Hej Patrick!

It seems a bit weird to me to use this definition at the (low) level of the 
absorption routines. Perhaps one solutions would be to have an option for this 
behaviour when ingesting concentration profile data? Perhaps by passing in a 
list of species that should be considered as not adding to the denominator for 
the VMR definition.

Note that for once the special thing about water is here not the fact that it’s 
condensible, I think, but just that there is so much of it, and at the same 
time very variable. Other gas species have also very variable concentrations, 
but it doesn’t matter for the total pressure.

All the best,


On 15 Sep 2021, at 20:19, Patrick Eriksson wrote:

> Stefan,
> Neither I had considered this definition of VMR. But would it not make sense 
> to follow it? Then a statement that the atmosphere contains 20.95% oxygen 
> makes more sense. You yourself pointed at that it would make sense to scale 
> N2 and O2 for low humid altitudes, where the amount of water can be several 
> %. In code preparing data for ARTS I normally do this adjustment. Should be 
> more correct!?
> A problem is to define what is the wet species when we go to other planets. 
> Or maybe there are even planets with several wet species?
> That is, I would be in favour to define VMR with respect to dry air, if we 
> can find a manner to handle other planets.
> Bye,
> Patrick
> On 2021-09-15 18:27, Stefan Buehler wrote:
>> Dear all,
>> Eli Mlawer brought up an interesting point in some other context:
>>> we recently had a LBLRTM user get confused on our vmr, which is 
>>> amount_of_gas / amount_of_dry_air. They weren’t sure that dry air was the 
>>> denominator instead of total air.  I’m too lazy to look at the link above 
>>> that @Robert Pincus provided, but I hope it is has dry air in the 
>>> denominator.  So much easier to simply specify evenly mixed gases, such as 
>>> 400 ppm CO2 (and, 20 years from now, 500 ppm CO2).
>> I’ve never considered that one could define it this way. Perhaps this 
>> convention explains, why VMRs in climatologies like FASCOD add up so poorly 
>> to 1.
>> I’m not suggesting that we change our behaviour, but want to make you aware 
>> that this convention is in use. (Or perhaps you already were, and just I 
>> missed it.)
>> All the best,
>> Stefan

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