Which of the following is a secondary air pollutant?

Options

A. PAN
B. CO
C. `NO_2`
D. `SO_2`

Answer

To determine which of the following is a secondary air pollutant, we need to understand the difference between primary and secondary air pollutants.

### Step-by-Step Solution:

1. **Define Primary and Secondary Air Pollutants**:
- Primary air pollutants are substances that are directly emitted from a source into the atmosphere. Examples include carbon monoxide (CO), nitrogen dioxide (NO2), and sulfur dioxide (SO2).
- Secondary air pollutants are not emitted directly. Instead, they form in the atmosphere through chemical reactions involving primary pollutants.

2. **Identify the Options**:
- We need to evaluate the given options to identify which one is a secondary air pollutant. The options are:
- Option 1: PAN (Peroxyacyl Nitrate)
- Option 2: Carbon Monoxide (CO)
- Option 3: Nitrogen Dioxide (NO2)
- Option 4: Sulfur Dioxide (SO2)

3. **Analyze Each Option**:
- **Option 1: PAN (Peroxyacyl Nitrate)**: This is formed through chemical reactions involving primary pollutants, specifically in photochemical smog. Therefore, it is a secondary air pollutant.
- **Option 2: Carbon Monoxide (CO)**: This is a primary pollutant as it is emitted directly from sources like vehicle exhaust.
- **Option 3: Nitrogen Dioxide (NO2)**: This is also a primary pollutant, produced from combustion processes.
- **Option 4: Sulfur Dioxide (SO2)**: This is another primary pollutant, primarily released from burning fossil fuels.

4. **Conclusion**:
- The only option that qualifies as a secondary air pollutant is **Option 1: PAN (Peroxyacyl Nitrate)**.

### Final Answer:
**PAN (Peroxyacyl Nitrate) is a secondary air pollutant.**

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