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| Last Updated:: 24/01/2019

Acid Rain

Acid Rain
Acid Rain is a term used for rain with a pH of less than 5.6. The acid characteristic of water is measured by pH (the negative logarithm of the hydrogen ion activity of the medium). The normal pH range is 6.5 to 7.5. Below pH 6.5 indicates acidic character. If the pH of rainfall is 6, it is considered as only ‘mildly’ or ‘weakly’ acidic. This level of acidity is not considered dangerous. The ‘normal’ rain is mildly acidic showing a pH value of around 6. This mild acidity is due to dissolution of CO2, that forms carbonic acid and due to the presence of chemicals produced by lightening and volcanic activity. When the falling rain exhibits a pH value of less than 5.6, it is termed “acid rain”. Further drop in the pH from ‘around 6 pH’ to less than pH 5.6 occurs when more acid forming gases present in the polluted air gets dissolved in the falling rain. Acid Rain represents one of the major consequences of air pollution. It has adverse effect on forest, agriculture, soil, microorganisms, wild animals and even buildings. The polluted air generally contains gases like Sulphur dioxide and nitrogen oxide at higher than normal concentrations. Other chemical present in the polluted air consists of hydrochloric acid vapours, phosphoric acid, formic acid, formaldehyde, carbon mono oxide, carbon dioxide, sulphuric acid and nitric acid. When rain falls through the polluted air different gases and vapours gets dissolved in the falling rain making it more acidic than natural rain.
Causes and Sources of acid rain
List of Compounds and their Source
Sl. No. Compounds that cause acidification of Rain Source of compound
1 Sulphur compunds and radicals
Sulphur dioxide (SO2)
Sulphur trioxide (SO3)
Hydrogen Sulphide (H2S)
Sulphate ions (SO4--)
Volcanic eruptions, forest fires, decomposition of organic matter by microorganism, burning of coal (contributes 60% of SO2) and petroleum products (contributes 30% of SO2). Industrial production of sulphuric acid in metallurgical and chemical industries.
2 Nitrogen compounds and radicals
Nitric Oxide (NO)
Nitrous Oxide (N2O)
Nitrogen Dioxide (NO2)
Nitric acid(HNO3)
Lightening, Volcanic eruption, biological activities, power station, vehicle exhausts and industries.
3 Chlorine and hydrochloric acid
Chloride (Cl2)
Hydrochloric acid (HCl)
Chlorination plants producing vinyl chloride and ethylene dichloride. HCl from electro plating industries, fertilizer industries etc.
4 Phosphoric Acid Phosphoric acid manufacturing plants.
5 Formic Acid Biomass burning due to forest fire.
6 Carbon mono oxide and Carbon dioxide Mainly from automobiles and fossil fuel buring.
Chemistry of acid rain
The acid rain is infact a cocktail of H2SO4 and HNO3 and the ratio of the of the two may vary depending on the relative quantities of oxides of sulphur and nitrogen emitted in the atmosphere. The NOX and SOX are gradually converted into HNO3 and H2SO4 respectively. Sunlight stimulates the formation of photo-oxidants such as ozone in the atmosphere, that interact with oxide of sulphur and nitrogen to produce H2SO4 and HNO3 by oxidation.
Nitrogen Oxides Sulphur Oxides
NO + O3 ® NO2 + O2 SO2 + hn ® SO2 (energized)
NO2 + O3 ® NO3 + O2 SO2 + O2 ® SO4 (energized)
NO2 + NO3 ® N2O5 SO4 + O2 ® SO3 +O3
N2O5 + H2O ® 2HNO3 SO3 + H2O ® H2SO4
These oxides of sulphur and nitrogen, and other gases like ammonia dissolve in rain water to produce acidic cations like H+ and NH4+, Sulphates (SO4--) and nitrates (NO3-).
Effect of acid rain
The acidification of rain water may cause extensive damage to materials and terrestrial as well as aquatic ecosystems. Acid rain often remain undetected until it cause damage. The organisms which are acid sensitive may serve as indicators of the initial stage of acidification e.g. Lichens serve as good bio-indicators of air pollution. In the vicinity of pH 6.0, several animals like fresh water shrimp, cray fish, snails and some mussels decline.
Effect on Soil
In India so far, there has not been much complaints of acid rain except for a couple of isolated cases in highly industrialized areas. The soil in most parts of the country (except the far northeast) is alkaline and the very fine dust emanating from these alkaline soil is scattered in the lower levels of our troposphere in all seasons. This dust plays a buffering role reducing the acidity of rain so that by the time the rain reaches the surface, it is no longer “acid rain” even if it had become acidic by dissolving pollutants in the atmosphere. The acid neutralizing ability of the soil of a given region greatly influences the extent of harm acid rain may cause in that region soils of different regions, have differing ability to process and neutralize acidic inputs.
  • The exchange between hydrogen ions and the nutrients cations like K+ and Mg++ in the soil cause leaching of the nutrients, making the soil infertile.
  • An increase in ammonia in the soil due to a decrease in other nutrients decreases in other nutrients decreases the rate of decomposition.
  • The nitrate level of soil also decreases.
  • Toxic metals to plants gradually accumulates in the soil due to increase in soil metal mobility.
Effect on Vegetation
Acid rain is harmful to forests trees and under growth in forests.
  • Growth of the leaves is suppressed due to high acidity.
  • Result in stunted and abnormal growth of the plant.
  • Discoloration and loss of foliar biomass.
  • Decreased annual increment (width of growth rings).
  • Increased susceptibility to secondary root and folia pathogens.
  • Death of herbaceous vegetation beneath affected trees.
  • Premature falling of leaves while still green.
  • Loss of feeder – root biomass in conifers.
  • Premature senescence of older needles in conifers.
  • Shedding of whole green shoots, especially in spruce.
  • Formation of stork’s nest crown in young white fir.
  • Excessive seed and cone productions, year after year.
Effect on Microorganisms
Acid rain also disturbs the microbial activity of soil. The microorganism is very sensitive to change in soil pH. Change in pH due to acid rain may cause a shift in the community of the microorganisms. The pH optima of most bacteria and protozoa are near neutrality, most fungi prefer an acidic environment, most blue-green bacteria prefer an alkaline environment.
  • Acid rain results in shifting of bacterial community to fungal community. This causes imbalance in the microflora.
  • Decay in the decomposition of soil organic material due to loss of bacterial community. The fungi are not as good decomposers as bacteria.
  • Increased incidence of fungal diseases.
  • Inhibition of mycorrhizal activity.
Effect on Waterbodies
  • In natural water bodies metals such as zinc, cadmium, nickel, manganese, chromium, copper and mercury form complexes with naturally occurring ligands such as fulvic acids and phosphates. In this complexed form the bioavailability and toxicity of these metals is greatly reduced (Abbasi 1995, Soni 1991) when a lake get acidified the metal-fulvic acid complexes are destablized releasing metal ions.
  • pH also effects the mobility of metals present in sediments, towards the overlaying water through direct dissolution.
  • The combined effect of above two phenomenon increases the concentration of metals in water often to levels that causes toxicity and even acute toxicity in aquatic organisms (Abbasi 1995).
  • Algae are susceptible to change in pH of the water. Many forms of algae do not survive at a pH of less than 4.5. Some blue green algae also decrease or are eliminated. The ultimately result is reduced primary productivity of the water body.