[CF-metadata] standard names for chemistry - MCM

From: Philip J. Cameronsmith1 <cameronsmith1>
Date: Wed, 22 Oct 2008 13:00:47 -0700 (PDT)

On Mon, 20 Oct 2008, Heinke Hoeck wrote:

> Dear All,
>
> I don't agree with Martin and Philip.
>>
>> Dear Stephen, Martin, et al.
>>
>> I generally agree with Martin.
>>
>> I will say, though, that the "two table approach" (or alternatively:
>> "a two orthogonal vector approach") is particularly clean for
>> chemistry. In particular:
> What do you mean with 'orthogonal' ?
>>
>> a) I cannot think of any "matrix elements" of quantity_for_species_X
>> which would be undefined (although they may be negligibly small).
> Please see:
> http:// wiki.esipfed.org/index.php/CF_Standard_Names_-_Construction_of_Atmospheric_Chemistry_and_Aerosol_Terms
> by Christiane Textor, Michael Schulz and Olivier Boucher.
> If you look through the 'Fluxes' list some quantities contain a X, G and
> A. This looks for me that some fluxes
> are only useful for gases and aerosols.

Hi,

This email is just to respond to Heinke's point about different 'specie'
categories: X, G, A. The issue is independent of our other discussions.

There are two obvious ways to specify the amount of a specie: The number
of molecules, or the mass. One can convert between the two by the 'molar
mass' for that specie (molar mass = mass/mole [kg/mol]). In that sense,
all gas and aerosol species can be specified using the same unit (if
desired).

In practice, the molar mass becomes less meaningful when talking about
mixtures of species, such as for 'lumped species', or aerosols. In this
case, if the ratio of species is constant, then an effective molar mass
can still be defined. If the ratio of the species can vary, then
an archetypal molar mass can still be defined, with either the number or
mass of the specie becoming an 'effective' quantity.

In practice, gas phase chemicals are typically recorded using number of
moles, while aerosols are usually recorded using mass. Hence, the G for
gases, A for aerosols, and X for the combined set of G and A. But because
of the discussion above, it would be possible to force the gases and
aerosols to always have the same units for CF (as long as the Molar Mass
is recorded or prescribed for each specie).

Both systems have advantages and disadvantages (primarily uniformity
versus preciseness), but the advantages and disadvantages are weak, so I
am fairly neutral.

There is a related issue. In part to avoid the imprecise conversion
between number and mass for mixtures (including many aerosols), the
counting (or mass) can be restricted to a single element, often carbon
atoms. This is particularly useful when the effect of the chemical is
proportional to the amount of that element. For example, the chemical
effect of a molecule with 10 carbons is similar to the effect of two
molecules each with 5 carbons.

Here again, one would have the option of introducing other groups eg C for
species measured by number of carbons. But one could also force them to
use a consistent unit, with a constant conversion factor.

In short, this is a choice for CF to make. So far, CF has considered
species individually, but we do have the option to force uniformity going
forward if we want to.

Another option is for CF to allow all possible units for each specie:
moles_of_X
mass_of_X
carbons_of_X
etc...

But this may lead to undesirable proliferation of names.

Best wishes,

      Philip





------------------------------------------------------------------------
Dr Philip Cameron-Smith Atmospheric, Earth, and Energy Division
pjc at llnl.gov Lawrence Livermore National Laboratory
+1 925 4236634 7000 East Avenue, Livermore, CA94550, USA
------------------------------------------------------------------------
Received on Wed Oct 22 2008 - 14:00:47 BST