Consider the following:

1. Carbon monoxide

2. Nitrogen oxide

3. Ozone

4. Sulphur dioxide

Excess of which of the above in the environment is/are cause(s) of acid rain?

The correct option is 2 and 4 only.

Acid rain is a type of precipitation that is unusually acidic, possessing a pH level lower than that of normal rainwater (which is slightly acidic with a pH of about 5.6 due to dissolved carbon dioxide). The primary causes of acid rain are the atmospheric emissions of sulfur dioxide (SO₂) and nitrogen oxides (NO_x).

When these gases are emitted into the atmosphere, they undergo chemical reactions with water vapor, oxygen, and other substances to form acidic compounds. Specifically, sulfur dioxide is oxidized and reacts with water to form sulfuric acid (H₂SO₄) according to the following reactions:

$$2{\mathrm{SO}}_2+O_2\to 2{\mathrm{SO}}_3$$

$${\mathrm{SO}}_3+H_{2}O\to H_2{\mathrm{SO}}_4$$

Similarly, nitrogen oxides react with water and oxygen in the air to produce nitric acid (HNO₃):

$$4{\mathrm{NO}}_2+O_2+2H_{2}O\to 4{\mathrm{HNO}}_3$$

These acids then dissolve in cloud water droplets and fall to the ground as rain, snow, or fog, significantly lowering the pH of the precipitation.

In contrast, Carbon monoxide (1) and Ozone (3) do not contribute directly to the formation of acid rain:
• Carbon monoxide (CO) is a toxic air pollutant and a greenhouse gas, but it does not react with water in the atmosphere to form a strong acid.
• Ozone (O₃) in the troposphere is a secondary pollutant and a major component of photochemical smog, but it does not directly yield acidic solutions when dissolved in water.