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 of identifying the odd one out with respect to primary air pollutants, we can follow these steps:

### Step 1: Understand the Definition of Primary Air Pollutants
Primary air pollutants are substances that are directly emitted into the atmosphere from a source. They do not undergo any chemical transformation before being released.

### Step 2: Identify the Options Given
The options provided are:
- Option A: Carbon Dioxide (CO2)
- Option B: Nitrogenous Oxides (NOx)
- Option C: Pollen
- Option D: PAN (Peroxyacetyl Nitrate)

### Step 3: Classify Each Option
- **Carbon Dioxide (CO2)**: This is a primary air pollutant as it is directly emitted from sources like vehicles and industrial processes.
- **Nitrogenous Oxides (NOx)**: These are also primary air pollutants, produced from combustion processes in vehicles and power plants.
- **Pollen**: Pollen grains are considered primary air pollutants as they are naturally occurring and released directly into the air.
- **PAN (Peroxyacetyl Nitrate)**: This is a secondary air pollutant, formed through chemical reactions in the atmosphere, particularly involving nitrogen oxides and volatile organic compounds.

### Step 4: Identify the Odd One Out
Since PAN is a secondary air pollutant while the others (CO2, NOx, and pollen) are primary air pollutants, PAN is the odd one out.

### Step 5: Conclusion
The answer to the question is Option D: PAN (Peroxyacetyl Nitrate).

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