The pollutants which come directly in the air from sources are called primary pollutes. Primary polluants are sometimes converted into secondary pollutants. Which of the following belongs to secondary air pollutants ?

Options

A. `CO`
B. Hydrocarbon
C. Peroxyacetyl nitrate
D. NO

Explanation

To determine which of the given options belongs to secondary air pollutants, we first need to understand the difference between primary and secondary pollutants.

### Step-by-Step Solution:

1. **Definition of Pollutants**:
- **Primary Pollutants**: These are pollutants that are directly emitted into the atmosphere from sources such as vehicles, industries, and natural sources. Examples include carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen oxides (NOx).
- **Secondary Pollutants**: These are not emitted directly. Instead, they form in the atmosphere through chemical reactions between primary pollutants and other atmospheric components.

2. **Formation of Secondary Pollutants**:
- Primary pollutants, such as hydrocarbons, can react with oxygen (O2) and nitrogen dioxide (NO2) in the presence of sunlight (photolysis).
- During these reactions, primary pollutants are converted into highly reactive intermediates, known as free radicals.

3. **Example of Secondary Pollutant Formation**:
- For instance, hydrocarbons (R) can react with oxygen to form a free radical (RCO).
- This free radical can further react with oxygen to form another intermediate (RCO3).
- The intermediate (RCO3) can then react with nitrogen dioxide (NO2) to produce peroxyacetyl nitrate (RCO3NO3).

4. **Identifying the Secondary Pollutant**:
- Peroxyacetyl nitrate (RCO3NO3) is a well-known secondary pollutant formed from the reactions of primary pollutants.
- Therefore, if the options provided include peroxyacetyl nitrate, it would be the correct answer as it is classified as a secondary pollutant.

5. **Conclusion**:
- Based on the above analysis, the answer to the question is that peroxyacetyl nitrate is a secondary air pollutant.

### Final Answer:
The secondary air pollutant among the options is **peroxyacetyl nitrate**.

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