Which substance is commonly emitted by vehicles and contributes to smog formation?

Carbon monoxide is a common emission from vehicles and plays a significant role in smog formation. When gasoline is burned in an internal combustion engine, carbon monoxide is produced as a byproduct of incomplete combustion. This gas is harmful because it can react with other atmospheric components, particularly sunlight and volatile organic compounds (VOCs), to form ground-level ozone, a key ingredient in smog.

In urban areas, where vehicle traffic is heavy, these reactions are exacerbated during warm weather, leading to the accumulation of smog. Unlike other choices, carbon monoxide is a direct product of fuel combustion in engines and is recognized for its contribution to both air quality degradation and health problems.

Water vapor, while a significant component of the atmosphere, does not contribute to smog in the same way. Carbon dioxide, although a major greenhouse gas, is less reactive in the context of smog formation. Nitrogen, in itself, is an inert gas and does not contribute to ozone formation without reacting in combination with other pollutants.