1. Acid Rain (Acid Deposition)

1.1 Meaning

• Acid rain refers to precipitation with a pH < 5.6
• Includes:
  • Rain
  • Snow
  • Fog
  • Mist
• Normal rainwater is slightly acidic due to carbonic acid (CO₂ + H₂O), but not acid rain

1.2 Causes

• Emission of acidic gases:
  • Sulphur dioxide (SO₂ / SOx)
  • Nitrogen oxides (NO, NO₂, N₂O collectively NOx)
• Released mainly from:
  • Fossil fuel burning
  • Thermal power plants
  • Smelting industries
  • Vehicular emissions

1.3 pH Scale (Prelims Favourite)

• Range: 0–14
• 7 → Neutral
• <7 → Acidic
• >7 → Basic
• Each unit decrease in pH → 10× increase in acidity

1.4 Gases Responsible for Acid Rain

1.5 Types of Acid Deposition

A. Wet Deposition

• Acidic rain, snow, fog, mist
• Directly affects soil, crops, and lakes

B. Dry Deposition

• Acidic gases & particles settle on:
  • Buildings
  • Vegetation
  • Soil
• Washed down later by rain → more acidic runoff
• Nearly 50% acidity comes via dry deposition

1.6 Chemistry of Acid Rain (Simplified)

1. Emission of SOx & NOx
   II. Sunlight forms photo-oxidants (e.g., ozone)
   III. Oxidation reactions occur
   IV. Formation of:

• Sulphuric acid (H₂SO₄)
• Nitric acid (HNO₃)
  Acidic ions fall as wet/dry deposition

2. Harmful Effects of Acid Rain

2.1 On Human Health

• Eye, skin, and respiratory irritation
• Aggravates:
  • Bronchitis
  • Emphysema
  • Lung cancer (indirectly)

2.2 On Soil

• Leaching of nutrients:
  • Calcium
  • Magnesium
  • Potassium
• Release of toxic metals:
  • Aluminium
  • Lead
• Indian soils → mostly alkaline → better buffering capacity

2.3 On Aquatic Ecosystems

• pH change kills:
  • Fish eggs
  • Amphibian larvae
• Heavy metals are released into water
• Microbial productivity declines

2.4 On Terrestrial Plants

• Leaf cuticle damage
• Reduced photosynthesis
• Metal toxicity through roots

2.5 On Microorganisms

• Shift from:
  • Bacteria-dominated → Fungi-dominated soil
• Slower decomposition
• Reduced soil fertility

2.6 On Buildings & Monuments

• Marble & limestone react with acids
• Causes:
  • Flaking
  • Black crust formation
• Example:
  • Taj Trapezium Zone – “Marble Cancer”

2.7 Acid Rain Regions

• Global:
  • Scandinavia
  • NE USA
  • Canada
  • NW Europe
• India:
  • First reported: Mumbai (1974)
  • NE India, coastal Karnataka, Kerala, Odisha, WB, Bihar

2.8 Control Measures

• Low-sulphur fuel
• Washed coal
• Use of natural gas
• Buffering (Liming):
  • CaO / CaCO₃ added to acidified lakes

3. Ocean Acidification

3.1 Meaning

• Ongoing decrease in ocean pH
• Due to the absorption of atmospheric CO₂
• Called:
  • “Evil twin of global warming”
  • “Other CO₂ problem”

3.2 Chemical Process

1. CO₂ dissolves in seawater
   II. Forms carbonic acid (H₂CO₃)
   III. Produces:

• Bicarbonate (HCO₃⁻)
• Hydrogen ions (H⁺)
  pH decreases → acidity increases

3.3 Major Contributors

• Fossil fuel emissions
• Deforestation
• Eutrophication:
  • Plankton blooms → decomposition → CO₂ increase

4. Effects of Ocean Acidification

4.1 On Marine Chemistry

• Decrease in carbonate ions (CO₃²⁻)
• Difficulty in forming:
  • Calcium carbonate (CaCO₃)

4.2 On Marine Life

• Affects calcifying organisms:
  • Corals
  • Molluscs
  • Calcareous plankton
• Leads to:
  • Coral bleaching
  • Weak shells
  • Reduced growth

4.3 On Fisheries

• Damage to the base of the food web
• Threatens:
  • Arctic fisheries
  • Commercial fisheries

4.4 On Climate Regulation

• Oceans absorb ~1/3rd anthropogenic CO₂
• Acidification reduces buffering capacity

5. Artificial Cloud Seeding

• Technique to induce rainfall
• Uses:
  • Silver iodide
  • Dry ice
• Encourages ice crystal formation in clouds