Which is true about secondary air pollutant?

Options

A. They are formed on the ground
B. Chemically they are of diverse nature
C. They are comparatively less toxic
D. They are generally formed through photochemical reactions

Answer

### Step-by-Step Solution:

1. **Understand Air Pollutants**:
- Air pollutants are substances that negatively impact the environment. They can be classified into two categories: primary and secondary pollutants.

2. **Identify Primary Pollutants**:
- Primary air pollutants are those that are emitted directly from a source into the atmosphere. Examples include sulfur dioxide, ammonia, and nitrogen dioxide.

3. **Define Secondary Pollutants**:
- Secondary air pollutants are not emitted directly. Instead, they are formed when primary pollutants react with other substances in the atmosphere, particularly hydrocarbons, often under the influence of sunlight.

4. **Examples of Secondary Pollutants**:
- Common examples of secondary pollutants include acid rain, nitrogen oxides, and peroxyacetyl nitrates (PAN).

5. **Evaluate Each Option**:
- **Option 1**: "They are formed on ground."
- This is incorrect because secondary pollutants are formed in the atmosphere, not on the ground.

- **Option 2**: "Chemically they are of diverse nature."
- This is also incorrect. While they can vary, they are generally harmful and toxic, not diverse in nature.

- **Option 3**: "They are comparatively less toxic."
- This is incorrect as well. Secondary pollutants can be very toxic due to their formation from primary pollutants and hydrocarbons.

- **Option 4**: "They are generally formed from photochemical reactions."
- This is the correct option. Secondary pollutants are formed from reactions involving primary pollutants and hydrocarbons, particularly under the influence of UV radiation, leading to photochemical smog.

6. **Conclusion**:
- The correct statement about secondary air pollutants is that they are generally formed from photochemical reactions.