Primary vs. Secondary Pollutants

Primary pollutants are emitted directly from a source into the atmosphere. Examples include carbon monoxide (CO) and sulfur dioxide (SO₂).

Secondary pollutants are formed in the atmosphere through chemical reactions involving primary pollutants and other atmospheric components, often initiated by sunlight.

Analysis of Options

• Carbon monoxide (CO): A primary pollutant, typically resulting from incomplete combustion of carbon-containing fuels.
• Sulphur dioxide (SO₂): A primary pollutant released mainly from burning fossil fuels containing sulfur.
• Ozone (O₃): A key secondary pollutant in the troposphere. It forms through complex photochemical reactions involving nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) in the presence of sunlight. The simplified concept is:

  $NO_x + VOCs + Sunlight \rightarrow O_3$

  Tropospheric ozone contributes to smog and respiratory problems.
• Nitrogen dioxide (NO₂): While NO₂ can be emitted directly as a primary pollutant from combustion sources, it also participates in reactions forming other secondary pollutants, including ozone. However, ozone (O₃) itself is the classic example of a secondary pollutant formed from reactions involving NOₓ and VOCs.

Conclusion

Based on the definitions and formation processes, Ozone (O₃) is the secondary pollutant among the given options found in the troposphere.