acid rain and ozone layer

acid rain
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1. What is Acid Rain?
"Acid rain" is a broad term referring to a mixture of wet and dry deposition (deposited
material) from the atmosphere containing higher than normal amounts of nitric and sulfuric
acids. The precursors, or chemical forerunners, of acid rain formation result from both natural
sources, such as volcanoes and decaying vegetation, and man-made sources, primarily
emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx)resulting from fossil fuel
combustion. In the United States, roughly 2/3 of all SO2 and 1/4 of all NOx come from
electric power generation that relies on burning fossil fuels, like coal. Acid rain occurs when
these gases react in the atmosphere with water, oxygen, and other chemicals to form various
acidic compounds. The result is a mild solution of sulfuric acid and nitric acid. When sulfur
dioxide and nitrogen oxides are released from power plants and other sources, prevailing
winds blow these compounds across state and national borders, sometimes over hundreds of
miles.

Wet Deposition
Wet deposition refers to acidic rain, fog, and snow. If the acid chemicals in the air are blown
into areas where the weather is wet, the acids can fall to the ground in the form of rain, snow,
fog, or mist. As this acidic water flows over and through the ground, it affects a variety of
plants and animals. The strength of the effects depends on several factors, including how
acidic the water is; the chemistry and buffering capacity of the soils involved; and the types
of fish, trees, and other living things that rely on the water.
Dry Deposition
In areas where the weather is dry, the acid chemicals may become incorporated into dust or
smoke and fall to the ground through dry deposition, sticking to the ground, buildings,
homes, cars, and trees. Dry deposited gases and particles can be washed from these surfaces

2. by rainstorms, leading to increased runoff. This runoff water makes the resulting mixture
more acidic. About half of the acidity in the atmosphere falls back to earth through dry
deposition.
Effects of Acid Rain
After studying the Hubbard Brook Forest and other areas today, there are several important
impacts of acid deposition on both natural and man-made environments. Aquatic settings are
the most clearly impacted by acid deposition though because acidic precipitation falls directly
into them. Both dry and wet deposition also runs off of forests, fields, and roads and flows
into lakes, rivers, and streams.
As this acidic liquid flows into larger bodies of water, it is diluted but over time, acids can
accrue and lower the overall pH of the body. Acid deposition also causes clay soils to release
aluminum and magnesium further lowering the pH in some areas. If the pH of a lake drops
below 4.8, its plants and animals risk death and it is estimated that around 50,000 lakes in the
United States and Canada have a pH below normal (about 5.3 for water). Several hundred of
these have a pH too low to support any aquatic life.
Aside from aquatic bodies, acid deposition can significantly impact forests. As acid rain falls
on trees, it can make them lose their leaves, damage their bark, and stunt their growth. By
damaging these parts of the tree, it makes them vulnerable to disease, extreme weather, and
insects. Acid falling on a forest’s soil is also harmful because it disrupts soil nutrients, kills
microorganisms in the soil, and can sometimes cause a calcium deficiency. Trees at high
altitudes are also susceptible to problems induced by acidic cloud cover as the moisture in the
clouds blankets them.
Damage to forests by acid rain is seen all over the world, but the most advanced cases are in
Eastern Europe. It’s estimated that in Germany and Poland, half of the forests are damaged,
while 30% in Switzerland have been affected.
Finally, acid deposition also has an impact on architecture and art because of its ability to
corrode certain materials. As acid lands on buildings (especially those constructed with
limestone) it reacts with minerals in the stones sometimes causing it to disintegrate and wash
away. Acid deposition can also corrode modern buildings, cars, railroad tracks, airplanes,
steel bridges, and pipes above and below ground.
What's Being Done?
Because of these problems and the adverse effects air pollution has on human health, a
number of steps are being taken to reduce sulfur and nitrogen emissions. Most notably, many
governments are now requiring energy producers to clean smoke stacks by using scrubbers
which trap pollutants before they are released into the atmosphere and catalytic converters in
cars to reduce their emissions. Additionally, alternative energy sources are gaining more
prominence today and funding is being given to the restoration of ecosystems damaged by
acid rain worldwide.

3. The ozone layer is a belt of naturally occurring ozone gas that sits 9.3 to
18.6 miles (15 to 30 kilometers) above Earth and serves as a shield from the harmful
ultraviolet B radiation emitted by the sun.
Ozone is a highly reactive molecule that contains three oxygen atoms. It is constantly being
formed and broken down in the high atmosphere, 6.2 to 31 miles (10 to 50 kilometers) above
Earth, in the region called the stratosphere.
Today, there is widespread concern that the ozone layer is deteriorating due to the release of
pollution containing the chemicals chlorine and bromine. Such deterioration allows large
amounts of ultraviolet B rays to reach Earth, which can cause skin cancer and cataracts in
humans and harm animals as well.
Extra ultraviolet B radiation reaching Earth also inhibits the reproductive cycle of
phytoplankton, single-celled organisms such as algae that make up the bottom rung of the
food chain. Biologists fear that reductions in phytoplankton populations will in turn lower the
populations of other animals. Researchers also have documented changes in the reproductive
rates of young fish, shrimp, and crabs as well as frogs and salamanders exposed to excess
ultraviolet B.
Chlorofluorocarbons (CFCs), chemicals found mainly in spray aerosols heavily used by
industrialized nations for much of the past 50 years, are the primary culprits in ozone layer
breakdown. When CFCs reach the upper atmosphere, they are exposed to ultraviolet rays,
which causes them to break down into substances that include chlorine. The chlorine reacts
with the oxygen atoms in ozone and rips apart the ozone molecule.
One atom of chlorine can destroy more than a hundred thousand ozone molecules, according
to the the U.S. Environmental Protection Agency.
The ozone layer above the Antarctic has been particularly impacted by pollution since the
mid-1980s. This region’s low temperatures speed up the conversion of CFCs to chlorine. In
the southern spring and summer, when the sun shines for long periods of the day, chlorine
reacts with ultraviolet rays, destroying ozone on a massive scale, up to 65 percent. This is
what some people erroneously refer to as the "ozone hole." In other regions, the ozone layer
has deteriorated by about 20 percent.
About 90 percent of CFCs currently in the atmosphere were emitted by industrialized
countries in the Northern Hemisphere, including the United States and Europe. These
countries banned CFCs by 1996, and the amount of chlorine in the atmosphere is falling now.
But scientists estimate it will take another 50 years for chlorine levels to return to their
natural levels.
The Causes of Ozone Depletion
Scientific evidence indicates that stratospheric ozone is being destroyed by a group of
manufactured chemicals, containing chlorine and/or bromine. These chemicals are called
"ozone-depleting substances" (ODS).
ODS are very stable, nontoxic and environmentally safe in the lower atmosphere, which is
why they became so popular in the first place. However, their very stability allows them to
float up, intact, to the stratosphere. Once there, they are broken apart by the intense
ultraviolet light, releasing chlorine and bromine. Chlorine and bromine demolish ozone at an

4. alarming rate, by stripping an atom from the ozone molecule. A single molecule of chlorine
can break apart thousands of molecules of ozone.
What's more, ODS have a long lifetime in our atmosphere — up to several centuries. This
means most of the ODS we've released over the last 80 years are still making their way to the
stratosphere, where they will add to the ozone destruction.
The main ODS are chlorofluorocarbons (CFCs), hydrochlorofluorcarbons (HCFCs), carbon
tetrachloride and methyl chloroform. Halons (brominated fluorocarbons) also play a large
role. Their application is quite limited: they're used in specialized fire extinguishers. But the
problem with halons is they can destroy up to 10 times as much ozone as CFCs can. For this
reason, halons are the most serious ozone-depleting group of chemicals emitted in British
Columbia.
Hydrofluorocarbons (HFCs) are being developed to replace CFCs and HCFCs, for uses such
as vehicle air conditioning. HFCs do not deplete ozone, but they are strong greenhouse gases.
CFCs are even more powerful contributors to global climate change, though, so HFCs are
still the better option until even safer substitutes are discovered.
The Main Ozone-Depleting Substances (ODS)
 Chlorofluorocarbons (CFCs)
 The most widely used ODS, accounting for over 80% of total stratospheric ozone
depletion.
 Used as coolants in refrigerators, freezers and air conditioners in buildings and cars
manufactured before 1995.
 Found in industrial solvents, dry-cleaning agents and hospital sterilants.
 Also used in foam products — such as soft-foam padding (e.g. cushions and
mattresses) and rigid foam (e.g. home insulation).
 Halons
 Used in some fire extinguishers, in cases where materials and equipment would be
destroyed by water or other fire extinguisher chemicals. In B.C., halons cause
greater damage to the ozone layer than do CFCs from automobile air conditioners.
 Methyl Chloroform
 Used mainly in industry — for vapour degreasing, some aerosols, cold cleaning,
adhesives and chemical processing.
 Carbon Tetrachloride
 Used in solvents and some fire extinguishers.
 Hydrofluorocarbons (HCFCs)
 HCFCs have become major, “transitional” substitutes for CFCs. They are much
less harmful to stratospheric ozone than CFCs are. But HCFCs they still cause
some ozone destruction and are potent greenhouse gases.