Choose the odd w.r.t primary air pollutants.

Options

A. `CO_2`
B. `NO_x`
C. Pollen
D. PAN

Answer

To solve the question "Choose the odd with respect to primary air pollutants," we will identify which of the given options does not belong to the category of primary air pollutants.

### Step-by-Step Solution:

1. **Understand the Types of Air Pollutants**:
- There are two main types of air pollutants: primary and secondary.
- Primary air pollutants are those that are emitted directly into the atmosphere from a source.

2. **List of Primary Air Pollutants**:
- Common examples of primary air pollutants include:
- Suspended Particulate Matter (SPM)
- Carbon Monoxide (CO)
- Carbon Dioxide (CO2)
- Hydrogen Sulfide (H2S)
- Sulfur Dioxide (SO2)
- Nitrogen Oxides (NOx)
- Fluorides
- Pollens and microbes

3. **Identify the Options**:
- The options provided are:
1. Carbon Dioxide (CO2) - Primary pollutant
2. Nitrogen Dioxide (NO2) - Primary pollutant
3. Pollen - Primary pollutant
4. Peroxyacyl Nitrate (PAN) - Secondary pollutant

4. **Determine the Odd One Out**:
- Among the options, Carbon Dioxide, Nitrogen Dioxide, and Pollen are all classified as primary air pollutants.
- Peroxyacyl Nitrate (PAN) is classified as a secondary pollutant.

5. **Conclusion**:
- Therefore, the odd one out with respect to primary air pollutants is **Peroxyacyl Nitrate (PAN)**.

### Final Answer:
The correct answer is **Option 4: Peroxyacyl Nitrate (PAN)**.

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