* *Acid Rain** The Causes, History, and Effects

Acid rain is rain consisting of water droplets
that are unusually acidic because of
atmospheric pollution - most notably the
excessive amounts of sulfur and nitrogen
released by cars and industrial processes.
Acid rain is also called acid deposition
because this term includes other forms of
acidic precipitation such as snow.
Acidic deposition occurs in two ways: wet
and dry. Wet deposition is any form of
precipitation that removes acids from the
atmosphere and deposits them on the
Earth ’s surface. Dry deposition polluting
particles and gases stick to the ground via
dust and smoke in the absence of
precipitation. This form of deposition is
dangerous however because precipitation
can eventually wash pollutants into streams,
lakes, and rivers.
Acidity itself is determined based on the pH
level of the water droplets. PH is the scale
measuring the amount of acid in the water
and liquid. The pH scale ranges from 0 to 14
with lower pH being more acidic while a
high pH is alkaline; seven is neutral. Normal
rain water is slightly acidic and has a pH
range of 5.3-6.0. Acid deposition is anything
below that scale. It is also important to note
that the pH scale is logarithmic and each
whole number on the scale represents a 10-
fold change.
Today, acid deposition is present in the
northeastern United States, southeastern
Canada, and much of Europe including
portions of Sweden, Norway, and Germany.
In addition, parts of South Asia, South Africa,
Sri Lanka, and Southern India are all in
danger of being impacted by acid
deposition in the future.
Causes and History of Acid Rain
Acid deposition can occur via natural
sources like volcanoes but it is mainly
caused by the release of sulfur dioxide and
nitrogen oxide during fossil fuel
combustion. When these gases are
discharged into the atmosphere they react
with the water, oxygen, and other gases
already present there to form sulfuric acid,
ammonium nitrate, and nitric acid. These
acids then disperse over large areas
because of wind patterns and fall back to
the ground as acid rain or other forms of
precipitation.
The gases responsible for acid deposition
are normally a byproduct of electric power
generation and the burning of coal. As such,
it began entering the atmosphere in large
amounts during the Industrial Revolution
and was first discovered by a Scottish
chemist, Robert Angus Smith, in 1852. In
that year, he discovered the relationship
between acid rain and atmospheric
pollution in Manchester, England.
Although it was discovered in the 1800s,
acid deposition did not gain significant
public attention until the 1960s and the
term acid rain was coined in 1972.


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.