Question

Considering the four common air pollutants found in urban environment -- NO, SO\(_2\), Soot and O\(_3\), which one of them is the secondary pollutant?

• A. NO
• B. SO\(_2\)
• C. Soot
• D. O\(_3\)

Hint:

Examples: \[ \boxed{ \begin{aligned} \text{Primary Pollutants} &: \mathrm{CO},\ \mathrm{SO_2},\ \mathrm{NO}\\ \text{Secondary Pollutants} &: \mathrm{O_3},\ \mathrm{PAN},\ \mathrm{HNO_3} \end{aligned} } \]

Answer 1

The Correct Option is D

Step 1: Define primary and secondary pollutants.
A primary pollutant is emitted directly from a source, a secondary pollutant is formed afterwards by a chemical reaction in the atmosphere.
Step 2: Classify each of the four options.
NO comes straight out of vehicle and combustion exhaust, $\mathrm{SO_2}$ comes directly from burning sulphur containing fuel, and soot is directly emitted particulate matter, all three are primary.
Step 3: Look at ozone separately.
Ground level $\mathrm{O_3}$ is not emitted by any source, it forms when NO and volatile organic compounds react in sunlight, so it is the secondary pollutant among the four.
\[ \boxed{\mathrm{O_3}} \]