Air pollution basics: Among the gases listed, which is most significant as a conventional air pollutant due to its health and environmental impacts from combustion sources?

Introduction / Context:
Air pollutants are substances in the atmosphere that, even at low concentrations, harm human health, ecosystems, or materials. Identifying key pollutants assists in control strategies and regulatory frameworks for ambient air quality management.

Given Data / Assumptions:

• Common gases present in ambient air or emitted from combustion.
• Evaluation focuses on direct pollutant status and health effects at typical ambient concentrations.

Concept / Approach:
Sulphur dioxide (SO2) is a classic criteria pollutant produced by combustion of sulfur-containing fuels. It causes respiratory irritation, contributes to acid deposition, and forms fine particulate sulfates that harm visibility and health. By contrast, oxygen and nitrogen are natural major constituents of air, and carbon dioxide, while a greenhouse gas of climate concern, is not treated as a primary toxic air pollutant in the same health-based sense as SO2 at ambient levels.

Step-by-Step Solution:
List candidate gases: CO2, O2, N2, SO2.Assess pollutant significance: SO2 is regulated for acute and chronic exposure impacts.Select SO2 as the most significant conventional air pollutant among the listed gases.

Verification / Alternative check:
Ambient air quality standards worldwide include SO2 as a criteria pollutant with prescribed limits to protect public health and ecosystems, underscoring its significance.

Why Other Options Are Wrong:
Carbon dioxide: addressed primarily under climate policy rather than acute ambient toxicity control.Oxygen, Nitrogen, Helium: not pollutants under normal conditions; these are essential or inert atmospheric components.

Common Pitfalls:

• Equating greenhouse gas status with conventional air pollutant toxicity.
• Overlooking secondary particulate formation from SO2 emissions.

Final Answer:
Sulphur dioxide