Question

Photochemical smog formation in urban areas is mainly due to the presence in the atmosphere of.

• A. Ozone, PAN, and Nitrogen dioxide.
• B. Fog and Smoke.
• C. Particulates and fog.
• D. Winter climate and high humidity

Hint:

To distinguish between smog types:
- Photochemical Smog (Los Angeles type) = Oxidizing, requires warm sunlight, characterized by $\text{NO}_x$, $\text{O}_3$, and PAN.
- Industrial Smog (London type) = Reducing, occurs in cool damp climates, characterized by $\text{SO}_2$ and soot.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Photochemical smog is a modern type of air pollution that forms in urban areas.
It is produced when solar ultraviolet light reacts with primary pollutants like nitrogen oxides ($\text{NO}_x$) and volatile organic compounds (VOCs) in the atmosphere.

Step 3: Detailed Explanation:
Let us analyze the chemical formation of photochemical smog:
Automobile exhaust and industrial emissions release primary pollutants, mainly nitric oxide ($\text{NO}$) and unburned hydrocarbons.
In the presence of sunlight, $\text{NO}$ is oxidized to nitrogen dioxide ($\text{NO}_2$), which absorbs UV light and splits into nitric oxide and highly reactive oxygen atoms ($\text{O}$):
\[ \text{NO}_2 + h\nu \rightarrow \text{NO} + \text{O} \]
These oxygen atoms react with atmospheric oxygen ($\text{O}_2$) to form ozone ($\text{O}_3$):
\[ \text{O} + \text{O}_2 \rightarrow \text{O}_3 \]
Ozone and nitrogen dioxide then react with volatile organic compounds (hydrocarbons) to form peroxyacetyl nitrates (PAN) and other organic oxidants.
Therefore, photochemical smog is characterized by a yellowish-brown haze made up of secondary pollutants, primarily ozone ($\text{O}_3$), PAN, and nitrogen dioxide ($\text{NO}_2$).
In contrast, classical smog (London-type smog) is formed by sulfur dioxide, fog, and smoke under cool, damp winter conditions.

Step 4: Final Answer:
Photochemical smog in urban areas is caused by the presence of ozone, PAN, and nitrogen dioxide.
Therefore, the correct option is (A).