, etc.) and electric
utilities, producing nitric acid. There is a certain degree of naturally
occurring acidity in rain water. This acid is from reaction with alkaline
chemicals, found in soils, lakes and stream, and can occasionally occur when a
volcano erupts as well. Bacterial action in soils and degasing from oceanic
plankton also contribute to the acidity found in rain. More than 90% of the
sulfur and 95% of the nitrogen emissions which occur in North America are due to
the pollution created by humans.1 How Is Acid Rain Formed? Acid rain consists
mainly of acids formed in the atmosphere. It consists of the oxides of sulfur,
SO2 and SO3, and of nitrogen NO and NO2. Let us examine the major contributor to
acid rain, sulfur oxides. Natural sources which emit sulfur dioxide include
volcanoes, sea spray, plankton and rotting vegetation. Despite these natural
occurrences, the burning of fossil fuels (such as coal and oil) can be largely
blamed for the emissions. The chemical reactions begin as energy from sunlight,
in the form of photons, hit ozone molecules (O3) to form free oxygen (O2), as
well as single reactive oxygen atoms (O). The oxygen atoms react with water
molecules (H2O), producing electrically charged, negative hydroxyl radicals
(HO). These hydroxyl radicals are responsible for oxidizing sulfur dioxide and
nitrogen dioxide, which produces sulfuric acid and nitric acid. Some particles
will settle to the ground (in the form of acid deposition) or vegetation can
absorb some of the SO2 gas directly from the atmosphere. When sulfur dioxide
comes in contact with the atmosphere, it oxidizes and forms a sulfate ion. It
becomes sulfuric acid as it joins with hydrogen atoms in the air and falls down
to earth. Oxidation occurs most in clouds, especially in heavily polluted air,
where other compounds such as ammonia and ozone help to catalyze the reaction,
increasing the amount of sulfur dioxide changing to sulfuric acid. Not all of
the sulfur dioxide is converted to sulfuric acid, and it is not uncommon for a
substantial amount to float up into the atmosphere, move to another area, and
return to earth as sulfur dioxide, unconverted. S (in fossil fuels) + O2 =* SO2
2 SO2 + O2 =* 2 SO3 Much of the sulfur dioxide is converted to sulfur trioxide
in the atmosphere SO3 + H2O =* H2SO4 The sulfur trioxide can then dissolve
within water to form sulfuric acid Nitric oxide and nitric dioxide are mainly
from power plants and exhaust fumes. Similar to sulfur dioxide, reactions are
heavily catalyzed in heavily polluted clouds where iron, manganese, ammonia and
hydrogen peroxide are present. Also, the formation of nitric acid can trigger
further reactions which release new hydroxyl radicals to generate more sulfuric
acid. The following is a typical reaction, which is direct combination of
nitrogen and oxygen at the high temperature inside a car engine. N2 + O2 + heat
=* 2NO 2NO + O2 =* 2NO2 This nitrogen monoxide immediately reacts with oxygen
and forms nitrogen dioxide in the following reaction 3NO2 + H2O =* 2HNO3 (aq) +
NO The nitrogen will then dissolve in water in the atmosphere and produce nitric
acid There are several other potential contributors to acid rain. These include
oxidation by products of alkene-ozone reactions, oxidation by reactions of NxOy
species and oxidation by peroxy radicals. Each of these reactions, however prove
to be minor contributors and are rather insignificant. How Is Acid Rain Harmful?
Environmental Hazards Aquatic Ecosystems Acid rain has an effect on virtually
all ecosystems it touches. Perhaps the most prominent, and equally as troubling
is the harmful results it produces when in contact with lakes, streams and
ponds. Scientists studying the effects of acid rain went to a lake about 135 km
away from the Ontario- Manitoba border called Lake 223. This lake, so far north
acid rain did not reach it, was extremely healthy, and was a perfect setting to
explore the effects of acid rain on aquatic ecosystems. In 1974, scientists
began to add sulfuric acid into the lake. The acid was added very slowly, and it
was four years later when they saw a major change. The freshwater shrimp began
to die out. Fathead minnows stopped reproducing and began to vanish. As the
scientists continued adding acid to Lake 223 in low amounts, large algae mats
began to form and crayfish became unhealthy and died. Seven years after the
beginning of the experiment, the lake trout stopped reproducing, and most of the
fish species, leeches, crawfish and mayflies began to die. In 1984, the
scientists stopped adding the acid. Without the addition of deadly sulfuric
acid, the lake slowly began to recover. Some of the fish species began to
recover, however some of the scientists estimated it would take one hundred
years for the lake to fully recover, even without the addition of any more acid.
Fish can still live in a lake with a low acid level, however they will get sick
and not grow to proper proportions. Often the fish will not reproduce, and
eventually, as the acid level increases, all the fish will die. The acid will
also ‘leach’ metals from the bottom of the lake. There are metals contained
within the mud and rocks of the lake bottom, however they remain not dangerous
as long as they are not released. The acid will draw out these harmful metals
and dissolve them in the water, resulting in the deterioration and disappearance
of a species. One of these damaging metals is aluminum, which will coat and burn
the gills of the fish as it intakes the polluted water. Some fish found in
acidic lakes contain higher levels of mercury in their bodies, which is harmful
to humans, resulting in the government telling society to limit the amount of
fish they eat from certain lakes and rivers. If the numbers of one species or
group of species changes in response to acidification, the ecosystem of the
entire body of water is likely to be affected through the predator-prey
relationships. Let us examine how acid rain is dangerous to fish. A freshwater
fish’s respiration consists of a ‘trade’ of hydrogen ions (H+) in their blood
for sodium ions (Na+) from the water around them. If the concentration of
hydrogen ions in the water is increased, which is essentially what happens when
pH falls, there are (proportionally) fewer sodium ions. Fish are forced to
absorb more hydrogen while finding it harder to obtain sodium. The acidity of
their blood increases, while the salt content drops. An experiment involving
brown trout showed that at a pH of 5.2 or lower, this process was fatal to this
species, and is likely deadly to many other trout species. The following chart
shows the steps typical to freshwater fish as the acidity increases. (Fig 1-1)
ACIDITY LEVEL (pH) EFFECTS ON AQUATIC LIFE 7 Neutral, H+ and H- are in balance
6.8 Shells of clams and snails become thinner, due to lack of hazardous calcium
ions in the water 6.6 The viability of eggs of the fathead minnow is reduced,
rain can have and fewer eggs hatch 6.5 Lake trout begin to have difficulty
reproducing, clams and snails become scarcer, green algae growth increases 6
Several clam and snail species disappear, several trout species populations
decrease, the smooth newt is gone, smallmouth bass, walleyes and spotted
salamanders have difficulty reproducing, several mayfly species cease to lay
eggs 5.8 Copepods (a critical link of crustaceans in the marine food chain) are
gone, crayfish have trouble regrowing exoskeleton after molting 5.7 Several
algae species decrease, while filamentous green algae increases, plankton
decreases 5.5 Rainbow trout, fathead minnows and smallmouth bass lose
considerable population, walleyes, brook trout, roach, lake trout and shiners
don’t reproduce, leeches and mayfly larvae vanish. 5.4 Crayfish reproductivity
is impaired. 5 Snail and clams are extinct. All but one species of crayfish are
extinct, brook trout, walleyes and most bullfrogs are gone, most fish species
experience reproduction difficulties, zooplankton population begins to drop,
green and green-blue algae mats have largely spread 4.8 Leopard frog numbers
decline 4.5 Mayflies and stoneflies vanish, a slowing in growth rate and oxygen
uptake of bacteria is notable 4.2 The common toad disappears 4 The oxygen output
of Lobelia plants declines 75% 3.5 Virtually all clams, snails, frogs, fish and
crayfish vanish 2.5 Only a few species of acid-tolerant midges, bacteria and
fungi are alive 2 In practical terms, the lake is sterile Two hundred and twenty
lakes in Ontario have been found acidified, meaning their pH is less that 5.1
year round.2 Terrestrial Plant Life It is much more difficult to solve the
mystery of forest destruction compared to that of a lake. This is partially
because trees live so much longer than fish do, and acid rain damage in trees
may not show up for thirty or forty years. It is also very difficult to
replicate forest conditions in a laboratory, such as insects, cold winters,
pollution, elevation and abrupt changes in rainfall. Each of these conditions
put stress on the trees and can be considered variables. Many scientists are
convinced that because of the complexity of a forest ecosystem, it is nearly
impossible to prove the death of forests is due to pollution in the form of acid
rain, but deduce from many experiments it is a main factor in forest
destruction. Deciduous trees are like air filters, and screen particles that
pass through the air around them. These particles collect on the leaves of the
tree, and studies have shown that when these particles contain acid they can
cause damage to the leaves. The leaves are the part of the tree that help make
food, hence any damage to the leaves will result in harm to the health of the
entire tree. Coniferous trees are vulnerable to the harmful effects of acid rain
as well. The tree’s needles are designed to nourish the tree after they fall to
the ground. Each needle houses whole colonies of microscopic bacteria and algae
that help the tree change nitrogen into food at the roots. Acid rain will often
burn away this material, thereby reducing adequate food supply, and weakening
the tree’s health. After the damage has been done to leaves and needles, acid
rain harms the trees even more through the soil. Soil has a level of acid. Acid
in the soil can do damage to the trees by releasing aluminum, which, once in
contact with acid, becomes highly poisonous to forests. The aluminum will enter
the tree’s hairlike roots, choking them, and when these become clogged, the
upper branches are no longer nourished. Even though there may be plenty of
moisture in the soil, the tree can die of thirst. Scientists have discovered
that the aluminum content in soil has tripled since the 1960s.3 Acid rain also
kills important organisms on the forest floor. The process of decomposition is
interrupted as the acid kills many of the bacteria and fungi that live on the
forest floor. At a pH level of 4.0, the earthworm dies, further damaging the
decomposition process. Without earthworms and bacteria to decompose the debris
consisting of animal and bird droppings, twigs and dead leaves, the materials
continue to build on the forest floor. When debris builds up, seedlings from the
trees are not able to survive, because they can not work their way down to the
soil to root. This causes the forest to slowly disappear, as older trees die,
and the forest will not be able to rejuvenate itself. Acid rain is hardest on
trees high up in mountains, because it is often covered in mist or fog,
literally bathing the trees in an acidic atmosphere. Trees also suffer because
of changes in the soil. Acid rains leach metals (draw metals out of mud and
rocks) in the soil, and the trees in turn intake these harmful metals through
their roots. Figure 1-2 shows the damage that acid rain can to do a forest Human
Health It is known that the earth contains many metals that are potentially
dangerous to humans, such as lead, mercury, and aluminum. Most of the time these
metals are harmless because they are in the soil, bonded to other elements. The
problem occurs when acid detaches these metals from the rocks and soils, and can
be carried deep into the ground and make their way to underground streams. These
streams eventually connect to our water sources. Medical researchers have found
these metals can be dangerous, and on rare occasion, is even fatal. Aluminum has
been found to kill people who have kidney problems, and can also collect in
brain tissue. Some scientists even suspect that aluminum deposits on the brain
cause Alzheimer’s disease. (A disease that results in memory loss, nervous
system problems, and death. Acid rain is known to irritate the whole respiratory
system, beginning with mucous membranes in the nose and throat, all the way to
tissue in the lungs. Consequently, acid rain has an increased effect on people
with respiratory problems. The U.S. Council on Environmental Quality estimates
health-related problems due to acid precipitation cost the United States $2
billion per year.4 In August 1987, over one hundred people were treated for eye,
throat, and mouth irritation when 1.8 metric tonnes of highly toxic sulfur
dioxide gas leaked from an Inco plant near Sudbury, Ontario. Even Fig 1-2 This
picture shows how a coniferous forest has been virtually destroyed. Acid rain is
blamed for the destruction of terrestrial ecosystems around the world. without
accidents, the sulfur dioxide regularly emitted from Inco smokestacks has been
linked to chronic bronchitis in Inco employees.5 Drinking Water Acid rain
damages drinking water in various ways. Thus far, amounts of metals in drinking
water have been minimal, however the fact that metals even leak into the water
is troubling to scientists. Since metals remain in the body once ingested, over
time, small amounts accumulate into large quantities, and it has yet to be
concluded how large an amount will prove to be harmful to humans. Acid rain
causes damage by loosening metals off metal water pipes. Modern plumbing uses
plastic tubing, but older systems have copper pipes. The copper pipes are held
together by a mixture of tin and lead. Lead is known to be extremely dangerous
to humans, even in small amounts, and will cause damage to the brain and nervous
system. A study that was done in Ontario found that water sitting in plumbing
pipes for ten days contained hazardous levels of copper and lead. This discovery
could be a widespread danger, since often people will go on vacation and not
shut off the plumbing, allowing water to sit and absorb these dangerous metals.
Acid rain can also dissolve the reinforcements that occur around large water
pipes. In some parts of the United States, asbestos is used to reinforce the
cement bases that hold water pipes. Asbestos is not dangerous when bound to the
cement, but is highly dangerous when separated, and has been linked to cancer
and other serious diseases. Many health officials worry that loose asbestos will
find it’s way to the city’s water when acid rain comes in contact with the
cement. Effects On Man Made Structures Scientists are becoming increasingly
concerned with acid rain’s destruction of the ‘built environment’. There are
objects in our built environment that are irreplaceable. Historic landmarks and
statues, old cathedrals and temples, paintings and sculpture – all are part of
the built environment and are slowly being damaged. Some of these objects are
practical, making life easier, safer or more comfortable. Many factors determine
how much damage acid rain will do, including the amount of rain, the location,
and direction of wind. All influence the amount of corrosion done. Areas that
have a large amount fog or humidity tend to suffer more than dry areas, which is
why many steel bridges located over water get rusted and corroded by acid. When
metal is decayed, it cannot take the same amount of stress of weight as when it
was originally created. Acid rain has been blamed in several collapses of
bridges around the world. Acid rain corrodes the steel track used on railroads,
thus the tracks must be constantly checked. Metal in air planes can also be
eaten away by acid rain. The United States Air Force spends more that $1 billion
every year to repair or replace damaged parts.6 A study done in Sweden showed
that metal rusts four times faster in areas that receive a lot of acid rain.
This figure is staggering, and yet, metal is not the only material damaged by
acid rain. Houses and buildings made of brick and stone are affected as well.
Acid rain can dissolve the mortar, which is used in cement to hold bricks
together. When the mortar is worn away, the bricks crumble more easily, because
they shift and cannot stay intact against the heavy weight of the bricks
pressuring from above. The corrosive effects of acid rain are particularly
obvious on limestone, because it is composed of calcium carbonate, which is
highly reactive with acid rain. Tombstones made of marble (which is
metamorphosed or heated limestone) have been badly damaged, while older
tombstones made of slate remain intact. Famous buildings such as the Taj Mahal,
The United States Capitol building and the Lincoln Memorial in Washington, are
all being continually damaged by acid rain. Statues made of bronze and copper
are particularly susceptible to corrosion. These statues turn green naturally,
and this covering, called a patina, acts as a protective shield against the
elements. Acid rain eats away at the patina, and where the acid dissolves the
green covering, it leaves a streaky black coat. This process ruins statues
throughout the world. How Does Acid Rain Affect the Economy? Canada/American
Relations Canada is particularly susceptible to the effects of acid rain. Its
geographical location places it directly in the path of a large amount of U.S.
emission, and the granite bedrock of the Canadian Shield has a poor buffering
quality. (A buffer is a material that can chemically weaken acid soil and is
less harmful to the environment, such as lime or baking soda.) The lack of such
a quality renders Eastern Canada highly vulnerable to damage due to United
States pollution. Canada suffers more from acid rain than the United States
does, even though much of the pollution originates in the United States. Acid
rain costs Canadians hundreds of millions of dollars every year. To try and
decrease the large amounts of money the pollution is costing tax payers, Canada
has passed laws to force its electrical companies to cut down on harmful
emissions. However, no matter what laws are passed in Canada, it is not possible
to stop U.S. power plants from sending acid in its direction. Figure 1-3
displays amounts of emissions created by the United States and Canada. The Gavin
power plant is an excellent example of how the United States sends tonnes of
acid to Canada every year. Every hour, this power plant burns 600 tonnes of
coal. The higher the smokestack, the further the dangerous gases will travel,
and the Gavin smokestack is 1 103 feet tall.7 Obviously, The Gavin can not be
solely blamed for the pollution, but it is power plants such as these that have
caused trouble between the two countries. It is estimated that about 50% of the
sulfate deposited in Canada derived from American sources.8 Sixty of the largest
plants and thus largest polluters are located in the Ohio Valley, a short
distance away from vulnerable Canadian land. In 1980, Canada and the United
States signed a Memorandum of Intent, an agreement that both countries would
make acid rain control a priority. They both promised to focus on developing
ideas to cut down the amount of sulfur dioxide and nitrogen oxide emissions
being pumped into the air. In the past, Canada has presented devastatingly large
figures to the United States, in an attempt to have them change laws and
regulations regarding pollution. Unfortunately, the attempts thus far have been
unsuccessful, as the US government requests more testing and studies instead of
altering laws. In the recent past, the negotiations between Canada and United
States representatives have been hardly reminiscent of efforts put forth by
Canadian officials. Many U.S. politicians still qualify acid rain as a ‘minor’
problem, and it is treated as such, according to Raymond Robinson, chairman of
the Canadian Environmental Assembly.
