Acid Rain
Definition:
Rainfall with a pH < 5.6, formed when oxides of sulphur and nitrogen react with atmospheric moisture. Damages ecosystems, materials, and human health.
Types of Acid Deposition:
- Wet Deposition: Acid chemicals fall with rain, snow, fog, or mist, impacting soil and water ecosystems.
- Dry Deposition: Acid particles/gases settle on surfaces in dry weather, later washed off into waterways.
Difference Between Normally Acidic and Anthropogenically Acidified Lakes
Major Sources:
- Sulphur: Oceans, volcanoes, organic decomposition, burning coal & petroleum, metal smelting.
- Nitrogen: Lightning, volcanic activity, combustion of fossil fuels.
- Formic Acid & Others: Forest fires, formaldehyde oxidation, chlorine, phosphoric acid, CO & CO₂ emissions.
pH Scale:
- pH 7 = neutral.
- <7 = acidic.
- Each unit drop = 10× increase in acidity.
Geographic Occurrence
- Industrialized Northern Hemisphere (Scandinavia, US Great Lakes, Northern Europe).
- In India: reported in Mumbai (1974) & other metros. Notable in NE India, Karnataka coast, Odisha, Bengal, Bihar.
Chemistry of Acid Rain
- SOx & NOx emitted.
- Some fall as dry deposition.
- Sunlight forms photo-oxidants.
- Oxidants convert SOx & NOx to acids.
- Acids deposited as wet deposition.
Impacts of Acid Rain
- Soil: Nutrient leaching, reduced fertility, impaired microbial activity.
- Vegetation: Reduced growth, foliage loss, premature aging, tree death.
- Microorganisms: Shift from bacteria-dominated to fungi-dominated systems.
- Wildlife: Egg/tadpole deformities, toxic metal ingestion, habitat loss.
- Humans: Respiratory diseases, skin irritation, heavy metal poisoning.
- Materials: Corrosion, erosion of buildings, metals, textiles, and leather.
Acid Rain Damage on Materials
| Material | Type of Impact | Principal Air Pollutants |
| Metals | · Corrosion, tarnishing | · Sulphur oxides and other acid gases |
| Building stone | · Surface erosion, soiling, black crust formation | · Sulphur oxides and other acid gases |
| Ceramics and glass | · Surface erosion, surface crust formation | · Acid gases, especially fluoride-containing |
| Paints and organic coatings | · Surface erosion, discolouration, soiling | · Sulphur dioxides, hydrogen sulphide |
| Paper | · Embrittlement, discolouration | · Sulphur oxides |
| Photographic materials | · Micro-blemishes | · Sulphur oxides |
| Textiles | · Fading, colour change | · Nitrogen oxides, ozone |
| Leather | · Weakening, powdered surface | · Sulphur oxides |
| Rubber | · Cracking | · Ozone |
Control Measures
- Buffering acidified water with lime.
- Switching to low-sulphur fuels.
- Emission reduction via desulphurization.
- Renewable energy adoption.
- Engine and industrial process modifications.
Categorization of Industrial Sectors
The MoEFCC classifies industries based on Pollution Index (PI):
This scientific re-categorization aligns pollution levels with regulatory requirements, ensuring ecologically fragile areas remain protected. |