Identify the primary (not secondary) air pollutant in the list Among the following, which pollutant is not formed in the atmosphere by secondary reactions but is emitted directly from sources?

Introduction / Context:
Air pollutants are often classified as primary (emitted directly from sources) or secondary (formed in the atmosphere via chemical reactions). Recognizing which is which guides policy and control strategies, because primary pollutants are controlled at the stack/tailpipe, while secondary pollutants require precursor controls and atmospheric chemistry management.

Given Data / Assumptions:

• Ozone and photochemical smog are classic secondary pollutants.
• Sulphur dioxide (SO₂) is emitted directly by combustion of sulphur-containing fuels and certain industrial processes.

Concept / Approach:
Tropospheric ozone forms via photochemical reactions involving NOx and VOCs; “photochemical smog” is a composite secondary pollution episode featuring ozone, peroxyacyl nitrates, and oxidants. By contrast, SO₂ emerges primarily from power plants, refineries, smelters, and ships burning sulphur-bearing fuels.

Step-by-Step Solution:

Verification / Alternative check:
Emission inventories show SO₂ directly measured at sources; atmospheric models treat ozone as a product of precursor chemistry, not as an emitted component.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing stratospheric ozone (beneficial) with tropospheric ozone (pollutant). Here we refer to ambient ground-level ozone.

Final Answer:
Sulphur dioxide