Question

One of the following acts as a secondary pollutant
A) $Br_2$
B) $o_3$
C) $Cl_2$
D) CO

Answer 1

Depending on whether or not a substance acts as a pollutant directly when it is released emission, or is formed when other pollutants react with each other, it can be defined as primary or secondary.

Complete answer:
Primary pollutants are of several kinds including car exhaust, dust particles, and volatile organic compounds whereas secondary pollutants include acids that form in the atmosphere and others formed due to photochemical reactions.

Halogens like bromine and chlorine are released as a result of many manufacturing processes. They are also primary pollutants; therefore options A and C are both incorrect.
Ozone or $o_3$ can be formed in more than one way. The UV rays in sunlight may split oxygen into two oxygen atoms, and then these single oxygen atoms react with molecules of $o_2$ to form $o_3$. Ozone is therefore a secondary pollutant. It can also be formed when volatile organic compounds like formaldehyde react with nitrogen oxides. Option B is the correct answer.
CO or carbon monoxide is commonly released with car exhaust as well as the burning of carbon based fuels like coal and wood. It is therefore a primary pollutant. Option D is incorrect.

Hence the correct answer is option ‘B’.

Note: While ozone is a pollutant in the lower layers of the atmosphere, it is an essential component of the upper atmosphere, helping to block the entry of harmful UV rays. Depletion of this ozone layer with the use of CFCs has been a major cause for concern over the last few decades.