Question

Which one of the following gases is primarily responsible for the formation of photochemical smog?

• A. Carbon monoxide
• B. Sulphur dioxide
• C. Nitrogen oxides
• D. Methane

Hint:

Photochemical smog is always linked with {NO$_x$ emissions + sunlight}. Vehicles are the major source.

Answer 1

The Correct Option is C

Step 1: Understanding photochemical smog.
Photochemical smog is a type of air pollution that forms due to chemical reactions between pollutants and sunlight. It is commonly observed in large cities with heavy vehicular traffic. This smog is characterized by the presence of ozone, nitrogen dioxide, aldehydes, and peroxyacetyl nitrate (PAN).
Step 2: Conditions required for photochemical smog formation.
The formation of photochemical smog requires three essential components: (i) Nitrogen oxides (NO and NO$_2$), (ii) Hydrocarbons or volatile organic compounds (VOCs), and (iii) Strong sunlight. Among these, nitrogen oxides act as the primary initiators of photochemical reactions.
Step 3: Role of nitrogen oxides in smog formation.
Nitrogen oxides are mainly released from automobile exhausts and combustion of fossil fuels. Under sunlight, nitrogen dioxide (NO$_2$) undergoes photodissociation to form nitric oxide (NO) and atomic oxygen. The atomic oxygen then reacts with molecular oxygen to produce ozone, which is the major harmful component of photochemical smog.
Step 4: Detailed analysis of options.
(A) Carbon monoxide: It is a toxic gas produced by incomplete combustion, but it does not participate in photochemical reactions leading to smog formation.
(B) Sulphur dioxide: This gas is mainly responsible for industrial smog and acid rain, not photochemical smog.
(C) Nitrogen oxides: Correct — These gases initiate sunlight-driven reactions that lead to the formation of ozone and other secondary pollutants in photochemical smog.
(D) Methane: Although methane is a hydrocarbon, it is relatively less reactive and does not play a primary role in smog formation.
Step 5: Final conclusion.
Since nitrogen oxides are the key gases that trigger photochemical reactions under sunlight, the correct answer is (C) Nitrogen oxides.