Which of the following compound is not a photochemical smog?

Options

A. `NO`
B. `NO_2`
C. `SO_2`
D. PAN

Answer

To determine which compound is not a part of photochemical smog, we need to understand what photochemical smog is and its components.

### Step-by-Step Solution:

1. **Definition of Photochemical Smog**:
Photochemical smog is a type of air pollution that occurs when sunlight reacts with pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). This reaction leads to the formation of secondary pollutants.

2. **Key Components of Photochemical Smog**:
- **Nitrogen Oxides (NOx)**: These are primary pollutants that contribute to the formation of photochemical smog.
- **Volatile Organic Compounds (VOCs)**: These compounds also play a significant role in the formation of photochemical smog.
- **Secondary Pollutants**: The primary products of the reactions between NOx and VOCs in the presence of sunlight are ozone (O3) and peroxyacyl nitrate (PAN).

3. **Identifying the Compounds**:
- **Ozone (O3)**: A key component of photochemical smog.
- **Peroxyacyl Nitrate (PAN)**: Another important secondary pollutant formed in photochemical smog.
- **Sulfur Dioxide (SO2)**: This is primarily associated with industrial pollution and is not a component of photochemical smog.

4. **Conclusion**:
Since sulfur dioxide (SO2) is not formed from the reactions of nitrogen oxides and volatile organic compounds in the presence of sunlight, it is not a part of photochemical smog. Therefore, the compound that is not a photochemical smog is **SO2**.

### Final Answer:
The compound that is not a photochemical smog is **Sulfur Dioxide (SO2)**.