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

Introduction / Context:
Air pollutants are classified as primary (emitted directly) or secondary (formed by atmospheric reactions). Understanding the distinction is foundational for control strategies and regulatory frameworks. This question asks you to pick a classic secondary pollutant from a short list.

Given Data / Assumptions:

• Focus on common urban pollutants.
• Secondary pollutant is formed by reactions in air.
• Primary pollutants are emitted from stacks, vehicles, or processes.

Concept / Approach:
Photochemical smog forms when sunlight drives reactions among nitrogen oxides and volatile organic compounds, producing oxidants such as ozone and peroxyacyl nitrates. In contrast, sulfur dioxide, nitrogen dioxide (to a large extent), and dust are typically emitted directly (primary), though NO2 also participates in secondary formation pathways.

Step-by-Step Solution:

Verification / Alternative check:
Urban air quality episodes with high ozone levels occur under sunny, stagnant conditions due to secondary chemistry, confirming the secondary nature of photochemical smog.

Why Other Options Are Wrong:
Sulfur dioxide: primary from combustion of sulfur-containing fuels.
Nitrogen dioxide: emitted directly and also formed secondarily, but the term photochemical smog more clearly represents a secondary mixture.
Dust particles: primary fugitives from roads, construction, or processes.

Common Pitfalls:
Equating presence of NO2 in smog with it being purely secondary; overlooking that the smog mixture is the hallmark secondary product.

Final Answer:
Photochemical smog