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Acid Essay, Research Paper

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 (H3O), 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 + H3O =* H3SO4 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 + H3O =*

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



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