Question

The primary pollutant that leads to photochemical smog is:
(A) Sulphur dioxide
(B) Acrolein
(C) Ozone
(D) Nitrogen oxide

Answer 1

Hint:. Smog is a mixture of fog and smoke. There are two types of smog : classical smog and photochemical smog. To answer this question we should be aware of both classical smog and photochemical smog.

Complete step by step answer:
Classical smog is a type of smog produced by combustion of fossil fuels.
Photochemical smog is a type of smog produced. It is oxidizing smog that is having high concentration of oxidants. This is caused due the release of pollutants from factories to the atmosphere, especially automobile industries and power plants. This can be seen in densely populated cities where the temperature condition is hot. In those cities, it is visible as a brown haze which can be noticed well in the morning and afternoon.

Reaction involved in formation of photochemical reaction:
1. Nitrogen oxide released from industries combine with oxygen molecules in the atmosphere forming nitrogen dioxide.
\[NO + {O_2} \to N{O_2} + O\]
2. The rays from sunlight break $N{O_2}$ giving out NO and a free oxygen atom.
\[N{O_2} + hv \to NO + O\]
3. The free oxygen yield in the above step combines with oxygen in the atmosphere and forms an ozone molecule.
 \[O + {O_2} \to {O_3}\]
Option A is sulphur dioxide. Sulphur dioxide and fog is the primary constituent of classical smog. Therefore option A is the wrong answer.

Option B is acrolein. Acrolein is involved in the formation of photochemical smog, but there is an alternate formation of photochemical smog where acrolein is involved at all. So, this is evident that it is not the primary pollutant that leads to photochemical fog. Therefore, Option B is the wrong answer.

Option C is ozone. Ozone is the secondary pollutant. From the above mentioned reaction we can observe ozone is formed from nitrogen dioxide but it is one of the pollutants produced at last in the chain of formation of photochemical smog. Therefore, option C is the wrong answer.

Option D is nitrogen oxide. Nitrogen oxide is the primary pollutant. Again, if we observe the reaction mentioned above we can observe that the whole set reaction in formation of photochemical smog revolves around nitrogen oxide. Without the presence of higher concentration of nitrogen oxide in one place, the formation of photochemical smog is impossible. Therefore, option D is the correct answer.
So, the correct answer is “Option D”.

Note: $NO$ and ${O_3}$ is the major pollutant of photochemical smog. Don’t confuse $NO$ and ${O_3}$. $NO$ is the primary pollutant and ${O_3}$ is the secondary pollutant. This is clearly evident from the illustration shown under chemical reaction involved in the formation of photochemical reaction.