Previous Year Question

Acid Rain is a form of precipitation in which atmospheric pollution increases the acidity of rainwater through the presence of Sulfuric Acid (H₂SO₄) and Nitric Acid (HNO₃). Normal rainwater has a pH of approximately 5.6 because atmospheric Carbon Dioxide (CO₂) dissolves in water and forms weak Carbonic Acid (H₂CO₃). When the pH of rainwater decreases significantly below its normal natural level, it is referred to as Acid Rain. The principal chemical precursors of Acid Rain are Sulfur Dioxide (SO₂) and Nitrogen Oxides (NOₓ).

Sulfur Dioxide (SO₂) is emitted mainly through the combustion of sulfur-containing fossil fuels such as coal and petroleum, Thermal Power Plants, and Metal Smelting. In the atmosphere, SO₂ contributes to the formation of Sulfuric Acid (H₂SO₄) through Oxidation and other chemical reactions. Similarly, Nitrogen Oxides (NOₓ), primarily including Nitric Oxide (NO) and Nitrogen Dioxide (NO₂), are emitted from vehicles, industrial plants, and high-temperature fuel combustion. Atmospheric reactions involving these gases can produce Nitric Acid (HNO₃). These acids reach the Earth's surface through rainfall, snowfall, and fog, a process known as Wet Deposition.

The scientific distinction between Nitrous Oxide (N₂O) and Nitrogen Oxides (NOₓ) is important. N₂O is a powerful Greenhouse Gas emitted mainly from agricultural activities and microbial processes in soils. In contrast, NO and NO₂ are the nitrogen oxides directly involved in the formation of Acid Rain.

Acid Rain affects Soil, Vegetation, Aquatic Ecosystems, and Historical Monuments. It can increase the Leaching of essential soil nutrients such as Calcium and Magnesium and enhance aluminium solubility, causing damage to plants and aquatic organisms. Sulfuric and Nitric Acids can react with Calcium Carbonate (CaCO₃) present in limestone and marble, accelerating the deterioration of stone surfaces. Among the other options, Carbon Dioxide contributes to the slight natural acidity of normal rain, whereas Carbon Monoxide (CO) and Methane (CH₄) are not major direct precursors of Acid Rain. Reducing SO₂ and NOₓ emissions is essential for controlling acid rain.