Primary air pollutant prominence: Among the following, which gas is typically the most abundant constituent of urban air pollution from mobile sources by volume?

Introduction / Context:
Air pollution inventories distinguish pollutants by mass and volume contributions. In urban areas dominated by traffic, certain primary pollutants dramatically outweigh others by concentration, even if their mass impacts differ. This question asks which listed gas is typically the most abundant by volume from mobile sources.

Given Data / Assumptions:

• Context is urban, on-road emissions without advanced controls.
• Mobile sources produce CO, NOx, hydrocarbons, and some SOx depending on fuel sulphur.
• Hydrogen sulphide is not a characteristic traffic pollutant.

Concept / Approach:
Carbon monoxide is produced in large quantities by incomplete combustion in spark-ignition engines. Near-roadway measurements frequently show CO in the parts-per-million range, far exceeding typical ambient NOx or SO2 levels. While oxides of nitrogen are critical for ozone formation, their volumetric concentrations are commonly lower than CO in traffic plumes. Hydrogen sulphide is associated with industrial or natural sources, not general urban traffic.

Step-by-Step Solution:
Identify the pollutant most directly linked to incomplete combustion in gasoline engines: CO.Compare typical near-road concentrations: CO > NOx by volume.Select “Carbon monoxide.”

Verification / Alternative check:
Historic EPA and other urban monitoring datasets show CO spikes of several ppm on roadsides, with NO2 commonly lower in ppm terms, affirming the selection, even though NOx may dominate certain impact metrics.

Why Other Options Are Wrong:
Oxides of sulphur: Dependent on fuel sulphur; generally much lower for transport emissions in many regions.Oxides of nitrogen: Important but typically lower by volume than CO in traffic plumes.Hydrogen sulphide: Not a primary urban traffic pollutant.

Common Pitfalls:

• Confusing concentration dominance with overall harm; NOx and SOx can have outsized environmental effects despite lower volume.
• Assuming diesel-dominated fleets mirror gasoline fleet CO profiles; diesel emits less CO.

Final Answer:
Carbon monoxide