Acid Rain and Acidification [Best Article]
Acid Rain

Acid Rain and Acidification [Best Article]

Acid Rain and Acidification


  • Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, which react with
    the water molecules in the atmosphere to produce acids.
  • Rain with a pH of < 5.6.
  • Acid rain is damaging to lakes, streams, and forests and the plants and animals that live
    in these ecosystems.

Acid rain is caused by the pollution of environment by
(a) carbon dioxide and nitrogen
(b) carbon monoxide and carbon dioxide
(c) ozone and carbon dioxide
(d) nitrous oxide and Sulphur dioxide
Prelims 2013

Types of acid deposition

Types of acid deposition

Wet deposition

Wet deposition of acids occurs when any form of precipitation (rain, snow, and so on.)
removes acids from the atmosphere and delivers it to the Earth’s surface.
As it flows it affects a variety of plants and animals.
Strength of the effects depends on –

  •  How acidic water is
  • types of fish, trees, and other living things that rely on the water

Dry deposition

if 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 and
vegetation, cars, etc.
Dry deposits can be washed by rainstorms.

Sources of compounds causing acid rain


Natural resources –

Seas and oceans
Volcanic eruptions
Biological processes in the soil. E.g. Decomposition of organic matter.

Man made sources –

Burning of coal and petroleum products
Smelting of metal sulfide ores to obtain the pure metals.
Industrial production of Sulfuric acid in metallurgical, chemical and fertilizer industries.


Natural sources:

Volcanic eruption
Biological activity.

Anthropogenic sources:

Forest fires
Combustion of oil, coal, and gas

Formic acid 

Biomass burning due to forest fires causes emission of formic acid (HCOOH) and
formaldehyde (HCHO) into the atmosphere.
Large fraction formaldehyde gets photo – oxidation and forms formic acid in the

Other Acids

Phosphoric acid
Hydrochloric acid (smokestacks).
Carbon monoxide and carbon dioxide (automobiles). These become carbonic acid.

Acid rain area common characteristics –

Concentrated in the industrialized belt
Often upland and / or mountainous areas, which are well-watered by rain and snow.
Possess numerous lakes and streams and also have more land covered with vegetation.
Thin soils and glaciated bedrock.

Formation of Acid rain

6 Steps –
  1. atmosphere receives oxides of sulfur and nitrogen from natural and man-made sources.
  2. Some of these oxides fall back as dry deposition
  3. Sunlight stimulates the formation of photo-oxidants (Ozone) in the atmosphere.
  4. photo-oxidants interact with the oxides of sulfur and nitrogen to produce
    H2S04 and HN03 by oxidation
  5. photo-oxidants interact with the oxides of sulfur and nitrogen to produce
    H2S04 and HN03 by oxidation
  6. oxides are of sulfur and nitrogen, photo-oxidants, and other gases (like NH3)
  7. Acid rain containing ions of sulfate, nitrate-ammonium and hydrogen falls as
    wet deposition.

Impact of acid rain


Leaching of Acid nutrients → make soil infertile.
Decrease in the respiration of soil organism.
Decrease in other nutrients → increase in ammonia → decreases the rate of
Decrease in nitrate level
Acid rain impact less in case of India because Indian soils are alkaline.


Causing reduced growth or abnormal growth
Growth decreasing symptoms are –
Discoloration and loss of foliar biomass

Loss of feeder-root biomass, especially in conifers
Premature senescence (aging) of older needles in conifers
Increase in susceptibility of damage to secondary root and foliar pathogens
Death of herbaceous vegetation beneath affected trees
Prodigious production of lichens affected trees.
Death of affected trees.


pH determines the proliferation of any microbial species and the rate at which it can
Optimum pH of most bacteria and protozoa – near neutrality
Most fungi prefer an acidic environment.
Most blue-green bacteria prefer an alkaline environment.
Microbial species shift from bacteria-bound to fungi-bound and cause an imbalance in
the microflora.
Causes a delay in the decomposition of soil organic material.
Increase in fungal disease in aquatic life and forests.


Effects – not very obvious
can directly affect the eggs and tadpoles of frogs and salamanders
indirectly affect wildlife (toxic substances may be ingested by animals, like birds, that
feed in such an environment)
Other indirect effects – loss or alteration of food and habitat resources.


Bad smells, reduced visibility, irritation of the skin, eyes and the respiratory tract.
Chronic bronchitis, pulmonary emphysema and cancer.
Indirect effect – food poisoning vis a vis drinking water and food.

Acid rain damage on materials

Trigger effect of Acid rain on pollutants

Methyl mercury and alkyl mercury accumulate in edible fish tissue (dangerous to
Increase the partitioning of methyl mercury into the water column.
Use of lime has helped in reducing the mercury levels in fish.
Leach substantial amounts of aluminum from watersheds
Cadmium can enter the drinking water supply through corrosion of galvanized pipe or
from the copper-zinc through corrosion of galvanized piper or from the copper-zinc
solder used in the distribution systems.
Lower than normal blood levels of lead can cause mental deficiencies and behavioural
Foetuses and infants are highly susceptible to drinking water lead contamination.
Asbestos in natural rock can be released by acidic waters.

Control measures

Controlling or eliminating sources of pollution by –
Adding a neutralizing agent to the acidified water to increase the pH. E.g. lime in the
form of calcium oxide and calcium carbonate is used.
Reducing the emission of S02 from power stations by burning less fossil fuel, using
alternate energy sources like tidal, wind, hydropower etc.
Using low sulphur fuel
decreasing emission of NOx from power stations
Modification of engines.
Stricter enforcement of bharat stage emission standards.
Emissions of SOx can be controlled by-

  • o Converting to sulphuric acid
    o Converting it to elemental Sulphur
    o Neutralizing it and using it in the manufacture of other products.
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