Carbon monoxide air quality: With a TLV under 50 ppm, which source contributes the largest share of carbon monoxide in ambient urban air under typical conditions?

Introduction:
Carbon monoxide, abbreviated as CO, is a colorless, odorless gas formed by incomplete combustion. Understanding primary sources informs monitoring, standards, and control strategies. The question references a threshold limit value under 50 ppm and asks for the main ambient source, particularly relevant to urban air quality management.

Given Data / Assumptions:

• Urban or peri urban settings with traffic.
• Conventional internal combustion fleets without universal zero emission replacement.
• Background photochemistry present but not dominant compared to direct emissions.

Concept / Approach:
CO forms when carbon containing fuels do not fully oxidize to CO2. Gasoline engines, especially during cold start, rich fueling, or poor maintenance, generate significant CO. While industries emit pollutants, their CO contribution is generally smaller compared to the aggregated vehicle fleet in cities. Domestic fuels contribute locally but not at the scale of traffic corridors.

Step-by-Step Solution:
Identify widespread incomplete combustion sources.Quantify relative contributions: urban traffic dominates CO inventories in many cities.Industrial stacks are more associated with NOx, SO2, and particulates than CO dominance.Therefore, automobiles exhaust is the main source under typical urban conditions.

Verification / Alternative check:
Emission inventory studies consistently show roadway emissions as the leading CO source where large vehicle fleets operate. Ambient CO peaks near busy intersections and during rush hours, matching traffic patterns.

Why Other Options Are Wrong:
Industrial chimney exhaust: not the dominant CO source in most cities. Photochemical reactions: CO can form secondarily but is minor compared to direct tailpipe emissions. Domestic fuel burning: localized and smaller scale than traffic. Natural soils: minor background source.

Common Pitfalls:
Confusing CO with NOx or SO2 source dominance. Traffic control measures reduce CO significantly through improved combustion and catalytic converters.

Final Answer:
Automobiles exhaust