What is Acid Rain and What Causes It?
“Acid rain” is a broad term used to describe several ways that acids fall out of
the atmosphere. A more precise term is acid deposition, which has two parts:
wet and dry.
Wet deposition refers to acidic rain, fog, and snow. As this acidic water flows
over and through the ground, it affects a variety of plants and animals. The
strength of the effects depend on many 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 refers to acidic gases and particles. About half of the acidity in
the atmosphere falls back to Earth through dry deposition. The wind blows
these acidic particles and gases onto buildings, cars, homes, and trees. Dry
deposited gases and particles can also be 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.
Prevailing winds blow the compounds that cause both wet and dry acid
deposition across state and national borders, and sometimes over hundreds
of miles. Scientists discovered, and have confirmed, that sulfur dioxide (SO2)
and nitrogen oxides (NOx) are the primary causes of acid rain. Almost all of
SO2 and some 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. Sunlight
increases the rate of most of these reactions. The result is a mild solution of
sulfuric acid and nitric acid.
Acid rain is measured using a scale called pH. The lower a substance's pH,
the more acidic it is. Pure water has a pH of 7.0, which is neutral. Normal rain
is slightly acidic because carbon dioxide dissolves into it, so it has a pH of
about 5.5. In the year 2000, the most acidic rain falling in North America had a
pH of about 4.3.
Effects of Acid Rain
Acid rain causes acidification of lakes and streams and contributes to damage
of trees at high elevations (for example, red spruce trees above 2,000 feet)
and many sensitive forest soils. In addition, acid rain accelerates the decay of
building materials and paints, including irreplaceable buildings, statues, and
sculptures that are part of a nation's cultural heritage. Prior to falling to the
Earth, SO2 and NOx gases and their particulate matter derivatives, sulfates
and nitrates, contribute to visibility degradation and harm human health.
7
Effects on materials and buildings: Acid rain and the dry deposition of
acidic particles contribute to the corrosion of metals (such as bronze) and the
deterioration of paint and stone (such as marble and limestone). These effects
seriously reduce the value to society of buildings, bridges, cultural objects
(such as statues, monuments, and tombstones), and cars. Dry deposition of
acidic compounds can also dirty buildings and other structures, leading to
increased maintenance costs.
Effects on water bodies: The ecological effects of acid rain are most clearly
seen in the aquatic or water environments such as streams, lakes, and
marshes. Acid rain flows into streams, lakes, and marshes after falling on
forests, fields, buildings, and roads. Acid rain also falls directly on aquatic
habitats. Most lakes and streams have a pH between 6 and 8, although some
lakes are naturally acidic even without the effects of acid rain. Acid rain
primarily affects sensitive bodies of water, which are located in watersheds
whose soils have a limited ability to neutralize acidic compounds (called
"buffering capacity"). Lakes and streams become acidic (pH value goes down)
when the water itself and its surrounding soil cannot buffer the acid rain
enough to neutralize it. In areas where buffering capacity is low, acid rain also
releases aluminum from soils into lakes and streams.
Many lakes and streams examined in developed countries suffer from chronic
acidity, a condition in which water has a constant low pH level. Streams
flowing over soil with low buffering capacity are as susceptible to damage
from acid rain as lakes. The acidification problem grows in magnitude if
"episodic acidification" is taken into account. Episodic acidification refers to
brief periods during which pH levels decrease due to runoff from melting snow
or heavy downpours. Lakes and streams in many areas are sensitive to
episodic acidification. A lot more lakes and streams become temporarily acidic
during storms and spring snowmelt.
Acid rain causes a cascade of effects that harm or kill individual fish, reduce
fish population numbers, completely eliminate fish species from a water body,
and decrease biodiversity. As acid rain flows through soils with low buffering
capacity in a watershed, aluminum is released from soils into the lakes and
streams located in that watershed. So, as pH in a lake or stream decreases,
aluminum levels increase. Both low pH and increased aluminum levels are
directly toxic to fish. In addition, low pH and increased aluminum levels cause
chronic stress that may not kill individual fish, but leads to lower body weight
and smaller size and makes fish less able to compete for food and habitat.
Some types of plants and animals are able to tolerate acidic waters. Others,
however, are acid-sensitive and will be lost as the pH declines. Generally, the
young of most species are more sensitive to environmental conditions than
8
adults. At pH 5, most fish eggs cannot hatch. At lower pH levels, some adult
fish die. Some acid lakes have no fish at all.
In an ecosystem, different organisms are interdependent on each other. So
even if some fish are able to tolerate the acidic waters, their food – consisting
of smaller fish or insects – may not survive the increased acidity.
Effects on forest floors: A spring shower in the forest washes leaves and
falls through the trees to the forest floor below. Some trickles over the ground
and runs into a stream, river, or lake, and some of the water soaks into the
soil. That soil may neutralize some or all of the acidity of the acid rainwater.
This ability is called buffering capacity, and without it, soils become more
acidic. Differences in soil buffering capacity are an important reason why
some areas that receive acid rain show a lot of damage, while other areas
that receive about the same amount of acid rain do not appear to be harmed
at all. The ability of forest soils to resist, or buffer, acidity depends on the
thickness and composition of the soil, as well as the type of bedrock beneath
the forest floor.
Effects on plants and trees: Acid rain does not usually kill plants and trees
directly. Instead, it is more likely to weaken them by damaging their leaves,
limiting the nutrients available to them, or exposing them to toxic substances
slowly released from the soil. Quite often, injury or death of trees is a result of
these effects of acid rain in combination with one or more additional threats.
Scientists know that acidic water dissolves the nutrients and helpful minerals
in the soil and then washes them away before trees and other plants can use
them to grow. At the same time, acid rain causes the release of substances
that are toxic to trees and plants, such as aluminum, into the soil. Scientists
believe that this combination of loss of soil nutrients and increase of toxic
aluminum may be one way that acid rain harms plants and trees. Such
substances also wash away in the runoff and are carried into streams, rivers,
and lakes. More of these substances are released from the soil when the
rainfall is more acidic.
However, trees can be damaged by acid rain even if the soil is well buffered.
Forests in high mountain regions often are exposed to greater amounts of
acid than other forests because they tend to be surrounded by acidic clouds
and fog that are more acidic than rainfall. Scientists believe that when leaves
are frequently bathed in this acid fog, essential nutrients in their leaves and
needles are stripped away. This loss of nutrients in their foliage makes trees
more susceptible to damage by other environmental factors, particularly cold
winter weather.
Although damaged by other air pollutants such as ground level ozone, food
crops are not usually seriously affected because farmers frequently add
fertilizers to the soil to replace nutrients that have washed away. They may
also add crushed limestone to the soil. Limestone is an alkaline material and
increases the ability of the soil to act as a buffer against acidity.
9
Ways of reducing the harmful effects of acid deposition
Awareness is being generated among the masses worldwide about acid rain
and its harmful effects. This in turn would lead to the politicians being
provided with more and better solutions as well as being pressurized to take
effective steps to control acid rain. The following are a few steps that could be
taken in this regard:
Clean up smokestacks and exhaust pipes: Almost all of the electricity that
powers modern life comes from burning fossil fuels like coal, natural gas, and
oil. Acid deposition is caused by two pollutants that are released into the
atmosphere, or emitted, when these fuels are burned: sulfur dioxide (SO2) and
nitrogen oxides (NOx).
Coal accounts for most for most of the sulfur dioxide (SO2) emissions and a
large portion of NOx emissions. Sulfur is present in coal as an impurity, and it
reacts with air when the coal is burned to form SO2. In contrast, NOx is formed
when any fossil fuel is burned.
There are several options for reducing SO2 emissions, including using coal
containing less sulfur, washing the coal, and using devices called scrubbers to
chemically remove the SO2 from the gases leaving the smokestack. Power
plants can also switch fuels; for example burning natural gas creates much
less SO2 than burning coal. Certain approaches will also have additional
benefits of reducing other pollutants such as mercury and carbon dioxide.
Understanding these co-benefits has become important in seeking costeffective air pollution reduction strategies. Finally, power plants can use
technologies that don't burn fossil fuels. But each of these options has its own
costs and benefits. Similar to scrubbers on power plants, catalytic converters
reduce NOx emissions from cars.
Use alternative energy sources: There are other sources of electricity
besides fossil fuels. They include: nuclear power, hydropower, wind energy,
geothermal energy, and solar energy. Of these, nuclear and hydropower are
used most widely. Wind, solar, and geothermal energy have not yet been
harnessed on a large scale, but are potential alternatives.
There are also alternative energies available to power automobiles, including
natural gas powered vehicles, battery-powered cars, fuel cells, and
combinations of alternative and gasoline powered vehicles.
All sources of energy have environmental costs as well as benefits. Some
types of energy sources are more expensive to harness than others, which
means that not all people can afford all types of energy. Nuclear power,
hydropower, and coal are the cheapest forms today, but changes in
technologies and environmental regulations may shift that in the future. All of
these factors must be weighed when deciding which energy source to use
today and which to invest in for tomorrow.
10
Restore a damaged environment: Acid deposition penetrates deeply into
the fabric of an ecosystem, changing the chemistry of the soil as well as the
chemistry of the streams and narrowing, sometimes to nothing, the space
where certain plants and animals can survive. Because there are so many
changes, it takes many years for ecosystems to recover from acid deposition,
even after emissions are reduced and the rain becomes normal again. For
example, while the visibility might improve within days, and small or episodic
chemical changes in streams improve within months, chronically acidified
lakes, streams, forests, and soils can take years to decades or even centuries
(in the case of soils) to heal.
However, there are some things that people do to bring back lakes and
streams more quickly. Limestone or lime (a naturally-occurring basic
compound) can be added to acidic lakes to neutralize the acidity. This
process, called liming, has been used extensively in Norway and Sweden but
is not used very often in the United States. Liming tends to be expensive, has
to be done repeatedly to keep the water from returning to its acidic condition,
and is considered a short-term remedy in only specific areas rather than an
effort to reduce or prevent pollution. Furthermore, it does not solve the
broader problems of changes in soil chemistry and forest health in the
watershed, and does nothing to address visibility reductions, materials
damage, and risk to human health. However, liming does often permit fish to
remain in a lake, so it allows the native population to survive in place until
emissions reductions reduce the amount of acid deposition in the area.
Take action as individuals: Like many environmental problems, acid
deposition is caused by the cumulative actions of millions of individual people.
Therefore, each individual can also reduce their contribution to the problem
and become part of the solution. So steps are being taken to create
awareness among the masses. People are being educated worldwide –
especially in the developed countries – regarding that individuals can
contribute directly by conserving energy, since energy production causes the
largest portion of the acid deposition problem. For example, one can:
• Turn off lights, computers, and other electric appliances when they are not
being used.
• Use energy efficient appliances: lighting, air conditioners, heaters,
refrigerators, washing machines, etc.
• Insulate the houses as best as possible so as to reduce energy losses.
• Use public transportation rather than personal cars, or even walk or bicycle
whenever doable.
• Buy vehicles with low NOx emissions, and maintain all vehicles well so that
they release lesser emissions.
Research by Ahmad Raza sahmadraza@yahoo.com
Reference contents URLs
http://www.epa.gov/airmarkets/acidrain/index.html
http://www.policyalmanac.org/environment/archive/acid_rain.shtml
11