Acid rain

Geography glossary

Also called: Acid deposition · Topic: Geographical Issues: Pollution, Urban Waste, Slums and Land Degradation · NCERT: Class 12, Ch 9 "Geographical Perspective on Selected Issues and Problems"

Meaning

Acid rain (also called acid deposition) is rain, snow or fog with a pH below about 5.6. It forms when oxides of sulphur (SOₓ) and nitrogen (NOₓ) from burning fossil fuels turn into acids in the air.

  • It matters because it harms soils, lakes, forests and buildings.
  • It also slowly damages marble monuments such as the Taj Mahal, which made it a court and policy issue in India.

Explanation

What the pH number means

  • pH is a scale from 0 to 14 that tells how acidic something is. Below 7 is acidic.
  • Clean rain is already a little acidic, with a pH of about 5.6.
  • Rain dissolves a little CO₂ from the air.

  • This makes a weak acid called carbonic acid.

  • So rain counts as "acid rain" only when its pH is below about 5.6. Plain CO₂ cannot push it that low. Stronger acids are needed.

How acid rain forms (cause → process → result)

  • Cause: coal, oil and vehicles release SO₂ and NOₓ.

  • These come from coal-based thermal power plants, refineries, smelters, cement plants, brick kilns and vehicle exhaust.

  • Process: these gases mix with water vapour and oxygen in the air.

  • SO₂ turns into sulphuric acid.

  • NOₓ turns into nitric acid.

  • Result: the acids come down with rain, snow or fog. This is why the wider term is acid deposition: the acid can arrive in many forms, not only as rain.

Why the first rain after summer is the most acidic (NCERT)

  • In cities, the first rain after summer always has a lower pH than later rains.
  • Through the dry summer, pollutants build up in the air.
  • The first shower washes all of them out at once, so it is the most acidic.
  • Later rains fall through cleaner air, so their pH is higher.

What acid rain damages

  • Soils: acid washes out nutrients such as calcium and magnesium, so soil becomes less fertile.
  • Lakes: the water turns acidic and fish die.
  • Forests: leaves are damaged.
  • Monuments: acid eats away marble and limestone, which slowly wear away.
  • Taj Mahal ("marble cancer"):
  • SO₂ from the Mathura refinery and nearby foundries turned into acid.
  • This acid began to yellow and pit (make small holes in) the white marble.

Where it occurs

  • Indian cities: acid rain is linked to city pollution. The first rain after summer is the most acidic (NCERT).
  • Agra, Uttar Pradesh: the Taj Mahal was damaged by SO₂ from the Mathura refinery and nearby foundries.
  • The Supreme Court created the Taj Trapezium Zone (TTZ) in the M.C. Mehta case (1996). This is a trapezium-shaped area around the Taj.

  • Inside the TTZ, polluting industries had to switch to natural gas or move out.

  • Places with a lot of SO₂ and NOₓ: coal-based thermal power plants, refineries, smelters and cement plants. Brick kilns on the edges of cities across the Indo-Gangetic plain also release SO₂.

  • Busy cities with many vehicles, such as Delhi, Mumbai, Kolkata and Bengaluru, have high NOₓ from vehicles.

Don't confuse with

  • Normal rain: it has a pH of about 5.6 because CO₂ makes weak carbonic acid. Acid rain is below 5.6, and its acids come from SOₓ and NOₓ, not from CO₂.
  • Classical (sulphurous) smog: coal smoke (SO₂ + soot) mixed with fog that hangs over a city in cold, damp winters, as in London in 1952 or Delhi in winter. Acid rain is acid that falls to the ground and damages soil, water and stone.
  • Photochemical smog: this forms when sunlight acts on NOₓ and hydrocarbons and makes ground-level ozone on warm, sunny days. NOₓ plays a part in both, but in acid rain NOₓ becomes nitric acid, not ozone.
  • Temperature inversion: a layer of cold air near the ground under warmer air, which traps pollution. It is a weather condition that makes smog worse. It is not a type of acid deposition.

Prelims Hooks

  • The acid rain threshold is a pH below about 5.6. Clean rain is about 5.6 because of carbonic acid.
  • The acids in acid rain are sulphuric and nitric acid, which come from SOₓ and NOₓ. A common trap is "CO₂ causes acid rain": this is wrong.
  • NCERT: in cities, the first rain after summer has the lowest pH.
  • Taj Trapezium Zone: created by the Supreme Court in the M.C. Mehta case, 1996. The polluter was the Mathura refinery.
  • Marble and limestone monuments are the most at risk. The damage to the Taj is called "marble cancer".
  • Acid deposition covers acid falling as rain, snow or fog.

Mains Points

  • Causes and spread (GS-I/III):
  • India has many sources of SO₂ and NOₓ: coal-based thermal power, refineries, smelters, brick kilns and city traffic.

  • Pollutants build up through the dry season, so the first rains of the monsoon carry the most acid. This hits soils, lakes, forests and city buildings.

  • Effects on resources:

  • Acid soils lose calcium and magnesium, which lowers farm output.
  • Acid lakes lose their fish.
  • Damaged leaves weaken forests.

  • Together, these link air pollution to land degradation and the health of ecosystems.

  • Acid rain and heritage (policy response):

  • The Taj Trapezium Zone (1996) shows how courts can control land use to protect the environment.
  • The polluter-pays and precautionary principles were applied through zoning: polluters had to switch to natural gas or move out.
  • It is a model for protecting other stone monuments near industrial areas.

Related concepts

Read more

Sources

  1. 1Class 12, Ch 9 "Geographical Perspective on Selected Issues and Problems" (primary)