Acid Rain

The rainfall that has been acidified when oxides of sulphur and nitrogen react with the moisture in the atmosphere. Acid rain has a pH of less than 5.6 (pH less than 7 is acidic). Acid rain is scientifically called as acid deposition, which may be a wet or dry deposition.

Wet Deposition

If the acid chemicals in the air are mixed with wet weather in the atmosphere, the acids can fall onto the ground in the form of acidic rain or other wet depositions of snow, fog, or mist.

Dry Deposition

If the acidic chemicals in the air are mixed with wet weather in the atmosphere, the acid mix with dust or smoke and falls onto the ground as dry deposition, sticking to the ground, buildings, vegetation, cars, etc.

Causes of Acid Rain

  • Anthropogenic activities including the burning of fossil fuels -coal, oil, natural gas- and the smelting of metal ores, are the major causes of acid deposition. For instance, electric utilities and fossil fuels burned by vehicles produce SO2 and NOx.
  • Coal-powered Thermal power stations are the biggest emission hotspots in India.

Impact of Acid Rains on the Environment

On Water bodies

  • Ocean Acidification: Acid rains or deposition lead to the acidification of oceans. It leads to a decline in the health of aquatic organisms such as fisheries and other marine organisms.
  • In wetlands and marshy ecosystems, the high acidity, especially from sulfur deposition, can fast-track the conversion of elemental mercury to methyl mercury, the deadliest neurological toxin. And it bio-accumulates in the fish tissue.
  • Acid rains react with harmful pollutants (like cadmium and lead) and contaminate water resources thereby causing harmful health risks.

On Vegetation

Acid rains adversely affect trees and undergrowth vegetation in the forest in numerous ways, causing reduced growth or abnormal growth. The symptoms of vegetation due to acid rains include:

  • Early ageing of older needles and loss of feeder-root biomass in conifers
  • Death of affected trees especially in poorly buffered soils.
  • Unusual Proliferation of lichens on affected trees.
  • Discolouration and loss of foliar biomass
  • Increased susceptibility of damage to the secondary root and foliar pathogens
  • Death of herbaceous vegetation beneath affected trees

On Soil

  • Buffered soils can withstand or neutralizes acidity. The poorly buffered soils are susceptible to acidification because they lack significant amounts of base cations (positively charged ions), which neutralize acidity.
  • The exchange between hydrogen ions and the nutrient cations like calcium, potassium and magnesium in the soil cause leaching of the nutrients, making the soil infertile. The impact of acid rain on Indian soil is less because Indian soils are typically alkaline with good buffering ability.

On Man-made Structures

  • Corrosion of statues.
  • Acid rain can dissolve marble and limestone through direct contact.
  • Discolorations of Monuments made of marble and limestone. For example, Acid rains caused discoloration of the Taj Mahal, which is made of white marble to pale yellow.

On Materials

  • Cracking of rubber,
  • Fading and discoloration of textiles,
  • Embrittlement, of paper,
  • Surface erosion of glassware and
  • Corrosion of metals etc.

Measures were taken by GoI to reduce the causes of Acid depositions

  • To regulate polluting industries, the Ministry of Environment, Forest and Climate Change (MoEF&CC) has developed the criteria for the categorization of industrial sectors based on the Pollution Index. The pollution index is a function of the emissions (air pollutants), effluents (water pollutants), hazardous wastes generated, and consumption of resources. Industries are categorized based on pollution index score as red (60 and above), orange(41-60), green (21-40), and white (<20).
  • To improve the Ambient Air Quality (AAQ), the Union Ministry Of Environment, Forests, and Climate Change released a notification in 2015, according to which thermal power plants across India, from 2017, will have to cut particulate matter emissions including P.M10, SOx, and NOx by as much as 40 percent and reduce their water consumption. This will reduce mercury pollution by 70% and reduce the energy requirement for the drawl of water.
  • In 2015, the Ministry of Environment, Forest and Climate Change (MoEF&CC) introduced SO2 and NOx emission limits for coal power stations by adopting flue-gas desulfurization (FGD) technology by June 30, 2020.
  • GoI actively promotes non-renewable sources of energy-solar and wind, etc.

Acid Rain


  • Most of the coal plants in India have not installed the flue-gas desulfurization (FGD) technology, which is necessary to scrub emissions clean of sulfur.
  • According to recent data compiled by Central Electricity Authority (CEA) for all power plants, Central, State, and Independent Power Producers (IPPs) show that most of the power plants would not be able to meet the deadline in spite of advanced efforts due to several constraints. FICCI viewed that at a time when the Power Ministry has made prior approval mandatory for the import of certain power equipment from China and Pakistan and the ministry said no to import from China, the FGD sourcing and supply capacity becomes a critical challenge.
  • Improving the buffering capacity of the soil to resist change in pH.

Way Forward

  • The government of India shall constitute a committee comprising the Ministry of Power and the Ministry of Environment (MOEFCC) to review the timelines for FGD implementation, by conducting an assessment of the requirements and prevailing bottlenecks.
  • time extension may be granted for the installation and commissioning of FGDs by captive power plants (CPP).
  • Creating an enabling environment for Indian manufacturers to ramp up capacity. 


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