Which is always present in photochemical smog ?

Options

A. Ozone
B. `CO_(2)`
C. `SO_(2)`
D. `CH_(4)`

Answer

To answer the question "Which is always present in photochemical smog?", we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Types of Smog**:
- There are two main types of smog: classical smog and photochemical smog.
- Classical smog is primarily associated with sulfur dioxide (SO2) and is typically found in industrial areas.

2. **Identify the Components of Photochemical Smog**:
- Photochemical smog is formed when sunlight reacts with pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs), which include hydrocarbons.
- One of the key components of photochemical smog is ozone (O3), which is formed during these reactions.

3. **Analyze the Options Given**:
- Option 1: Ozone (O3) - This is a significant component of photochemical smog.
- Option 2: Carbon Dioxide (CO2) - Not a primary component of photochemical smog.
- Option 3: Sulfur Dioxide (SO2) - Related to classical smog, not photochemical.
- Option 4: Methane (CH4) - A hydrocarbon that can contribute to the formation of photochemical smog but is not always present.

4. **Determine the Correct Answer**:
- Since ozone is a direct product of the reactions that occur in photochemical smog and is always present in this type of smog, the correct answer is **Option 1: Ozone (O3)**.

### Final Answer:
The component that is always present in photochemical smog is **Ozone (O3)**.

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