Acid rain describes any form of precipitation with
high levels of nitric and sulfuric acids. It can also occur in the form of
snow, fog, and tiny bits of dry material that settle to Earth.
Rotting vegetation and erupting volcanoes release
some chemicals that can cause acid rain, but most acid rain falls because of
human activities. The biggest culprit is the burning of fossil fuels by
coal-burning power plants, factories, and automobiles.
When humans burn fossil fuels, sulfur dioxide (SO2)
and nitrogen oxides (NOx) are released into the atmosphere. These chemical
gases react with water, oxygen, and other substances to form mild solutions of
sulfuric and nitric acid. Winds may spread these acidic solutions across the
atmosphere and over hundreds of miles. When acid rain reaches Earth, it flows
across the surface in runoff water, enters water systems, and sinks into the
Acid rain has many ecological effects, but none is
greater than its impact on lakes, streams, wetlands, and other aquatic
environments. Acid rain makes waters acidic and causes them to absorb the
aluminum that makes its way from soil into lakes and streams. This combination
makes waters toxic to crayfish, clams, fish, and other aquatic animals.
Some species can tolerate acidic waters better than
others. However, in an interconnected ecosystem, what impacts some species
eventually impacts many more throughout the food chain—including non-aquatic
species such as birds.
Acid rain also damages forests, especially those at
higher elevations. It robs the soil of essential nutrients and releases
aluminum in the soil, which makes it hard for trees to take up water. Trees'
leaves and needles are also harmed by acids.
The effects of acid rain, combined with other
environmental stressors, leave trees and plants less able to withstand cold
temperatures, insects, and disease. The pollutants may also inhibit trees'
ability to reproduce. Some soils are better able to neutralize acids than
others. In areas where the soil's "buffering capacity" is low, the
harmful effects of acid rain are much greater.
The only way to fight acid rain is by curbing the
release of the pollutants that cause it. This means burning fewer fossil fuels.
Many governments have tried to curb emissions by cleaning up industry smokestacks
and promoting alternative fuel sources. These efforts have met with mixed
results. But even if acid rain could be stopped today, it would still take many
years for its harmful effects to disappear.
Individuals can also help prevent acid rain by conserving
energy. The less electricity people use in their homes, the fewer chemicals
power plants will emit. Vehicles are also major fossil fuel users, so drivers
can reduce emissions by using public transportation, carpooling, biking, or
simply walking wherever possible.
A stand of withered red spruce and Fraser fir trees
blights a green vista in North Carolina's Mount Mitchell State Park. The trees
fell victim to balsam wooly aphids after being weakened by the effects of acid
Over the years acid deposition, commonly referred to
as "acid rain," has rendered dozens of lakes in the Adirondacks
uninhabitable for fish and other wildlife. Now, Rensselaer Polytechnic
Institute researchers at the Darrin Fresh Water Institute (DFWI) have indicated
that some of the most severely affected lakes in that region are showing signs
"In about half of the 30 lakes under study, an
increase in the pH has been observed, a sign that acidic levels are
decreasing," says Director Sandra Nierzwicki-Bauer.
Levels of nitrogen influenced by nitric oxide, a
primary source of acid rain, have decreased moderately in 18 of the 30 lakes
the DFWI has monitored since 1994 through its federally funded Adirondack
Effects Assessment Program. There also has been an overall reduction of
sulfuric acid, another main contributor of acid rain that comes from industry
The reductions may be correlated with the 1990 Clean
Air Act, a federal mandate to significantly reduce emissions that cause
acidification, says Nierzwicki-Bauer.
More research is needed to pinpoint the exact reasons
for the apparent changes seen in the lakes in the southwestern part of the
Adirondack Park, an area hardest hit by acid rain.
"Recovery doesn't happen overnight," says
Charles Boylen, professor of biology and DFWI associate director. "One of
the reasons we need long-term data is that other factors can come into play.
More or less rainfall in a year, for instance, can lead to a temporary shift in
acid-rain levels. You need to track specific data over 10 to 15 years."
The DFWI's long-term strategy recently has led to a
$2.36 million grant from the Environmental Protection Agency. The five-year
grant will allow the DFWI and its collaborators to study acid rain effects in
four more lakes in addition to monitoring the other 30. During a workshop in
June at the Institute, researchers will announce the most up-to-date results in
the acid-rain studies to leading scientists around the country.
The Darrin Fresh Water Institute, established more than
25 years ago, has helped increase public awareness concerning the protection of
land, water and air. The Institute's all-encompassing study of fresh water
systems and ecological processes has earned it high regard in the national
scientific community and high marks from the general public.
Rensselaer Polytechnic Institute, founded in 1824, is
the nation's oldest technological university. The school offers degrees in
engineering, the sciences, information technology, architecture, management,
and the humanities and social sciences. Institute programs serve
undergraduates, graduate students, and working professionals around the world.
Rensselaer faculty are known for pre-eminence in research conducted in a wide
range of research centers that are characterized by strong industry
partnerships. The Institute is especially well known for its success in the
transfer of technology from the laboratory to the marketplace so that new
discoveries and inventions benefit human life, protect the environment, and
strengthen economic development.
Since the Industrial Revolution, emissions of sulfur
dioxide and nitrogen oxides to the atmosphere have increased.  In 1852,
Robert Angus Smith was the first to show the relationship between acid rain and
atmospheric pollution in Manchester, England. Though acidic rain was
discovered in 1852, it was not until the late 1960s that scientists began
widely observing and studying the phenomenon. The term "acid rain"
was generated in 1972. Canadian Harold Harvey was among the first to
research a "dead" lake. Public awareness of acid rain in the U.S
increased in the 1970s after the New York Times promulgated reports from the
Hubbard Brook Experimental Forest in New Hampshire of the myriad deleterious
environmental effects demonstrated to result from it.
Occasional pH readings in rain and fog water of well
below 2.4 (the acidity of vinegar) have been reported in industrialized
areas. Industrial acid rain is a substantial problem in Europe, China,
Russia and areas down-wind from them. These areas all burn sulfur-containing
coal to generate heat and electricity. The problem of acid rain not only
has increased with population and industrial growth, but has become more
widespread. The use of tall smokestacks to reduce local pollution has
contributed to the spread of acid rain by releasing gases into regional
atmospheric circulation. Often deposition occurs a considerable
distance downwind of the emissions, with mountainous regions tending to receive
the greatest deposition (simply because of their higher rainfall). An example
of this effect is the low pH of rain (compared to the local emissions) which
falls in Scandinavia.
 History of Acid Rain in the United States
In 1980, the U.S. Congress passed an Acid Deposition
Act. This Act established a 10-year research program under the direction of the
National Acidic Precipitation Assessment Program (NAPAP). NAPAP looked at the
entire problem. It enlarged a network of monitoring sites to determine how
acidic the precipitation actually was, and to determine long term trends, and
established a network for dry deposition. It looked at the effects of acid rain
and funded research on the effects of acid precipitation on freshwater and
terrestrial ecosystems, historical buildings, monuments, and building
materials. It also funded extensive studies on atmospheric processes and
potential control programs.
In 1991, NAPAP provided its first assessment of acid
rain in the United States. It reported that 5% of New England Lakes were
acidic, with sulfates being the most common problem. They noted that 2% of the
lakes could no longer support Brook Trout, and 6% of the lakes were unsuitable
for the survival of many species of minnow. Subsequent Reports to Congress have
documented chemical changes in soil and freshwater ecosystems, nitrogen
saturation, decreases in amounts of nutrients in soil, episodic acidification,
regional haze, and damage to historical monuments.
Meanwhile, in 1990, the US Congress passed a series
of amendments to the Clean Air Act. Title IV of these amendments established a
program designed to control emissions of sulfur dioxide and nitrogen oxides.
Title IV called for a total reduction of about 10 million tons of SO2 emissions
from power plants. It was implemented in two phases. Phase I began in 1995, and
limited sulfur dioxide emissions from 110 of the largest power plants to a
combined total of 8.7 million tons of sulfur dioxide One power plant in New
England (Merrimack) was in Phase I. Four other plants (Newington, Mount Tom,
Brayton Point, and Salem Harbor) were added under other provisions of the
program. Phase II began in 2000, and affects most of the power plants in the
During the 1990s, research has continued. On March
10, 2005, EPA issued the Clean Air Interstate Rule (CAIR) . This rule provides
states with a solution to the problem of power plant pollution that drifts from
one state to another. CAIR will permanently cap emissions of SO2 and NOx in the
eastern United States. When fully implemented, CAIR will reduce SO2 emissions
in 28 eastern states and the District of Columbia by over 70 percent and NOx
emissions by over 60 percent from 2003 levels.
 Emissions of chemicals leading to
The most important gas which leads to acidification
is sulfur dioxide. Emissions of nitrogen oxides which are oxidized to form
nitric acid are of increasing importance due to stricter controls on emissions
of sulfur containing compounds. 70 Tg(S) per year in the form of SO2 comes from
fossil fuel combustion and industry, 2.8 Tg(S) from wildfires and 7-8 Tg(S) per
year from volcanoes.
 Natural phenomena
The principal natural phenomena that contribute
acid-producing gases to the atmosphere are emissions from volcanoes and those
from biological processes that occur on the land, in wetlands, and in the
oceans. The major biological source of sulfur containing compounds is dimethyl
Acidic deposits have been detected in glacial ice
thousands of years old in remote parts of the globe.
 Human activity
The coal-fired Gavin Power Plant in Cheshire, OhioThe
principal cause of acid rain is sulfur and nitrogen compounds from human
sources, such as electricity generation, factories, and motor vehicles. Coal
power plants are one of the most polluting. The gases can be carried hundreds
of kilometres in the atmosphere before they are converted to acids and
deposited. In the past, factories had short funnels to let out smoke, but this
caused many problems locally; thus, factories now have taller smoke funnels.
However, dispersal from these taller stacks causes pollutants to be carried
farther, causing widespread ecological damage.
 Chemical processes
Combustion of fuels creates sulfur dioxide and nitric
oxides. They are converted into sulfuric acid and nitric acid.
 Gas phase chemistry
In the gas phase sulfur dioxide is oxidized by
reaction with the hydroxyl radical via an intermolecular reaction :
SO2 + OH· → HOSO2·
which is followed by:
HOSO2· + O2 → HO2· + SO3
In the presence of water, sulfur trioxide (SO3) is
converted rapidly to sulfuric acid:
SO3 (g) + H2O (l) → H2SO4 (l)
Nitrogen dioxide reacts with OH to form nitric acid:
NO2 + OH· → HNO3
 Chemistry in cloud droplets
When clouds are present, the loss rate of SO2 is
faster than can be explained by gas phase chemistry alone. This is due to
reactions in the liquid water droplets.
Sulfur dioxide dissolves in water and then, like
carbon dioxide, hydrolyses in a series of equilibrium reactions:
SO2 (g) + H2O
SO2·H2O H+ +
HSO3- H+ +
There are a large number of aqueous reactions that
oxidize sulfur from S(IV) to S(VI), leading to the formation of sulfuric acid.
The most important oxidation reactions are with ozone, hydrogen peroxide and
oxygen (reactions with oxygen are catalyzed by iron and manganese in the cloud
For more information see Seinfeld and Pandis
 Acid deposition
Processes involved in acid deposition (note that only
SO2 and NOx play a significant role in acid rain).
 Wet deposition
Wet deposition of acids occurs when any form of
precipitation (rain, snow, etc.) removes acids from the atmosphere and delivers
it to the Earth's surface. This can result from the deposition of acids
produced in the raindrops (see aqueous phase chemistry above) or by the
precipitation removing the acids either in clouds or below clouds. Wet removal
of both gases and aerosols are both of importance for wet deposition.
 Dry deposition
Acid deposition also occurs via dry deposition in the
absence of precipitation. This can be responsible for as much as 20 to 60% of
total acid deposition. This occurs when particles and gases stick to the
ground, plants or other surfaces.
 Adverse effects
This chart shows that not all fish, shellfish, or the
insects that they eat can tolerate the same amount of acid; for example, frogs
can tolerate water that is more acidic (i.e., has a lower pH) than trout.Acid
rain has been shown to have adverse impacts on forests, freshwaters and soils,
killing insect and aquatic life-forms as well as causing damage to buildings
and having impacts on human health.
 Surface waters and aquatic animals
Both the lower pH and higher aluminum concentrations
in surface water that occur as a result of acid rain can cause damage to fish
and other aquatic animals. At pHs lower than 5 most fish eggs will not hatch
and lower pHs can kill adult fish. As lakes and rivers become more acidic
biodiversity is reduced. Acid rain has eliminated insect life and some fish
species, including the brook trout in some lakes, streams, and creeks in
geographically sensitive areas, such as the Adirondack Mountains of the United
States. However, the extent to which acid rain contributes directly or
indirectly via runoff from the catchment to lake and river acidity (i.e.,
depending on characteristics of the surrounding watershed) is variable. The
United States Environmental Protection Agency's (EPA) website states: "Of
the lakes and streams surveyed, acid rain caused acidity in 75 percent of the
acidic lakes and about 50 percent of the acidic streams".
Soil biology and chemistry can be seriously damaged
by acid rain. Some microbes are unable to tolerate changes to low pHs and are
killed. The enzymes of these microbes are denatured (changed in shape so
they no longer function) by the acid. The hydronium ions of acid rain also
mobilize toxins such as aluminium, and leach away essential nutrients and
minerals such as magnesium.
2 H+ (aq) + Mg2+ (clay) 2 H+ (clay) + Mg2+ (aq)
Soil chemistry can be dramatically changed when base
cations, such as calcium and magnesium, are leached by acid rain thereby affecting
sensitive species, such as sugar maple (Acer saccharum).
 Forests and other vegetation
Effect of acid rain on a forest, Jizera Mountains,
Czech RepublicAdverse effects may be indirectly related to acid rain, like the
acid's effects on soil (see above) or high concentration of gaseous precursors
to acid rain. High altitude forests are especially vulnerable as they are often
surrounded by clouds and fog which are more acidic than rain.
Other plants can also be damaged by acid rain, but
the effect on food crops is minimized by the application of lime and
fertilizers to replace lost nutrients. In cultivated areas, limestone may also
be added to increase the ability of the soil to keep the pH stable, but this
tactic is largely unusable in the case of wilderness lands. When calcium is
leached from the needles of red spruce, these trees become less cold tolerant
and exhibit winter injury and even death.
 Human health
Scientists have suggested direct links to human
health. Fine particles, a large fraction of which are formed from the same
gases as acid rain (sulfur dioxide and nitrogen dioxide), have been shown to
cause illness and premature deaths such as cancer and other diseases. For
more information on the health effects of aerosols see particulate health
 Other adverse effects
Effect of acid rain on statuesAcid rain can also
cause damage to certain building materials and historical monuments. This
results when the sulfuric acid in the rain chemically reacts with the calcium
compounds in the stones (limestone, sandstone, marble and granite) to create
gypsum, which then flakes off.
CaCO3 (s) + H2SO4 (aq) CaSO4 (aq) + CO2 (g) + H2O (l)
This result is also commonly seen on old gravestones
where the acid rain can cause the inscription to become completely illegible.
Acid rain also causes an increased rate of oxidation for iron. Visibility
is also reduced by sulfate and nitrate aerosols and particles in the
 Affected areas
Particularly badly affected places around the globe
include most of Europe (particularly Scandinavia with many lakes with acidic
water containing no life and many trees dead) many parts of the United States
(states like New York are very badly affected) and South Western Canada. Other
affected areas include the South Eastern coast of China and Taiwan.
 Potential problem areas in the future
Places like much of South Asia (Indonesia, Malaysia
and Thailand), Western South Africa (the country), Southern India and Sri Lanka
and even West Africa (countries like Ghana, Togo and Nigeria) could all be
prone to acidic rainfall in the future.
 Prevention methods
 Technical solutions
In the United States, many coal-burning power plants
use Flue gas desulfurization (FGD) to remove sulfur-containing gases from their
stack gases. An example of FGD is the wet scrubber which is commonly used in
the U.S. and many other countries. A wet scrubber is basically a reaction tower
equipped with a fan that extracts hot smoke stack gases from a power plant into
the tower. Lime or limestone in slurry form is also injected into the tower to
mix with the stack gases and combine with the sulfur dioxide present. The
calcium carbonate of the limestone produces pH-neutral calcium sulfate that is
physically removed from the scrubber. That is, the scrubber turns sulfur
pollution into industrial sulfates.
In some areas the sulfates are sold to chemical
companies as gypsum when the purity of calcium sulfate is high. In others, they
are placed in landfill. However, the effects of acid rain can last for
generations, as the effects of pH level change can stimulate the continued
leaching of undesirable chemicals into otherwise pristine water sources,
killing off vulnerable insect and fish species and blocking efforts to restore
Automobile emissions control reduces emissions of
nitrogen oxides from motor vehicles.
The term acid rain refers to what scientists call
acid deposition. It is caused by airborne acidic pollutants and has
highly destructive results.
Scientists first discovered acid rain in 1852, when
the English chemist Robert Agnus invented the term. From then until now,
acid rain has been an issue of intense debate among scientists and policy
Acid rain, one of the most important environmental
problems of all, cannot be seen. The invisible gases that cause acid rain
usually come from automobiles or coal-burning power plants.
Acid rain moves easily, affecting locations far
beyond those that let out the pollution. As a result, this global
pollution issue causes great debates between countries that fight over
polluting each other's environments.
For years, science studied the true causes of acid rain. Some scientists concluded that human
production was primarily responsible, while others cited natural causes as
well. Recently, more intensive research has been done so that countries
have the information they need to prevent acid rain and its dangerous effects.
The levels of acid rain vary from region to
region. In Third World nations without pollution restrictions, acid rain
tends to be very high. In Eastern Europe, China, and the Soviet Union, acid rain levels have also
risen greatly. However, because acid rain can move about so easily, the
problem is definitely a global one.
For many years, there was considerable debate and
disagreement over what caused acid rain. Recent scientific work, however,
has helped to clarify this
The primary causes of acid rain are sulfur dioxide
and nitrogen oxides. These chemicals are released by certain industrial
processes, and as a result, the more industrialized nations of Europe as well as the US suffer severely from acid rain.
Most sulfur dioxide comes from power plants that use
coal as their fuel. These plants emit 100 million tons of sulfur dioxide,
70% of that in the world.
Automobiles produce about half of the world's
nitrogen oxide. As the number of automobiles in use increases, so does
the amount of acid rain. Power plants that burn fossil fuels also contribute significantly to nitrogen oxide emission.
Though human causes are primarily responsible for
acid rain, natural causes exist as well. Fires, volcanic eruptions,
bacterial decomposition, and lightening also greatly increase the amount of
nitrogen oxide on the planet. However, even the gigantic explosion of Mt. St. Helens released only about what one coal power plant emits in a
Once the tiny pollutant molecules have entered the
atmosphere, they can travel for thousands of miles. Eventually, the
particles will combine with other compounds to produce new, often harmful,
Acid rain comes down to the earth in the form of
rain, snow, hail, fog, frost, or dew. Once it reaches the ground, the
acidity in the substance can harm and even destroy both natural ecosystems and man-made products, such as car finishes.
Acid rain is having harmful effects both on people
and on the natural ecosystems of the world. Scientists today are
convinced that acid rain is severe in many areas, and that it is having an
adverse effect on the environments of those locations.
The problem of acid rain is rapidly spreading.
Because it is mainly caused by industrial processes, automobiles, and power
plants, those countries that are developed have the most severe acid rain
problems. However, as the undeveloped nations begin to
industrialize, acid rain will increase greatly.
Determining just how much the planet is being hurt by
acid rain is very difficult because the ecosystems that it affects are so diverse and complex.
Many ecosystems are affected by acid rain.
Bodies of water, such as lakes and rivers, see many of their inhabitants die
off due to rising acidity levels.
Acidic water also ruins plant nutrients, hurting
plants' ability to survive and to give life to other organisms.
Human-made products are also experiencing degradation
from acid rain. Cars can lose their finishes, and outdoor statues are
beginning to rust.
Acid rain's effects are destructive and long
lasting. Though scientists have studied lakes, streams, and many other
natural ecosystems to prove its negative effects, acid rain continues to be
produced and is increasing in many parts of the world.
Modern science has proven that acid rain is a
dangerous and highly destructive problem. As a result, various ways to
limit acid rain have been invented, and some are now being used.
Debate over acid rain and ways of preventing it have
raged between environmentalists and corporations. Businesses such as
power companies and car makers oppose controlling acid rain because they fear
the effects on their profits.
But in some cases, industries have attempted to curb
acid rain production. The Northern States Power company began working to
reduce acid rain in the 1980s, and has invested over a billion dollars to that
There are many ways that power plant companies like
Northern States can reduce acid rain creation. They can use coal with a
low sulfur content, they can remove the sulfur from smoke their plants release,
and they can limit processes known to generate high levels of acid rain.
Policy makers and environmental experts are now
looking into the best methods to limit acid rain.
Environmentalists advocate the installation of sulfur
cleaning scrubbers in factories, washing sulfur out of coal, and finding new
methods of burning coal. Power plant operators are looking for less
expensive solutions to the problem.
Individuals can help by conserving energy or driving
their cars less. Governments can pass laws restricting pollution levels,
or can use a variety of methods such as tradable emission
permits to reduce acid rain.
Whatever way it is done, acid rain will certainly have to be limited in the
Acid Rain--A Contemporary World Problem
Acid rain is one of the most dangerous and widespread forms of pollution.
Sometimes called "the unseen plague," acid rain can go undetected in
an area for years. Technically, acid rain is rain that has a larger amount of
acid in it than what is normal. The acidity of rain in parts of Europe and
North America has dramatically increased over the past few decades. It is now
common in many places for rain to be ten to seventy times more acid than
unpolluted rain. Many living and non-living systems become harmed and damaged
as a result of acid rain. This website gives an informational, in-depth look at
acid rain--it's causes and effects; and solutions to the acid rain problem.
Causes of Acid Rain
Acid rain is caused by smoke and gases that are given off by factories and cars
that run on fossil fuels. When these fuels are burned to produce energy, the
sulfur that is present in the fuel combines with oxygen and becomes sulfur
dioxide; some of the nitrogen in the air becomes nitrogen oxide. These
pollutants go into the atmosphere, and become acid.
Sulfur dioxide and nitrogen oxide are produced especially when coal is burnt
for fuel. Burning coal produces electricity, and the more electricity that
people use, the more coal is burnt. Of course, nowadays people probably
couldn't live without electricity, so coal will continue to be burnt; but
electricity and energy are constantly being overused. Think of it this way:
every time you turn on a light switch or the television set without really
needing to, you're indirectly contributing to the acid rain problem.
Automobiles produce nitrogen oxides (which cause acid rain), so every time you
don't carpool when you can, you are helping to cause acid rain. So now that we
know what causes acid rain, here's a look at how acid rain can hurt you and the
world around you. . .