Acid Rain

Acid rain, or acid deposition, refers to any form of precipitation—rain, snow, fog, or even dry particles—that contains higher than normal amounts of nitric and sulfuric acids. While normal rainwater is slightly acidic with a pH of approximately 5.6 (due to dissolved CO₂ forming weak carbonic acid), acid rain typically has a pH between 4.2 and 4.4.

Chemical Formation and Pathways

Acid rain is a secondary pollutant phenomenon caused by the atmospheric transformation of primary emissions.

• Primary Precursors: The two main precursors are Sulphur Dioxide (SO₂) and Nitrogen Oxides (NO_x).
• Sources: Natural sources include volcanoes and decaying vegetation; however, the majority is caused by human activity, specifically the burning of fossil fuels in thermal power plants and vehicular combustion.
• Chemical Reactions:
  • SO₂ + OH · → HSO₃
  • HSO₃ + O₂ → HO₂ + SO₃
  • SO₃ + H₂O → H₂SO₄ (Sulfuric Acid)
  • NO₂ + OH · → HNO₃ (Nitric Acid)

Forms of Acid Deposition

Acidic pollutants can reach the Earth’s surface in two distinct ways:

1. Wet Deposition: Acidic chemicals are incorporated into rain, snow, fog, or mist and fall to the ground.
2. Dry Deposition: In areas of low humidity, acidic gases and particles may stick to the ground, buildings, vegetation, or cars. These can be washed away by rainstorms, leading to highly acidic runoff.

Environmental and Economic Impacts

Acid rain acts as a systemic stressor that affects both biological and physical environments.

Impact on Aquatic Ecosystems

• Aluminum Leaching: Acid rain leaches aluminum from soil into water bodies, which is highly toxic to fish.
• Biodiversity Loss: Most fish eggs cannot hatch at pH levels below 5. Many insects and aquatic plants (the base of the food chain) perish, leading to ecosystem collapse.

Impact on Forests and Soil

• Nutrient Depletion: It washes away essential nutrients like Calcium, Magnesium, and Potassium from the soil before plants can use them.
• Direct Damage: Acid fog and clouds at high altitudes strip nutrients from tree foliage, leaving them brown and unable to photosynthesize.

Impact on Architecture and Heritage

• Marble Cancer: This refers to the corrosion of monuments made of marble or limestone (CaCO₃).
• The Taj Mahal Case: Emissions from the Mathura Refinery and local industries led to the formation of H₂SO₄, which reacted with the Taj’s marble to form Calcium Sulphate (gypsum), causing yellowing and pitting.
  • Equation: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

Monitoring and Control Strategies

Reducing the precursors of acid rain requires both technological and policy-level interventions.

Global and Regional Context

• The 1979 Geneva Convention: The first international legally binding instrument to deal with problems of air pollution on a broad regional basis (Long-range Transboundary Air Pollution).
• Acid Rain in India: While India’s soil is generally alkaline (which helps neutralize acidity), the rapid increase in coal-based power generation has made regions like the Indo-Gangetic Plain and parts of Maharashtra vulnerable to episodic acidification.

Trivia and Key Facts for UPSC

• The pH Scale: Remember that the pH scale is logarithmic. A pH of 4 is ten times more acidic than a pH of 5 and a hundred times more acidic than a pH of 6.
• Bio-indicators: Like lichens for general air pollution, certain aquatic mosses and the disappearance of mayflies in streams serve as early indicators of water acidification.
• Transboundary Nature: Pollutants causing acid rain can travel thousands of kilometers, meaning the “polluter” and the “victim” are often in different countries (a major issue in Europe and North America).