Q. Acid rain is caused by the pollution of environment by: (UPSC Prelims 2013)
Answer: nitrous oxide and sulphur dioxide
Notes: The correct answer is [D] nitrous oxide and sulphur dioxide. Acid rain refers to any form of precipitation—rain, snow, fog, or hail—that contains high levels of nitric and sulfuric acids. It is a significant environmental issue primarily caused by the release of specific air pollutants.

• Sulphur Dioxide (SO2): This gas is primarily released into the atmosphere from the burning of fossil fuels in power plants and industrial facilities. When SO2 reacts with water, oxygen, and other chemicals in the atmosphere, it forms sulfuric acid (H2SO4).
• Nitrogen Oxides (NOx): These are released from the exhaust of vehicles, power plants, and off-road equipment. Nitrous oxide and other nitrogen oxides react in the air to form nitric acid (HNO3).
• Chemical Reaction: The formation of acid rain can be simplified as follows:
  • SO2 + H2O → H2SO4 (Sulfuric Acid)
  • NOx + H2O → HNO3 (Nitric Acid)
• Normal Rain vs. Acid Rain: Normal rainwater is slightly acidic with a pH of about 5.6 because carbon dioxide (CO2) dissolves into it to form weak carbonic acid. However, acid rain typically has a pH between 4.2 and 4.4, which is harmful to the environment.
• Impact of Acid Rain:
  • Aquatic Ecosystems: It makes waters acidic and causes them to absorb aluminum from the soil, which is toxic to fish and other aquatic animals.
  • Forests: It strips nutrients from the soil and damages the leaves of trees, making them vulnerable to diseases and extreme weather.
  • Architecture: It causes the corrosion of metals and the deterioration of paint and stone (especially limestone and marble), famously leading to the "marble cancer" of the Taj Mahal.

Why other options are incorrect: While Carbon Dioxide (Option A, B, and C) contributes to the very slight natural acidity of rain, it does not cause "acid rain" in the ecological sense. Carbon monoxide and Ozone are harmful pollutants, but they are not the primary precursors for the acidic compounds that fall as precipitation.