Which of the following is a secondary pollutant?

Options

A. ` SO_2`
B. ` CO_2`
C. ` CO`
D. ` O_3`

Answer

To determine which of the following is a secondary pollutant, we need to understand the difference between primary and secondary pollutants.

### Step-by-Step Solution:

1. **Define Primary Pollutants**:
- Primary pollutants are substances that are directly emitted into the atmosphere from a source. Examples include sulfur dioxide (SO2), carbon dioxide (CO2), and carbon monoxide (CO). These pollutants are produced from processes such as combustion.

2. **Define Secondary Pollutants**:
- Secondary pollutants are not emitted directly. Instead, they form in the atmosphere through chemical reactions between primary pollutants and other substances. These reactions can occur in the presence of sunlight, heat, or other environmental conditions.

3. **Identify the Options**:
- The options provided are SO2, CO2, CO, and ozone (O3).

4. **Analyze Each Option**:
- **SO2**: This is a primary pollutant as it is directly emitted from combustion processes.
- **CO2**: This is also a primary pollutant, produced from the burning of fossil fuels.
- **CO**: This is another primary pollutant, resulting from incomplete combustion of carbon-containing fuels.
- **Ozone (O3)**: Ozone is formed when primary pollutants like volatile organic compounds (VOCs) and nitrogen oxides (NOx) react in the presence of sunlight. Therefore, ozone is classified as a secondary pollutant.

5. **Conclusion**:
- Among the given options, ozone is the only secondary pollutant. Thus, the correct answer is ozone.

### Final Answer:
The secondary pollutant among the options is **ozone (O3)**.