Short note: Acid rain

Acid rain

Acid rain is rain (or snow, fog or dust) with a pH below 5.6. Normal rain water has a pH of about 5.6; it is slightly acidic because of dissolved CO₂ (CO₂ + H₂O ⇌ H₂CO₃). Acid rain is caused by the oxides of sulphur and nitrogen released into the air.

Causes

SO₂ comes from burning coal and oil and from smelters; NO and NO₂ come from vehicle engines and power plants (N₂ + O₂ → 2NO at high temperature). In the air they are oxidised and dissolve in cloud water:

2SO₂ + O₂ → 2SO₃          SO₃ + H₂O → H₂SO₄
2NO + O₂ → 2NO₂           4NO₂ + O₂ + 2H₂O → 4HNO₃

These strong acids fall as acid rain.

Effects

• Acidifies lakes and rivers, killing fish and other aquatic life.
• Damages forests and leaches nutrients (Ca²⁺, Mg²⁺) and releases toxic Al³⁺ from the soil.
• Corrodes metals and attacks marble and limestone buildings (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂), for example the Taj Mahal.
• Causes respiratory problems in humans.

Control

• Use low-sulphur coal and oil; remove sulphur before burning (desulphurisation).
• Flue gas desulphurisation: scrubbing with lime, CaCO₃ + SO₂ + ½O₂ → CaSO₄ + CO₂.
• Catalytic converters in vehicles; control of NOx by modern combustion.
• Use renewable energy (hydropower, solar), and treat acidified lakes with lime.