Acid Rain: Key Gas Precursors Explained

Acid rain is a type of precipitation—like rain, snow, or fog—that contains higher than normal amounts of nitric and sulfuric acids. It forms when certain gases released into the atmosphere react with water, oxygen, and other chemicals.

Role of Sulphur Dioxide ($SO_2$) in Acid Rain

The primary gases responsible for causing acid rain are sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$). These gases are released into the atmosphere mainly from burning fossil fuels (like coal, oil, and natural gas) in power plants, factories, and vehicles. They can also come from natural sources like volcanoes.

Once in the atmosphere, these gases undergo chemical reactions:

• Sulfur dioxide ($SO_2$) reacts with water and oxygen to form sulfuric acid ($H_2SO_4$).
• Nitrogen oxides ($NO_x$) react with water and oxygen 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.

Why Other Gases Aren't Primary Precursors

Let's look at the other options provided:

• Chlorofluorocarbons (CFCs): These are synthetic chemicals primarily known for depleting the ozone layer in the stratosphere. They do not directly contribute to the formation of acid rain.
• Methane ($CH_4$): Methane is a potent greenhouse gas that contributes to climate change. While it plays a role in atmospheric chemistry, it is not considered a principal precursor to acid rain.
• Ozone ($O_3$): Ozone is a gas that exists in the Earth's stratosphere, protecting us from harmful UV radiation. However, at ground level, ozone is a pollutant formed from reactions involving sunlight and other pollutants (like $NO_x$ and volatile organic compounds). While related to air quality issues, ground-level ozone itself is not a precursor *to* acid rain in the same way $SO_2$ and $NO_x$ are.
• Sulphur dioxide ($SO_2$): As discussed above, $SO_2$ is a major component released from burning fossil fuels, and it is essential for forming sulfuric acid, a key component of acid rain.

Identifying the Principal Precursor

The question asks for the gas that, acting *together with nitrogen oxides*, is a principal precursor of acid rain. Based on the chemistry of acid rain formation, sulphur dioxide ($SO_2$) fits this description perfectly.

Therefore, sulphur dioxide ($SO_2$) is the correct answer.