Depends what you are trying to measure. If you are trying to measure how irresponsible society is in general than a per capita value needs to be used. If you are trying to measure how noticeable the impact of their actions are then a per km^2 is more useful. Obviously the more you spread people out the less they will notice what the other people are doing.
Exactly, it's not a matter of whether we as a society in whole are adding substantially to the global air degradation, but the fact that if everyone in the world lived like us, it would be very bad news for the environment.