Identifying particulate air pollutants “Particulate” air pollutants comprise finely divided solids or liquids suspended in air. Which of the following is not a particulate class when stated as a pair?

Introduction / Context:
Air pollutants are often grouped by physical form: gases/vapours and particulates (solids or liquid droplets). Correct classification matters for selecting control technologies (e.g., cyclones, fabric filters for particulates; scrubbers or catalysts for gases). This item checks understanding of typical particulate categories and a notable exception.

Given Data / Assumptions:

• Particulates include dust, smoke, fumes, mist, aerosol, and soot (black carbon).
• Photochemical smog is a complex gaseous mixture formed by sunlight-driven reactions (e.g., ozone, peroxyacyl nitrates) with some aerosol contribution.
• The options are presented as pairs.

Concept / Approach:
Dust (solid), smoke/fumes (very fine solids from combustion/condensation), mists/fog (liquid droplets), and soot (solid carbon) are particulate forms. Photochemical smog, however, is primarily a gaseous-phase pollution episode with oxidants like ozone, although it can include aerosols secondarily. As a pair, “photochemical smog and soot” mixes a non-particulate class (smog) with a particulate (soot), making the pair not purely particulate.

Step-by-Step Solution:

Verification / Alternative check:
Air quality textbooks define photochemical smog by ozone and NOx-VOC chemistry, not particulate identity, confirming the mismatch in the pair.

Why Other Options Are Wrong:

• Dust and mists / Smoke and fumes: Both terms in each pair are particulate categories.
• None of these: Incorrect because option (c) contains a non-particulate term.
• Fog and haze: Both are aerosols (liquid and mixed-phase particulates), fitting particulate classes.

Common Pitfalls:
Thinking “smog” equals particulate matter only; photochemical smog is dominated by gaseous oxidants with secondary aerosols.

Final Answer:
Photochemical smog and soot