Acid rain is caused by sulfur and nitrogen emissions from fossil fuel combustion reacting with water and oxygen in the atmosphere to form acids. Natural rain has a pH of around 5.5-7 due to carbon dioxide, but acid rain has a pH below 5. It damages ecosystems, aquatic life, forests, and infrastructure. Effects include acidified lakes and streams unable to support fish and other species, and increased corrosion of buildings and statues. Preventing additional acid rain requires controlling sulfur dioxide and nitrogen oxide emissions through regulations on power plants and vehicles.

1. Acid Rain
Rain is naturally acidic because carbon dioxide in the atmosphere combines with water molecules
to form carbonic acid. Acidic precipitation occurs when sulfur dioxide and nitrogen oxides in the
atmosphere react with oxygen in the air to form sulfuric acid (H2SO4) and nitric acid (HNO), which
falls to the surface as rain, snow, or dust. This phenomenon is termed acid deposition or acid
precipitation.
If the precipitation has a pH of 5.0 or lower, it is considered to be acidic precipitation.
CHEMICAL COMPOSITION OF PRECIPITATION
Natural precipitation
Pure water has a pH of 7 determined by dissociation of H2O molecules:
Rainwater falling in the atmosphere always contains impurities, even in the absence of
human influence. It equilibrates with atmospheric CO2, a weak acid, following the
reactions presented in chapter 6:

2. The corresponding equilibrium constants in dilute solution at 298 K are KH =
[CO2H2O]/PCO2 = 3x10-2 M atm-1, K1 = [HCO3-][H+]/[CO2H2O] = 4.3x10-7 M (pK1
= 6.4), and K2= [CO32-][H+]/[HCO3-] = 4.7x10-11M (pK2 = 10.3). From these constants
and a preindustrial CO2 concentration of 280 ppm one calculates a rainwater pH of 5.7.
Other natural acids present in the atmosphere include organic acids emitted by the
biosphere, HNO3 produced by atmospheric oxidation of NOx originating from lightning,
soils, and fires and H2SO4 produced by atmospheric oxidation of reduced sulfur gases
emitted by volcanoes and by the biosphere. The natural acidity of rain is partly balanced
by natural bases present in the atmosphere, including NH3 emitted by the biosphere and
CaCO3 from suspended soil dust.
When all of these influences are taken into account, the pH of natural rain is found to be in
the range from 5 to 7. The term acid rain is customarily applied to precipitation with a pH
below 5. Such low pH values are generally possible only in the presence of large amounts
of anthropogenic pollution.
The median pH at that site is 4.34, typical of acid rain in the northeastern United States.
The H+ ion is the dominant cation and is largely balanced by SO42- and NO3-, which are
the dominant anions. We conclude that H2SO4 and HNO3 are the dominant contributors
to the precipitation acidity. Both are strong acids which dissociate quantitatively in water
to release H+:
Types of Acid deposition:
Acid deposition, which has two parts: wet and dry.
Wet deposition
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.

3. Dry deposition
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.
Causes of Acid rain
Scientists discovered, and have confirmed, that sulfur dioxide (SO2) and nitrogen oxides (NOx)
are the primary causes of acid rain. In the US, About 2/3 of all SO2 and 1/4 of all NOx comes 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. Combustion of fuels produces 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 (aq)
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.
Hydrolysis
Sulfur dioxide dissolves in water and then, like carbon dioxide, hydrolyses in a series
of equilibrium reactions:

4. SO2 (g) + H2O ⇌ SO2·H2O
SO2·H2O ⇌ H+ + HSO3
−
HSO3
− ⇌ H+ + SO3
2−
Oxidation
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 withozone, hydrogen
peroxide and oxygen (reactions with oxygen are catalyzed by iron and manganese in the cloud
droplets
How Do We Measure Acid Rain?
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. Normal rain is slightly acidic because carbon dioxide dissolves into
it, so it has a pH of about 5.5. As of the year 2000, the most acidic rain falling in the US has a pH
of about 4.3.
Adverse 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 our 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 public health.
Effect of acidic precipitation on aquatic ecosystems
Most biological life thrives best within a narrow range of pH levels, near neutral or 7.0. Aquatic
vegetation and animal life vary in their susceptibility to changes in pH; some species are more
acid-tolerant than others. Aquatic systems that are well-buffered may not be as affected by acidic
runoff as those that lack a buffering capacity. In lakes and streams whose waters become acidified,
microorganisms may be affected. As organisms lower in the food chain are reduced, species higher
up the food chain that rely on these organisms for food will be affected. Plankton and invertebrates
are most sensitive to changes in pH. If the pH levels drop below 5.0 most fish species are affected.
Effect of acidic precipitation on soils and plant growth
Some plants are tolerant of acidic conditions, while others are not. Acidic soils may affect
microorganisms in the soil, which play important roles in plant growth. Acidity affects the

5. availability of nutrients that are essential for plant growth and increases leaching of aluminum and
mercury, which are toxic to plants at high levels. Nitrogen is a nutrient and at certain levels,
nitrogen deposition from air emissions has increased growth of vegetation; however, at higher
levels, excess nutrients can reduce plant growth.
Effect of acid rain on buildings and materials
Acidic precipitation is corrosive of metals and alkaline building materials such as marble and
limestone. Urban areas subject to high levels of automobile exhaust and other sources of acidic
precipitation have experienced significant weathering of statues and building materials.
 Limestone and Marble
o CaCO3(s) + H2SO4(aq) -> CO2(g) + H2O(l),, + Ca(NO3)2(aq)
 Metal Corrosion
o Metal + Acid -> Salt + Water
o i.e. Fe(II)(s) + H2SO4(aq) -> FeSO4(aq) + 2H+
Effect of acid rain on health
Acidic precipitation does not affect human health directly; however, the particulate
matter associated with acid precipitation has been shown to have adverse health effects,
particularly among those who have respiratory disorders. There is also some concern that acidic
precipitation could contribute to leaching of toxins such as mercury that could be carried by runoff
into bodies of water, contributing to environmental sources of this toxin.
Effects on animals and birds
All living organisms are interdependent on each other. If a lower life form is killed, other species
that depended on it will also be affected. Every animal up the food chain will be affected. Animals
and birds, like waterfowl or beavers, which depended on the water for food sources or as a habitat,
also begin to die. Due to the effects of acid rain, animals which depended on plants for their food
also begin to suffer. Tree dwelling birds and animals also begin to languish due to loss of habitat.
Effects on human beings
Mankind depends upon plants and animals for food. Due to acid rain the entire fish stocks in certain
lakes have been wiped out. The economic livelihood of people who depended on fish and other
aquatic life suffers as a result. Eating fish which may have been contaminated by mercury can
cause serious health problems. In addition to loss of plant and animal life as food sources, acid
rain gets into the food we eat, the water we drink, as well as the air we breathe. Due to this

6. asthmatic people and children are directly affected. Urban drinking water supplies are generally
treated to neutralise some of the effects of acid rain and therefore city dwellers may not directly
suffer due to acidified drinking water. But out in the rural areas, those depending upon lakes, rivers,
and wells will feel the effects of acid rain on their health. The acidic water moving through pipes
causes harmful elements like lead and copper to be leached into the water. Aluminium which
dissolves more easily in acid rain as compared to pure rainfall, has been linked to Alzheimer’s
disease. The treatment of urban water supplies may not include removal of elements like
Aluminium, and so is a serious problem in cities too.
Prevention method:
Technical solutions
 Many coal-firing power stations use flue-gas desulfurization (FGD) to remove sulfur-
containing gases from their stack gases. For a typical coal-fired power station, FGD will
remove 95% or more of the SO2 in the flue gases.
 An example of FGD is the wet scrubber which is commonly used. 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 tore store native life.
 Fluidized bed combustion also reduces the amount of sulfur emitted by power production.
 Vehicle emissions control reduces emissions of nitrogen oxides from motor vehicles
References:
1. Likens, Gene E.; Keene, William C.; Miller, John M.; Galloway, James N. (1987).
"Chemistry of precipitation from a remote, terrestrial site in Australia". Journal of
Geophysical Research 92: 13299. doi:10.1029/JD092iD11p13299.
2. Jump up^E. S. de Beer, ed. The Diary of John Evelyn, III, 1955 (19 September 1667)
p. 495.
3. US EPA: Effects of Acid Rain – Surface Waters and Aquatic Animals