Question

Which of the following is a secondary air pollutant?

• A. Ozone
• B. Carbon dioxide
• C. Carbon monoxide
• D. Sulphur dioxide

Hint:

Remember that secondary pollutants are formed through chemical reactions in the air. Ground-level ozone, PAN (Peroxyacetyl nitrate), and acid rain components are common examples of secondary pollutants.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Air pollutants are classified into two primary categories based on how they enter the atmosphere:
1. Primary pollutants: Emitted directly from a source, such as volcanic eruptions or industrial chimneys.
2. Secondary pollutants: Formed in the atmosphere through chemical reactions between primary pollutants and other environmental factors, such as sunlight and moisture.

Step 2: Detailed Explanation:
Let us analyze the pollutants listed in the options:
- Carbon dioxide (\( \text{CO}_2 \)):
Emitted directly from cellular respiration, fossil fuel combustion, and volcanic activity (Primary pollutant).
- Carbon monoxide (\( \text{CO} \)):
Emitted directly from the incomplete combustion of fossil fuels (Primary pollutant).
- Sulphur dioxide (\( \text{SO}_2 \)):
Emitted directly from volcanic eruptions and coal-burning power plants (Primary pollutant).
- Ozone (\( \text{O}_3 \)):
Ground-level ozone is not emitted directly from any source.
Instead, it is formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight.
These chemical reactions make ozone a classic example of a secondary air pollutant.
While stratospheric ozone protects the earth from UV radiation, tropospheric (ground-level) ozone is a harmful secondary pollutant and a major component of photochemical smog.

Step 3: Final Answer:
Ozone is a secondary air pollutant formed through photochemical reactions in the atmosphere.
Thus, the correct choice is (A).