Question

Which of the following is a secondary pollutant?
$\begin{align}
  & a)S{{O}_{2}} \\
 & b)CO \\
 & c){{O}_{3}} \\
 & d)NO \\
\end{align}$

Answer 1

Hint: A secondary pollutant is formed by the combination of two primary pollutants. With this in mind, analyse the given options and try to figure out which of them fits the required criterion.

Complete step-by-step answer:
Let us first analyse primary and secondary pollutants, before coming to answering this question.
Primary pollutants are produced from nature and anthropogenic sources (results from Manual cause) in the environment.
For example, a primary pollutant produced naturally are the ashes from a volcanic eruption, whereas anthropogenic sources​ can be CO (carbon monoxide) from fuel burning vehicles.
On the other hand, secondary pollutants are formed by combining the primary pollutants. They are not emitted directly to the environment. These result from the interaction of primary pollutants in the atmosphere.
For example, ozone is a secondary pollutant. Ozone is a result of the combination of \[N{{O}_{X}}\] with hydrocarbons in the presence of Sunlight. For this example, \[N{{O}_{X}}\] and hydrocarbons are primary pollutants and are interacting with each other in the atmosphere to result in the production of Ozone.
The majority of tropospheric ozone formation occurs when nitrogen oxides (\[N{{O}_{X}}\]), carbon monoxide (CO) and volatile organic compounds (VOCs), react in the atmosphere in the presence of sunlight, specifically the UV spectrum. \[N{{O}_{X}}\], CO, and VOCs are considered ozone precursors.
Thus, the answer to this question is c).

Note: The term anthropogenic means ‘man-made’. Do not be confused with the meaning of this word while referring to this answer. Remember that a free radical reaction takes place between the oxides of nitrogen and CO to produce the hydroxyl radical which then turns into a peroxy radical.
\[\begin{align}
  & \bullet OH\text{ }+\text{ }CO\text{ }\to \text{ }\bullet HOCO \\
 & \bullet HOCO\text{ }+\text{ }{{O}_{2}}~\to \text{ }H{{O}_{2}}\bullet \text{ }+\text{ }C{{O}_{2}} \\
\end{align}\]