The photochemical smog is essentially caused by presence by

Options

A. `O_(2) and O_(3)`
B. Oxides of nitrogen and hydrocarbons
C. Oxides of sulphur and nitrogen
D. `O_(2) and N_(2)`.

Answer

**Step-by-Step Solution:**

1. **Understanding Photochemical Smog**: Photochemical smog is a type of air pollution that is formed when sunlight reacts with pollutants such as volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the atmosphere. This reaction leads to the formation of secondary pollutants, including ozone and peroxyacyl nitrate (PAN).

2. **Components of Photochemical Smog**: The main components of photochemical smog include:
- Ozone (O3)
- Nitrogen oxides (NO and NO2)
- Hydrocarbons (especially unsaturated hydrocarbons)
- Peroxyacyl nitrate (PAN, with the formula RCO3NO2)

3. **Formation Process**:
- Unsaturated hydrocarbons react with ozone in the presence of sunlight, leading to the formation of aldehydes and ketones.
- These aldehydes and ketones can further react with nitrogen dioxide (NO2) to form peroxyacyl nitrate (PAN).

4. **Identifying the Cause**: The question asks what causes photochemical smog. The key components that lead to its formation are:
- Nitrogen oxides (NOx), particularly NO2
- Hydrocarbons, especially unsaturated hydrocarbons

5. **Evaluating Options**:
- Option A: O2 (Oxygen) is not a primary cause of photochemical smog.
- Option B: Oxides of nitrogen (NOx) and hydrocarbons are indeed the primary contributors to photochemical smog.
- Option C: Oxides of sulfur are not a significant factor in the formation of photochemical smog.
- Option D: N2 and O2 are not responsible for photochemical smog.

6. **Conclusion**: The correct answer is Option B, which states that photochemical smog is essentially caused by the presence of nitrogen oxides and hydrocarbons.