Question

Example of a secondary pollutant is:

• A. CFCS's
• B. $\text{CH}_4$
• C. PAN
• D. CO

Hint:

Primary Pollutants $\rightarrow$ Emitted directly ($\text{CO}$, $\text{CH}_4$, $\text{SO}_2$, $\text{CFCs}$).
Secondary Pollutants $\rightarrow$ Formed in the air via chemical reactions ($\text{O}_3$, $\text{PAN}$, Acid Rain).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Air pollutants are classified into two primary categories based on their origin and formation pathways in the atmosphere:
1. Primary Pollutants: Emitted directly into the atmosphere from identifiable sources, such as volcanic eruptions, industrial chimneys, or vehicle exhausts.
2. Secondary Pollutants: Not emitted directly into the atmosphere.
Instead, they are formed in the air through chemical and photochemical reactions between primary pollutants and other normal atmospheric constituents.

Step 2: Detailed Explanation:
Let us evaluate each of the listed chemical species to identify its pollutant class:
(A). CFCs (Chlorofluorocarbons):
CFCs are synthetic compounds released directly into the atmosphere from air conditioners, refrigerators, and aerosol sprays, making them primary pollutants.
(B). \(\text{CH}_4\) (Methane):
Methane is a potent greenhouse gas emitted directly from diverse natural and anthropogenic sources (such as wetlands, rice paddies, and enteric fermentation in livestock), making it a primary pollutant.
(C). PAN (Peroxyacetyl Nitrate):
PAN is a highly toxic component of photochemical smog.
It is not emitted directly by any industrial source or vehicle.
Instead, it is formed in the atmosphere through complex photochemical reactions between nitrogen oxides (\(\text{NO}_x\)) and volatile organic compounds (VOCs) in the presence of solar ultraviolet light.
Specifically, the photolysis of aldehydes generates acyl peroxy radicals, which react with nitrogen dioxide (\(\text{NO}_2\)) to produce PAN:
\[ \text{CH}_3\text{CO(O)O} \cdot + \text{NO}_2 \rightarrow \text{CH}_3\text{CO(O)O-ONO}_2 \text{ (PAN)} \]
Therefore, PAN is a classic secondary pollutant.
(D). CO (Carbon Monoxide):
Carbon monoxide is a toxic gas emitted directly from the incomplete combustion of fossil fuels in vehicle exhausts and biomass burning, making it a primary pollutant.

Step 3: Final Answer:
Consequently, the chemical species that represents a secondary air pollutant is PAN.