Automobile emissions — identify the principal pollutant in exhaust In the context of typical petrol/gasoline engine exhaust without after-treatment, which pollutant is the dominant concern?

Introduction / Context:
Road vehicles emit a mixture of pollutants. Historically, before widespread catalytic converters and stringent standards, one compound dominated public-health concerns from gasoline engines: carbon monoxide (CO).

Given Data / Assumptions:

• Spark-ignition engines operating near stoichiometric conditions without advanced after-treatment.
• Question asks for the main pollutant by concern and typical prominence in raw exhaust.
• Diesel engines differ; focus here is gasoline-type exhaust.

Concept / Approach:
CO forms from incomplete combustion of carbon in fuel. In spark-ignition engines, CO fractions in raw exhaust are relatively high compared with many other toxic components. CO is colorless, odorless, and interferes with oxygen transport in blood, making it a critical pollutant historically targeted by automotive catalysts.

Step-by-Step Solution:

Verification / Alternative check:
Emission inventories and textbook data show high CO percentages in untreated gasoline engine exhaust, validating CO as the principal pollutant historically addressed.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing climate relevance (CO2) with acute toxicity (CO), and conflating diesel vs gasoline emission profiles.

Final Answer:
CO