8.3.1 Acid Deposition

What is acid deposition?

• Rain is naturally acidic because of dissolved CO₂ which forms carbonic acid

H₂O (l) + CO₂ (g)  ⇌  H₂CO₃ (aq)

• Carbonic acid is a weak acid and dissociates in the following equilibrium reaction giving a pH of 5.6

H₂CO₃ (aq)  ⇌  H⁺ (aq) +  HCO₃^- (aq)

• For that reason acid rain is defined as rain with a pH of below 5.6
• Acid deposition includes all processes by which acidic components leave the atmosphere
  • This could be gases or precipitates

• There are two types of deposition: wet acid deposition and dry acid deposition
  • Wet acid deposition refers to rain, snow, sleet, hail, fog, mist and dew
  • Dry acid deposition refers to acidic particles and gases that fall to the ground as dust and smoke

• Acid deposition is formed when nitrogen or sulfur oxides dissolve in water to form HNO₃, HNO₂, H₂SO₄ and H₂SO₃

Formation of sulfur based acids

• Fossil fuels are often contaminated with small amounts of sulfur impurities
• When these contaminated fossil fuels are combusted, the sulfur in the fuels get oxidised to sulfur dioxide

 S (s) + O₂ (g) → SO₂ (g)

• Sulfur dioxide may be further oxidised to sulfur trioxide

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

• The sulfur dioxide and sulfur trioxide then dissolve in rainwater droplets to form sulfurous acid and sulfuric acid

SO₂(g) + H₂O  (l) → H₂SO₃ (aq)

SO₃ (g) + H₂O (l) → H₂SO₄ (aq)

• These acids are components of acid rain which has several damaging impacts on the environment

Formation of acid rain by nitrogen oxides

• The temperature in an internal combustion engine can reach over 2000 °C
• Here, nitrogen and oxygen, which at normal temperatures don’t react, combine to form nitrogen monoxide:

N₂ (g)+ O₂ (g) ⇌ 2NO (g)

• Nitrogen monoxide reacts further forming nitrogen dioxide:

2NO (g) + O₂ (g) ⇌ 2NO₂ (g)

• Nitrogen dioxide gas reacts with rain water to form a mixture of nitrous and nitric acids, which contribute to acid rain:

2NO₂ (g) + H₂O (l)  → HNO₂ (aq) + HNO₃ (aq)

• Lightning strikes can also trigger the formation of nitrogen monoxide and nitrogen dioxides in air
• Nitrogen dioxide gas reacts with rain water and more oxygen to form nitric acid

4NO₂ (g) + 2H₂O (l) + O₂ (g)→ 4HNO₃ (aq)

• When the clouds rise, the temperature decreases, and the droplets get larger
• When the droplet containing these acids are heavy enough, they will fall down as acid rain

The diagram shows the formation of acid rain by the oxidation of nitrogen dioxide