What are secondary pollutants?

Explanation

### Step-by-Step Solution

1. **Definition of Secondary Pollutants**:
Secondary pollutants are not directly emitted from a source. Instead, they are formed in the atmosphere through chemical reactions involving primary pollutants and other atmospheric components.

2. **Formation Process**:
- Primary pollutants, which are directly emitted from sources (like factories, vehicles, etc.), react with other gases present in the atmosphere.
- These other gases can include water vapor, molecular oxygen, and various hydrocarbons.
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