Why are pollutant emissions measured in g/km?

[ksio89]

When we read about how much carbon dioxide (or any other air pollutant) an internal combustion engine in a certain vehicle produces, the measure unit employed is grams per kilometre, it is, mass per unit of distance. However, in my understanding, it would make much more sense to measure emissions in grams per second or per minute, it is, to use mass flow rate units. One of the reasons is that I believe it is much easier to set an emissions target and to measure it if the unit chosen is in mass per unit of time.

Correct me if I'm wrong, but stating how much CO₂ an engine produces per kilometre does not make any sense at all, because we don't know how the gas production rate has variated. It seems more intuitive and useful to know how much pollutant gas is produced per unit of time, considering some additional parameters such as engine rpm or gasoline octane rating.

Do vehicle manufacturers and regulatory agencies stating emissions in units mass/distance for simplification? I I mean, does it make easier for consumers to compare emissions between different models?

 

[rcgldr]

Since the normal purpose of a vehicle to transport someone from one location to another, then emissions and efficiency are rated versus distance instead of versus time.

In the USA and some other countries, pollution versus unit time is also taken into account, such as a car idling while stopped at an intersection.