I think that delta-14CO2 is not the same thing as the mole fraction. Rather, it is an expression of isotopic enrichment/depletion with respect to a standard. Whilst I have no experience of atmospheric 14C, I have come across delta notation a lot with other isotopes in geology and oceanography such as 13C and 18O. There, delta is an expression of the ratio of the target isotope to another isotope in the sample relative to some standard - ((sample 13C/12C ratio / standard 13C/12C ratio) - 1) * 1000 to give a result scaled to per mil. I presume that delta-14C is no different.
I am unaware (i.e. I couldn't find) a precedent for delta values in CF Standard Names. The issue of describing these things has been addressed at length in the BODC parameter descriptions with almost 400 measurement descriptions. A typical example is:
Enrichment with respect to Vienna Pee Dee Belemnite (VPDB) of carbon-13 in carbonate in the sediment
This particular example includes information on the specific standard used. Many do not because the information is often unavailable for older data.
A straw man alternative to Kate's proposal could be
enrichment_of_14C_in_carbon_dioxide_in_air
If information on the standard is available then that could be added as an 'enrichment_with_respect_to_whatever' clause or the information could be confined to the long name. The better solution depends upon the use case (e.g. does it require inclusion of data where standard is unknown).
Another approach could be to adopt community vocabulary such as:
delta14C_in_carbon_dioxide_in_air
Others may have alternative suggestions.
I went for 'enrichment of x' in the BODC dictionary because it provides a better fit to a normalised semantic model for mapping purposes. One only has to include one 'enrichment' rather than a long list of 'deltas' in the semantic element.
Cheers, Roy.
I have now retired but will continue to be active through an Emeritus Fellowship using this e-mail address.
________________________________
From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of Katherine Pugsley <katherine.pugsley at bristol.ac.uk>
Sent: 08 February 2019 10:46
To: cf-metadata at cgd.ucar.edu
Subject: [CF-metadata] New standard name for 14CO2
Dear All,
I'd like to request an addition to the standard name list for atmospheric 14CO2 measurements. Here are the details of the proposed standard name.
Proposal for a new standard variable name
Name: mole_fraction_of_14C_dioxide_in_air
Canonical Units: 1
Description: Atmospheric 14CO2 measurements are reported in ?14C notation with units of per mil, the deviation from the absolute radiocarbon reference standard. ?14C is used to calculate fossil fuel CO2 content. The long name will contain information that the variable is ?14C.
Thanks,
Katherine
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