Previous Year Question

2. Nitrogen oxide is a cause of acid rain.

• Nitrogen oxides are collectively called NOₓ and mainly include nitric oxide and nitrogen dioxide.

• Their major anthropogenic sources are the high-temperature combustion of fossil fuels in vehicles, thermal power plants and industrial units.

• In the atmosphere, nitric oxide may undergo oxidation to form nitrogen dioxide.

• Nitrogen dioxide reacts with water, oxygen and other oxidising agents to form nitric acid.

• Nitric acid can reach the Earth’s surface through rain, snow, fog or dry particles.

• Therefore, nitrogen oxides are major precursors of acidic deposition.

4. Sulphur dioxide is a cause of acid rain.

• Sulphur dioxide is emitted primarily from the combustion of sulphur-containing fossil fuels such as coal and oil.

• Thermal power plants, metal smelting and certain industrial processes are its major anthropogenic sources.

• Volcanic activity can be a natural source of sulphur dioxide.

• In the atmosphere, SO₂ may undergo oxidation to form sulphur trioxide and sulphate compounds.

• These compounds react with water to form sulphuric acid.

• Sulphuric acid reaches the Earth’s surface through rainfall or other forms of deposition.

• Therefore, sulphur dioxide is a major precursor of acid rain.

1. Carbon monoxide is not a major cause of acid rain.

• Carbon monoxide is a colourless and odourless gas produced by incomplete combustion.

• It can bind with haemoglobin in the blood and reduce the oxygen-carrying capacity of the body.

• It may indirectly influence atmospheric chemical reactions and the formation of ground-level ozone.

• However, it is not a major precursor that forms sulphuric acid or nitric acid.

• Therefore, it is not regarded as a principal cause of acid rain.

3. Ozone is not a major cause of acid rain.

• Ground-level ozone is formed through photochemical reactions involving nitrogen oxides and volatile organic compounds in sunlight.

• It is a secondary pollutant and a powerful oxidising agent.

• It can damage the respiratory system, crops and vegetation.

• However, ozone itself is not a major acidic precursor responsible for producing acid rain.

• Therefore, it is not included among the principal causes of acid rain.

Important facts concerning acid rain

• Normal rainwater is slightly acidic because of weak carbonic acid formed from dissolved carbon dioxide.

• The pH of normal rainwater is considered to be approximately 5.6.

• Precipitation having a pH below 5.6 is generally called acid precipitation.

• Acidic deposition can occur in the forms of both wet deposition and dry deposition.

• Wet deposition includes acidic rain, snow, hail and fog.

• Dry deposition consists of acidic gases and particles deposited on surfaces without precipitation.

• Acidic deposition can lower the pH of lakes and rivers, increase the leaching of nutrients from soil and make metals such as aluminium more soluble.

• It can damage aquatic organisms, forests, crops, buildings and monuments made of limestone or marble.

• SO₂ and NOₓ can travel long distances through the atmosphere; therefore, acid rain can also become a transboundary environmental problem.