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A secondary pollutant is
B) $CO_2$
D) Aerosol

Last updated date: 13th Jun 2024
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Hint:A substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource is known as a pollutant.

Complete Answer:
We can start with understanding the concept of secondary pollutants-
The pollutants which form in the atmosphere are called Secondary pollutants. Secondary pollutants do not emit directly from a source like vehicles or power plants. It forms as a result of the pollutants the sources emit and reacts with the molecules in the atmosphere. Pollutants which are emitted into the environment directly from the main source which are known as the primary pollutants.
Let's talk about all the choices in the question individually-
1) $CO_2$ differs from secondary pollutants because secondary pollutants must form in the atmosphere, whereas primary pollutants do not. Types of primary pollutants include: Nitrogen oxides (NOx) Carbon monoxide (CO).
2) Peroxyacetyl nitrate (PAN) and ozone are the two important secondary photochemical air pollutants found in the urban atmosphere.
3) PANs are secondary pollutants, which means they are not directly emitted as exhaust from power plants or internal combustion engines, but they are formed from other pollutants by chemical reactions in the atmosphere.
4) Aerosol pollutants are formed within the atmosphere as a result of gas-phase reactions which are followed by condensation of the products, or by reactions that take place in the existing atmospheric aerosol phase.

Therefore, the correct answer is option C (PAN).

Note:Secondary pollutants can be more harmful compared to primary pollutants and cause severe human health issues.Smog is an example of a secondary pollutant.