I think your reasoning is correct for CO2, but the OP asked about "emissions" in general. For some of these, the dispersion is less, and thus the high altitude

I think your reasoning is correct for CO2, but the OP asked about "emissions" in general. For some of these, the dispersion is less, and thus the high altitude does make a difference. I've only skimmed it, but this seems to be a good paper describing the different effects: http://www.anjakollmuss.com/wp-content/uploads/2017/04/SEI_Air_Travel_Emissions_Paper2_June_09.pdf http://www.anjakollmuss.com/wp-content/uploads/2017/04/SEI_A... While agreeing that a multiplier is not strictly correct, they recommend applying multiplier "greater than 2" to properly account for the non-CO2 effects of air traffic on global warming: Though science-based reasoning discourages the use of a simple multiplier to account for non-CO2 effects, such a multiplier is desirable from a policy and climate protection point of view. We elaborate on a number of scientific and value-related issues and conclude that a multiplier of 2 or greater should be used for air travel emissions calculators to account for non-CO2 warming effects.