Air pollution classification: which of the following is a secondary air pollutant formed in the atmosphere rather than emitted directly from a stack?

Introduction / Context:
Air pollutants are categorized as primary (emitted directly) or secondary (formed in the atmosphere by chemical reactions). Identifying secondary pollutants helps target precursor controls and effective abatement strategies.

Given Data / Assumptions:

• Typical urban atmosphere with sunlight, VOCs, and NOx.
• Understanding of smog formation chemistry.

Concept / Approach:
Photochemical smog forms when volatile organic compounds (VOCs) react with nitrogen oxides under sunlight, producing oxidants like ozone, peroxyacyl nitrates, and aldehydes. This mixture is not directly emitted; it arises from atmospheric photochemistry, hence it is a secondary pollutant complex.

Step-by-Step Solution:
Classify each option: SO2, NO2, and dust are primarily emitted from sources (combustion, processes, fugitive dust).Photochemical smog results from VOC + NOx + sunlight, producing secondary oxidants.Therefore, the secondary pollutant among the options is photochemical smog.

Verification / Alternative check:
Air-quality textbooks consistently list ozone and photochemical oxidants as secondary, formed through chain reactions initiated by solar radiation.

Why Other Options Are Wrong:
Sulphur dioxide: directly emitted from combustion of sulfur-containing fuels.
Nitrogen dioxide: primarily emitted directly or formed very near sources from NO oxidation; typically treated as primary for classification purposes.
Dust particles: directly emitted (primary) from handling, roads, and grinding.

Common Pitfalls:
Confusing secondary smog with primary NO2; smog is a complex mixture dominated by secondary oxidants, especially ozone.

Final Answer:
Photochemical smog