Which of the following is an example of a secondary air pollutant, formed in the atmosphere as a result of chemical reactions involving primary pollutants?

Introduction / Context:
Air pollution can be divided into primary and secondary pollutants. Primary pollutants are emitted directly from sources such as factories, vehicles, and power plants. Secondary pollutants, on the other hand, are not emitted directly but are formed in the atmosphere through chemical reactions involving primary pollutants. This question asks you to identify which example among the options is a secondary air pollutant, which is an important distinction in environmental science.

Given Data / Assumptions:

• Primary pollutants include gases and particulate matter emitted directly into the air.
• Secondary pollutants form in the atmosphere from reactions between primary pollutants and natural components such as water vapour.
• The options given are acid rain, sulfur dioxide (SO2), carbon dioxide (CO2), and dust particles.
• We must identify which option is clearly secondary in nature.

Concept / Approach:
Sulfur dioxide and nitrogen oxides are common primary pollutants emitted from burning fossil fuels. When these gases react with water vapour and other chemicals in the atmosphere, they can form acids such as sulfuric acid and nitric acid. These acids then fall to the ground as acid rain. Because acid rain is not directly emitted but produced through atmospheric reactions, it is classified as a secondary pollutant. In contrast, SO2, CO2, and dust are all emitted directly and are therefore primary pollutants.

Step-by-Step Solution:
Step 1: Recall the definition of a secondary pollutant: it is formed in the atmosphere by chemical reactions involving primary pollutants. Step 2: Recognise that acid rain results from sulfur dioxide and nitrogen oxides reacting with water vapour to form acids. Step 3: Note that sulfur dioxide itself is directly emitted from factories and power plants, so it is a primary pollutant. Step 4: Remember that carbon dioxide is released directly from respiration, combustion, and industrial activity, and dust particles are directly suspended in air. Step 5: Conclude that acid rain is the only option that is clearly a secondary pollutant.

Verification / Alternative check:
To verify, think of how each substance appears in the environment. Carbon dioxide is exhaled by organisms and produced by burning fuels, so it enters the atmosphere as is. Dust is lifted into the air from soil, roads, and industrial processes. Sulfur dioxide comes from chimneys and exhausts. Acid rain, however, does not leave chimneys directly as rain; it forms after gases have reacted in clouds. This difference confirms its classification as a secondary pollutant.

Why Other Options Are Wrong:
Sulfur dioxide (SO2) is a primary pollutant because it is released directly from combustion of coal and oil. Carbon dioxide (CO2) is also a primary emission from various natural and human activities. Dust particles represent particulate matter that is physically injected into the air rather than being formed by chemical reactions after emission. None of these three fit the definition of a secondary pollutant.

Common Pitfalls:
A typical mistake is to confuse the seriousness of a pollutant with whether it is primary or secondary. Students may think that because sulfur dioxide has strong environmental impacts, it must be secondary. Another pitfall is to assume that any pollutant associated with clouds or the atmosphere is secondary. The key is not severity but whether it is chemically transformed in the air before causing harm.

Final Answer:
The correct example of a secondary air pollutant is acid rain.