Acid Rain

Acid rain is rain that is more acidic than
normal. Acid rain is a complicated problem.

Caused by air pollution, acid rain’s spread
and damage involves weather, chemistry,
soil, and the life cycles of plants and animals
on the land and from acid rain in the water.
Scientists have discovered that air pollution
from the burning of fossil fuels is the major
cause of acid rain. Power plants and
factories burn coal and oil. Power plants use
that coal and oil to produce the electricity we
need to heat and light our homes and to run
our electric appliances. We also burn
natural gas, coal, and oil to heat our homes.
The smoke and fumes from burning fossil
fuels rise into the atmosphere and combine
with the moisture in the air to form acid rain.

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The main chemicals in air pollution that
create acid rain are sulfur dioxide and
nitrogen oxides. Acid rain usually forms high
in the clouds where sulfur dioxide and
nitrogen oxides react with water, oxygen,
and oxidants. This forms a mild solution of
sulfuric acid and nitric acid. Sunlight
increases the rate of most of these
reactions. Rainwater, snow, fog, and other
forms of precipitation containing those mild
solutions of sulfuric and nitric acids fall to the
earth as acid rain.
Water moves through every living plant and
animal, streams, lakes, and oceans in the
hydrologic cycle. In that cycle, water
evaporates from the land and sea into the
atmosphere. Water in the atmosphere then
condenses to form clouds. Clouds release
the water back to the earth as rain, snow, or
fog. When water droplets form and fall to the
earth they pick up particles and chemicals
that float in the air. Even clean, unpolluted
air has some particles such as dust or
pollen. Clean air also contains naturally
occurring gases such as carbon dioxide.

The interaction between the water droplets
and the carbon dioxide in the atmosphere,
and to a lesser extent, from chlorine which is
derived from the salt in the sea, gives rain
an average pH of about 5.6, making even
clean rain slightly acidic. Other natural
sources of acids and bases in the
atmosphere may lower or raise the pH of
unpolluted rain. However, when rain
contains pollutants, especially sulfur dioxide
and nitrogen oxides, the rain water can
become very acidic.
Acid rain does not account for all of the
acidity that falls back to earth from
pollutants. About half the acidity in the
atmosphere falls back to the earth through
dry deposition as gases and dry particles.

The wind blows these acidic particles and
gases onto buildings, cars, homes and
trees. In some instances, these gases and
particles can eat away the things on which
they settle. Dry deposited gases and
particles are sometimes washed from trees
and other surfaces by rainstorms. When that
happens, the runoff water adds those acids
to the acid rain, making the combination
more acidic than the falling rain alone. The
combination of acid rain plus dry deposited
acid is called acid deposition.
The chemical reactions that change air
pollution to acid rain can take from several
hours to several days. Years ago, when
smokestacks were only a few stories high,
pollution from smokestacks usually stayed
near the ground and settled on land nearby.

This caused unhealthy conditions for plants
and animals near the smokestacks. To
reduce this pollution, the government
passed a law permitting the construction of
very tall smokestacks. At that time, people
thought that if the pollution were sent high
into the air it would no longer be a problem.

Scientists now know that this is incorrect.

Sending pollution high into the sky
increases the time that the pollution stays in
the air. The longer the pollution is in the air,
the greater are the chances that the
pollutants will form acid rain. In addition, the
wind can carry these pollutants for hundreds
of miles before they become joined with
water droplets to form acid rain. For that
reason, acid rain can also be a problem in
areas far from the polluting smokestacks.

Dry deposition is usually more abundant
near the cities and industrial areas where
the pollutants are released.
There are also natural sources of acids such
as volcanoes, natural geysers and hot
springs. Nature has developed ways of
recycling these acids by absorbing and
breaking them down. These natural acids
contribute to only a small portion of the
acidic rainfall in the world today. In small
amounts, these acids actually help dissolve
nutrients and minerals from the soil so that
trees and other plants can use them for
food. The large amounts of acids produced
by human activities overload this natural

Acid rain is poorly understood and we will
probably spend billions on research ending
up with two opposing groups of experts.

One of the biggest money contributers to
our congressmen is a lobby group
dedicated to defeating or diluting any acid
rain controls.

“Acid Rain,” or more precisely acid precipitation, is
the word used to describe rainfall that has a pH level
of less than 5.6. This form of air pollution is currently
a subject of great controversy because of it’s
worldwide environmental damages. For the last ten
years, this phenomenon has brought destruction to
thousands of lakes and streams in the United
States, Canada, and parts of Europe. Acid rain is
formed when oxides of nitrogen and sulfite combine
with moisture in the atmosphere to make nitric and
sulfuric acids. The two primary sources of acid rain
are sulfur dioxide, and oxides of nitrogen. Sulfur
dioxide is a colourless, prudent gas released as a
by-product of combusted fossil fuels containing
sulfur. A variety of industrial processes, such as the
production of iron and steel, utility factories, and
crude oil processing produce this gas. In iron and
steel production, the smelting of metal sulfate ore,
produces pure metal. This causes the release of
sulfur dioxide. Metals such as zinc, nickel, and
copper are commonly obtained by this process.

Sulfur dioxide can also be emitted into the
atmosphere by natural disasters. Ten percent of all
sulfur dioxide emission comes from volcanoes, sea
spray, plankton, and rotting vegetation. Overall, 69.4
percent of sulfur dioxide is produced by industrial
combustion. Only 3.7 percent is caused by
The other chemical that is also chiefly responsible
for the make-up of acid rain is nitrogen oxide.

Oxides of nitrogen is a term used to describe any
compound of nitrogen with any amount of oxygen
atoms. Nitrogen monoxide and nitrogen dioxide are
all oxides of nitrogen. These gases are by-products
of firing processes of extreme high temperatures
(automobiles, utility plants), and in chemical
industries (fertilizer production). Natural processes
such as bacterial action in soil, forest fires, volcanic
action, and lightning make up five percent of nitrogen
oxide emission. Transportation makes up 43
percent, and 32 percent belongs to industrial
combustion. Nitrogen oxide is a dangerous gas by
itself. As mentioned before, any precipitation with a
pH level less than 5.6 is considered to be acid
rainfall. The difference between regular precipitation
and acid precipitation is the pH level. A pH scale is
used to determine if a specific solution is acidic or
basic. Any number below seven is considered to be
acidic. Any number above seven is considered to be
basic. The scale is color coordinated with the pH
level. Most pH scales use a range from zero to
fourteen. Seven is the neutral point (pure water). A
pH from 6.5 to 8, is considered the safe zone.

Between these numbers, organisms are in very little
or no harm.
Not only does the acidity of acid precipitation
depend on emission levels, but also on the chemical
mixtures in which sulfur dioxide and nitrogen oxides
interact in the atmosphere. Sulfur dioxide and
nitrogen oxides go through several complex steps of
chemical reactions before they become the acids
found in acid rain. The steps are broken down into
two phases, gas phase and aqueous phase. There
are various potential reactions that can contribute to
the oxidation of sulfur dioxide in the atmosphere
each having varying degrees of success. The most
common process is when sulfur dioxide reacts with
moisture found in the atmosphere. When this
happens, sulfate dioxide immediately oxidizes to
form a sulfite ion.
Afterwards, it becomes sulfuric acid when it joins
with hydrogen atoms in the air. A common reaction
for sulfur dioxide to becomes sulfuric acid is by
oxidation by ozone. This reaction occurs at a
preferable rate and sometimes is the main
contributor to the oxidation of sulfuric acid. There are
other insignificant reactions that contribute to acid
rain, but contribute to little to mention. These
reactions unfortunately prove to be insignificant for
various reasons. These reactions mentioned above,
are gas phase reactions.
Sulfur dioxide oxidation is most common in clouds
and especially in heavily polluted air where
compounds such as ammonia and ozone are in
abundance. These catalysts help convert more sulfur
dioxide into sulfuric acid. But not all of the sulfur
dioxide is converted to sulfuric acid. In fact, a
substantial amount can float up into the atmosphere,
transport to another area and return to earth
Like sulfur dioxide, nitrogen oxides rise into the
atmosphere and are oxidized in clouds to form nitric
or nitrous acid. These reactions are catalyzed in
heavily polluted clouds where traces of iron,
manganese, ammonia, and hydrogen peroxide are
present. Nitrogen oxides rise into the atmosphere
mainly from automobile exhaust. In the atmosphere
it reacts with water to form nitric or nitrous acid.
Over the years, scientists have noticed that some
forests have been growing more and more slowly
without reason. Trees do not grow as fast as they
did before. Leaves and pines needles turn brown and
fall off when they are supposed to be green.

Eventually, after several years of collecting and
recording information on the chemistry and biology
of the forest, researchers have concluded that this
was the work of acid rain. A rainstorm occurs in a
forest. The summer spring washes the leaves of the
branches and fall to the forest floor below. Some of
the water is absorbed into the soil. Water run-off
enters nearby streams, rivers, or lakes. That soil
may have neutralized some or all of the acidity in the
acid rainwater. This ability of neutralization is called
buffering capacity. Without buffering capacity, soil
pH would change rapidly. Midwestern states like
Nebraska and Indiana have soil that is well buffered.

Nonetheless, mountainous northwest areas such as
the Adirondack mountains are less able to buffer
acid. High pH levels in the soil help accelerate soil
weathering and remove nutrients. It also makes
some toxic elements, for example aluminum, more
soluble. High aluminum concentrations in soil can
prevent plants from using the nutrients in the soil.

Acid rain does not kill trees immediately or directly.

Instead, it is more likely to weaken the tree by
destroying its leaves, thus limiting the nutrients
available to it. Or, acid rain can seep into the
ground, poisoning the trees with toxic substances
that are slowly absorbed through the roots. When
acid rain falls, the acidic rainwater dissolves the
nutrients and helpful minerals from the soil. These
minerals are then washed away before trees and
other plants can use them to grow. Not only does
acid rain strip away the nutrients from the plants,
they help release toxic substance such as
aluminum into the soil. This occurs because these
metals are bound to the soil under normal
conditions, but the additional dissolving action of
hydrogen ions causes rocks and small bound soil
particles to break down. When acid rain is frequent,
leaves tend to lose their protective waxy coating,
When leaves lose their coating, the plant itself is
open to any possible disease. By damaging the
leaves, the plant can not produce enough food
energy for it to remain healthy. Once the plant is
weak, it can become more vulnerable to disease,
insects, and cold weather which may ultimately kill
Acid rain does not only effect organisms on land, but
also effect organisms in aquatic biomes. Most lakes
and streams have a pH level between six and eight.

Some lakes are naturally acidic even without the
effects of acid rain.

There are several routes through which acid rain can
enter the lakes. Some chemical substances exist
as dry particles in the atmosphere, while others
enter directly into the lake in a form of precipitation.

Acid rain that has fallen on land can be drained
through sewage systems leading to lakes. Another
way acids can enter the lake is by spring acid
shock. When acid snow melts in the spring, the
acids in the snow seeps into the ground. Some
run-off the ground and into streams and lakes.
Spring is a vulnerable time for many species since
this is the time for reproduction. The sudden change
in pH level is dangerous because the acid can cause
serious deformities in their young. Generally, the
young of most species are more sensitive older
animals of the same apeices. But not all species
can tolerate the same amount of acid. For example,
frogs may tolerate relatively high levels of acidity,
while snails are more sensitive to pH changes.
Acid molecules can cause mucus to form in the gills
of fish, preventing the fish to absorb oxygen well.

Also, a low pH level will throw off the balance of salt
in the fish’s tissue. Calcium levels of some fish
cannot be maintained due to the changes in pH
level. This causes a problem in reproduction: the
eggs are too brittle or weak. Lacking calcium
causes weak spines and deformities in bones.

Sometimes when acid rainfall runs off the land, it
carries fertilizers with it. Fertilizer helps stimulate
the growth of algae because of the amount of
nitrogen in it. However, because of the increase in
the death of fish the decomposition takes up even
more oxygen. This takes away from surviving fish. In
other terms, acid rain does not help aquatic
ecosystems in anyway.
Acid rain does not only damage the natural
ecosystems, but also man-made materials and
structures. Marble, limestone, and sandstone can
easily be dissolved by acid rain. Metals, paints,
textiles, and ceramic can easily be corroded. Acid
rain can also downgrade leather and rubber.

Man-made materials slowly deteriorate even when
exposed to unpolluted rain, but acid rain helps
speed up the process. Acid rain causes carvings,
monuments, and statues carved in stones to lose
their features.
In limestone, acidic water reacts with calcium to
form calcium sulfate. For iron, the acidic water
produces an additional proton giving iron a positive
charge. When iron reacts with more oxygen it forms
iron oxide (rust).
The repairs on building and monuments can be quite
costly. In 1990, the United States spent thirty-five
billion dollars on paint damage. In 1985, the Cologne
Cathedral cost the Germans approximately twenty
million dollars in repairs. The Roman monuments
cost the Romans about two hundred million dollars
from acid rain repairs.
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. The toxic metals
might be absorbed by the drinking water, crops, or
animals that a human may consume. These foods
that are consumed could cause nerve damage to
children, severe brain damage or even death.

Scientists believe that aluminum is somehow related
to Alzheimer’s disease.
One of the serious side effects of acid rain on
humans is respiratory problems. The sulfur dioxide
and nitrogen oxide emission gives risk to respiratory
problems such as dry coughs, asthma, headaches,
and eye, nose, and throat irritation. Polluted rainfall
is especially harmful to those who already suffer
from asthma or those who have a hard time
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 could
even lead to death.
In 1991, the United States and Canada signed an air
quality agreement. Ever since that time, both
countries have taken actions to reduce sulfur dioxide
emission. The United States agree to reduce their
annual sulfur dioxide emission by about ten million
tons by the year 2000. A year before the agreement,
the Clean Air Pact Amendment tried to reduce
nitrogen oxide by two million tons. This program
focused on the source that emits nitrogen oxide,
automobiles and coal-fired electric utility boilers.
Since most nitrogen oxide emissions are from cars,
catalytic converters must be install on cars to
reduce this emission. The catalytic converter is
mounted on the exhaust pipe, forcing all the exhaust
to pass though it. This converter looks like a dense
honeycomb, but it is coated with either platimun,
palladium, or rhodium. This converts nitrogen oxides,
carbon dioxides and unburned hydrocarbons into a
cleaner state.
To reduce sulfur dioxide emission, utility plants are
required to do several steps by the Clean Air Act
Amendment. Before combustion, these utilities
plants have to go through a process call coal
cleaning. Another way to reduce sulfur dioxide
before combustion is by burning coal with low sulfur
content. Low sulfur content coals are called
subituminous coal. This process in reducing sulfur
dioxide is very expensive due to the high demand of
subituminous coal.
During combustion, a process called Fluidized Bed
Combustion, is used to reduce sulfur dioxide
emissions into the atmosphere. This process
contains limestone or a sandstone bed that are
crushed and diluted into the fuel. Flue gas is the
mixture of gases resulting from combustion and
other reactions in a chamber. This enables the
limestones to react with sulfur dioxide and reduce
emission by 90 percent. After combustion, a
process known as wet flue gas desulfurization is
taken into action. This process requires a web
scrubber at the downward end of the boiler. This
process is very similar to Fluidized Bed Combustion.

This scrubber can be made of either limestone or
sodium hydroxide. Limestone is more commonly
used. As sulfur dioxide enters this area it reacts with
the limestone.
After being scrubbed, which is the term used for the
phase after coal has past the wet scrubber, the flue
gas is re-emmited and the waste solids are
Acid rain is an issue that can not be over looked.

This phenomenon destroys anything it touches or
interacts with it. When acid rain damages the forest
or the environment it affects humans in the long run.

Once forests are totally destroyed and lakes are
totally polluted animal populations begin to decrease
because of lack of food and shelter. If all the
animals, which are our food source, die out, humans
too would die out. Acid rain can also destroy our
homes and monuments that we hold dearly.
What humans can do, as citizens, to reduce sulfur
and nitrogen dioxide emission is to reduce the use
of fossil fuels. Car pools, public transportation, or
walking can reduce tons of nitrogen oxide
emissions. Using less energy benefits the
environment because the energy used comes from
fossil fuels which can lead to acid rain. For example,
turning off lights not being used, and reduce air
conditioning and heat usage. Replacing old
appliances and electronics with newer energy
efficient products is also an excellent idea. An
alternative power source can also be used in power
plants to reduce emissions. These alternatives are:
geothermal energy, solar power energy, wind
energy, and water energy.
In conclusion, the two primary sources of acid rain is
sulfur dioxide and nitrogen oxide. Automobiles are
the main source of nitrogen oxide emissions, and
utility factories are the main source for sulfur dioxide
emissions. These gases evaporate into the
atmosphere and then oxidized in clouds to form
nitric or nitrous acid and sulfuric acid. When these
acids fall back to the earth they do not cause
damage to just the environment but also to human
health. Acid rain kills plant life and destroys life in
lakes and ponds. The pollutants in acid rain causes
problem in human respiratory systems. The
pollutants attack humans indirectly through the
foods they consumed. They effected human health
directly when humans inhale the pollutants.

Governments have passed laws to reduce emissions
of sulfur dioxide and nitrogen oxide, but it is no use
unless people start to work together in stopping the
release of these pollutants. If the acid rain destroys
our environment, eventually it will destroy us as well.


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