Hej,

No time for writing a lot. Right now just want to make a basic check of our understanding.

With our present definition of VMRs, we agree on that having 78% N2, 21% O2 and e.g. 3% H2O is unphysical? That with a lot of H2O (or any other non-fixed gas) the standard values of the fixed gases should be scaled downwards. In the example above, with 0.97. Do you agree?

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.

If we agree on the above, then this is the simplest (but not most theoretically correct) solution.

Bye,

Patrick








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,

Stefan

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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