Carbon monoxide
Explanation
Acid rain is a phenomenon where precipitation becomes unusually acidic (pH less than 5.6) due to the presence of specific atmospheric pollutants. These pollutants react with water, oxygen, and other chemicals in the atmosphere to form strong acids, primarily sulfuric acid and nitric acid.
Option Analysis:
- Sulphur dioxide Sulphur dioxide ($SO_2$): This is a primary contributor. It is released from burning fossil fuels (coal and oil) and metal smelting. In the atmosphere, it oxidizes and reacts with water to form sulfuric acid ($H_2SO_4$), the main component of acid rain.
- Nitrogen dioxide Nitrogen dioxide ($NO_2$): This is a primary contributor. Emitted from vehicle exhaust and power plants, nitrogen oxides react with water to form nitric acid ($HNO_3$), significantly lowering the pH of rainwater.
- Carbon dioxide Carbon dioxide ($CO_2$): While $CO_2$ reacts with water to form weak carbonic acid ($H_2CO_3$), this process is responsible for the natural acidity of rain (pH ~5.6). It is generally not classified as a pollutant driving the "Acid Rain" phenomenon, which implies excess anthropogenic acidity. However, chemically, it is still an acidic oxide.
- Carbon monoxide Carbon monoxide ($CO$): This is the correct answer (it is NOT a contributor). Carbon monoxide is a neutral oxide. It is insoluble in water and does not react to form an acid. While it is a toxic air pollutant, it does not directly contribute to the acidification of rainwater.
Key Takeaway:
Acid rain is primarily caused by Sulphur Dioxide ($SO_2$) and Nitrogen Oxides ($NO_x$), which form strong acids. Carbon Monoxide ($CO$) is a neutral oxide and does not contribute to acid rain.