Acid rain; sources and control of CO and SO₂

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.

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: it acidifies lakes and kills fish, damages forests and soil, corrodes buildings and metals (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂) and harms health.

Sources of CO and SO₂

• CO: incomplete combustion of carbon fuels in vehicle engines (the largest source), burning of wood, coal and charcoal in poorly ventilated stoves, forest fires, industrial processes (steel).
• SO₂: burning coal and oil containing sulphur, smelting of sulphide ores, thermal power plants, brick kilns, oil refineries; volcanoes naturally.

Control

• CO: use catalytic converters (2CO + O₂ → 2CO₂ over Pt/Pd), keep engines well tuned for complete combustion, use cleaner fuels (LPG, CNG, electricity), improve ventilation and stoves, public transport.
• SO₂: use low-sulphur fuels, remove sulphur from fuels before burning, flue gas desulphurisation (lime scrubbing: SO₂ + CaCO₃ + ½O₂ → CaSO₄ + CO₂), tall chimneys with scrubbers, and renewable energy.