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Acid Rain
Introduction
Since the beginning of time, humans have
learned to make use of many things in nature such as fire and electricity. From
the early times through the Industrial Revolution to the Space Age, humans have
produced inventions that use many of the earth's varied energy resources to make
living easier. In many cases the energy comes from burning fossil fuels -- coal,
oil and natural gas.
Some of the inventions that make our lives
easier are also causing pollution. Pollution is the release of harmful
substances into the environment. One form of pollution is acid rain. Acid rain
can damage plants, animals, soil, water, building materials, and people.
Scientists have discovered that burning fossil fuels creates acid rain through
air pollution. People burn fossil fuels such as coal and oil to make
electricity. Electricity heats and lights buildings and runs appliances such as
televisions and video recorders. Fossil fuels power our cars, buses, and
airplanes. The air pollution created when these fuels burn does not stay in the
air forever. It can return to the earth as acid rain. And when it does, it may
weaken the plant and animal life it contacts. Acid rain is only one form of
pollution that results from burning fossil fuels. It is one of particular
interest, however, because it can be transported over long distances.
Scientists, engineers, and researchers are learning how to measure the amount
and effects of pollution in the air, forests, water, and soil. They are
inventing ways to reduce the amount of pollution that enters the environment and
prevent new damage in the future.
What is 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.
Air Pollution Creates Acid Rain
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. Cars, trucks, and airplanes use gasoline, another fossil fuel.
The smoke and fumes from burning fossil
fuels rise into the atmosphere and combine with the moisture in the air to
form acid rain. 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.
Acid Precipitation
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 gives rain
a pH of 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.
Dry Deposition
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.
Acid Rain is a Problem That Can Travel
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.
Natural Acids
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
acidity.
What is Acidity?
Acidic and basic are two extremes that
describe chemicals, just like hot and cold are two extremes that describe
temperature. Mixing acids and bases can cancel out their extreme effects, much
like mixing hot and cold water can even out the water temperature. A substance
that is neither acidic nor basic is neutral. The pH scale measures how acidic
or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is
neutral. A pH less than 7 is acidic. A pH greater than 7 is basic. Each whole
pH value below 7 is ten times more acidic than the next higher value. For
example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10)
more acidic than pH 6. The same holds true for pH values above 7, each of
which is ten times more alkaline (another way to say basic) than the next
lower whole value. For example, pH 10 is ten times more alkaline than pH 9 and
100 times (10 times 10) more alkaline than pH 8.
Pure water is neutral. But when
chemicals are mixed with water, the mixture can become either acidic or basic.
Examples of acidic substances are vinegar and lemon juice. Laundry detergents
and ammonia are examples of basic substances. Chemicals that are very basic or
very acidic are reactive. These chemicals can cause severe burns. Automobile
battery acid is an acidic chemical that is reactive. Automobile batteries
contain a stronger form of some of the same acid that is in acid rain.
Household drain cleaners often contain lye, a very alkaline chemical that is
reactive.
The following diagram
shows the pH scale and the pH of some common items:
This diagram shows that a strong pH
can harm various things:
U.S. Environmental Protection
Agency
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