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[UDUNITS #XJX-726297]: Request for addition of new 'molecule' unit

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  • Subject: [UDUNITS #XJX-726297]: Request for addition of new 'molecule' unit
  • Date: Thu, 21 Mar 2013 14:36:29 -0600
Alison,

> I am writing to ask if it is possible for a new unit of 'molecule' to be
> added to the UDUNITS-2 XML database. The unit would be defined as follows:
> 
> >    <unit>
> >        <def>mol/6.02214179e23</def>

Isn't the Avogadro constant 6.02214129e23? See 
<http://physics.nist.gov/cgi-bin/cuu/Value?na|search_for=physchem_in!>.

> >        <name>
> >            <singular>molecule</singular>
> >            <plural>molecules</plural>
> >        </name>
> >        <aliases>
> >            <name><singular>molec</singular></name>
> >        </aliases>
> >    </unit>
> 
> This request is a result of a recent discussion on the CF-Metadata mailing
> list in which we have agreed to create standard names for eight column
> trace gas quantities with canonical units of mole m-2. However, the
> common practice in the atmospheric chemistry community is to use units
> of molecule cm-2. These latter units could be used in the netCDF file
> and converted to the CF canonical unit if a definition for 'molecule'
> were to be added to UDUNI TS.

It's a possibility.

You could, of course, add this definition to a "local definitions" file (say 
"$prefix/share/udunits/udunits2-local.xml"; where "$prefix" is the top-level 
installation directory) and then include that file in the file 
"$prefix/share/udunits/udunits2.xml". I presume, however, that you would want 
this definition to be an "official" unit (i.e., one supported by the UDUNITS-2 
package and, consequently, distributed with the package). Correct?

> (The  CF discussion started in 2012 at
> 
> http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2012/056052.html
> 
> and following posts in the same thread and recently concluded in
> 
> http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2013/056207.html
> 
> and following posts).

Incidentally, I'm a little surprised to see that you included the word "mole" 
in the names of the physical quantities. The NIST frowns on this because it 
includes a unit in the name (see section 8.6.2 of 
<http://physics.nist.gov/Pubs/SP811/sec08.html>). I would have gone with names 
like

    areic_amount_of_substance_of_tropospheric_nitrogen_dioxide

Oh well. Not my call. :-)

I'll see about putting "molecules" in the next release -- providing we can 
agree on the Avogadro constant.

> Kind regards,
> Alison Pamment (CF Standard Names manager)
> 
> ------
> Alison Pamment                          Tel: +44 1235 778065
> NCAS/British Atmospheric Data Centre    Email: address@hidden
> STFC Rutherford Appleton Laboratory
> R25, 2.22
> Harwell Oxford, Didcot, OX11 0QX, U.K.

Regards,
Steve Emmerson

Ticket Details
===================
Ticket ID: XJX-726297
Department: Support UDUNITS
Priority: Normal
Status: Closed