Acid Rain: Identifying Primary Pollutants

Acid rain is a type of precipitation—such as rain, snow, fog, or hail—that contains higher than normal amounts of nitric and sulfuric acids. Normal rain is slightly acidic, with a pH of about 5.6, due to carbon dioxide ($CO_2$) dissolving in it to form weak carbonic acid. However, when pollutants like sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$) are released into the atmosphere, they react with water, oxygen, and other chemicals to form more acidic compounds.

Role of Sulphur Dioxide and Nitrogen Oxides

The main pollutants responsible for causing acid rain are:

• Sulphur dioxide ($SO_2$): This gas is released primarily from the burning of fossil fuels (like coal and oil) in power plants and industrial facilities. It can also come from natural sources like volcanoes.
• Nitrogen oxides ($NO_x$): These gases are emitted mainly from vehicle exhaust, power plants, and other combustion processes.

Once in the atmosphere, these gases undergo chemical reactions:

• Sulphur dioxide reacts with oxygen and water to form sulfuric acid ($H_2SO_4$). The process can be summarized as:
  • $SO_2 + O_2 \rightarrow SO_3$
  • $SO_3 + H_2O \rightarrow H_2SO_4$
• Nitrogen oxides react in the atmosphere to form nitric acid ($HNO_3$).

These acids then mix with water vapor and fall to the Earth as acid rain, snow, fog, or dry particles.

Analysis of Other Pollutants

Let's look at why the other options are not the primary causes of acid rain:

• Carbon monoxide ($CO$): While carbon monoxide is a harmful air pollutant resulting from incomplete combustion, it does not directly contribute to the formation of acid rain. It is not acidic and does not readily form acids in the atmosphere.
• Methane ($CH_4$): Methane is a significant greenhouse gas that contributes to climate change. However, it does not play a primary role in the chemical processes that lead to acid rain.
• Ozone ($O_3$): Ground-level ozone is a major component of smog and is formed when nitrogen oxides ($NO_x$) and volatile organic compounds (VOCs) react in sunlight. While $NO_x$ is involved, ozone itself is not the primary acidifying agent in acid rain. It's a secondary pollutant with different environmental impacts.

Conclusion

Based on the chemical processes involved, Sulphur dioxide and Nitrogen oxides are the principal pollutants released into the atmosphere that lead to the formation of acid rain.