I think it doesn't make sense to switch to this in ARTS.  In fact, I don't
think it makes sense to use it at at all unless you want a fixed
low-altitude model.

It does make a lot of sense to provide some level of automation to take
care of the adjustments for fixed low-altitude models, but it's not that
easy elsewhere.

First issue.  Even on Earth, a constant VMR is not OK at higher altitudes.
You have only 20% O2 at 80 km, and less above.  So the dry values has to be
provided as a profile to make any sense.  This is a minor inconvenience.

Second issue, is the pressure grid/field going to be the dry pressure?  It
has to be, otherwise x * P / k T is no longer the number density required
for absorption strength.  I don't see how you can work around this.

Third issue, related strongly to the second issue.  This means that
pressure broadening calculations can no longer rescale to 1-VMR, and that
the VMR profile must be 1 or higher at all altitudes for each of the
absorption lines pressure broadening species.  This is a deep change, as we
can no longer have the meaning of air be "everything required to add up to
1-VMR".  It would be a lot of effort to fix this.

Lastly, and perhaps a bit too philosophical.  Given the same temperature
profile, if you add water to the atmosphere, the actual mixing ratio of all
the other species goes down.  Their number densities or relative pressure
are the ones that are supposed to remain somewhat constant.  It seems to me
therefore that if anything should be changed in ARTS, it's that we
shouldn't be using VMR but instead number densities.  This would certainly
simplify absorption strength calculations, but take quite a bit of effort
to fix in broadening calculations.

With hope,

Den ons 15 sep. 2021 kl 20:19 skrev Patrick Eriksson <>:

> 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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