Hi Paul, Roy, et al.,
A couple of points regarding fugacity.
1) In the proposed definition of fugacity, the first sentence could be
misunderstood. I suggest changing it to the following:
The fugacity is the measured pressure (or partial pressure) of a real gas
corrected for the intermolecular forces of that gas, which allows that corrected
quantity to be treated like the pressure of an ideal gas in the ideal gas
equation PV = nRT.
2) The air-sea flux of a gas does not depend on its fugacity, only its partial
pressure (i.e., in both the atmosphere and ocean).
Cheers,
Jim
On Mon, 24 Sep 2018, Halloran, Paul wrote:
> Dear Jonathan, Roy and others,
>
> Thanks for your questions.
>
> Jonathan, as Roy highlights, partial pressure and fugacity are not the same thing (see next comment). Unless additional variables have been observed or output from a model, the conversion can not be performed, and while in some situations it may be acceptable to assume pCO2 and fCO2 are interchangeable or crudely convertible, given that air-sea CO2 flux is the result of small differences between atmospheric and surface ocean CO2 values, not getting these numbers spot on can have a big impact (i.e. flux of CO2 from the ocean to the atmosphere rather than the other way round).
>
> Roy, I had not appreciated that the canonical unit and actual unit of measurement could both be used. Using Pastal and micro atmospheres respectively would make sense. I also see what you are saying about defining the fugacity with respect to partial pressure. Your proposed definition (included again below) sounds good to me.
>
> Revised definition:
>
> Fugacity is the equivalent for a real gas to partial pressure for an ideal gas. The partial pressure of a dissolved gas in sea water is the partial pressure in air with which it would be in equilibrium. The partial pressure of a gaseous constituent of air is the pressure which it alone would exert with unchanged temperature and number of moles per unit volume.
>
> Canonical unit: Pascal
> Typical unit of measurement: micro atmospheres
>
> Thanks again,
> Paul
>
>> On 20 Sep 2018, at 18:36, Dorothee Bakker (ENV - Staff) <D.Bakker at uea.ac.uk> wrote:
>>
>>
>> Dear Matthias, cc Paul,
>>
>> Thank you. How many participants does this email discussion have?
>>
>> For a definition of fugacity I suggest checking e.g. Stumm and Morgan, 1981 and Zeebe and Wolf-Gladrow, 1991 (or 1992?).
>>
>> Kind regards from the UEA Marine Sciences Field Course in Oban,
>>
>> Dorothee.
>>
>>
>> From: Matthias Tuma <mtuma at wmo.int>
>> Sent: 20 September 2018 17:03
>> To: Dorothee Bakker (ENV - Staff)
>> Subject: Fwd: [CF-metadata] New name: fugacity of CO2
>>
>> Dear Dorothee,
>>
>> FYI, I thought I might make you aware of the following discussion thread that started 20min ago on the CF-metadata mailing list, which is dedicated to discussion around the CF (Climate and Forecast) metadata standards for NetCDF files.
>>
>> Probably some of your community are on the list and may even comment, but I thought I'd forward nonetheless FYI.
>>
>> Best regards, Matthias
>>
>>
>> ---------- Forwarded message ----------
>> From: Lowry, Roy K. <rkl at bodc.ac.uk>
>> Date: Thu, Sep 20, 2018 at 10:01 AM
>> Subject: Re: [CF-metadata] New name: fugacity of CO2
>> To: "Halloran, Paul" <P.Halloran at exeter.ac.uk>, "cf-metadata at cgd.ucar.edu" <cf-metadata at cgd.ucar.edu>
>>
>>
>> Dear Paul,
>>
>> I'm pretty sure the Canonical Unit should be Pascal, as with the existing partial pressure Standard Names. Note that this doesn't mean the data need to be in Pascals, it is just a way of expressing dimensionality in terms of SI. There is a field for the actual units of measure in the parameter attributes.
>>
>> There are existing partial pressure Standard Names for CO2 and CH4, with partial pressure defined using the following text.
>>
>> The partial pressure of a dissolved gas in sea water is the partial pressure in air with which it would be in equilibrium. The partial pressure of a gaseous constituent of air is the pressure which it alone would exert with unchanged temperature and number of moles per unit volume.
>>
>> What is needed for fugacity is this with a description of the difference between partial pressure and fugacity. What do you think of the following?
>>
>> Fugacity is the equivalent for a real gas to partial pressure for an ideal gas. The partial pressure of a dissolved gas in sea water is the partial pressure in air with which it would be in equilibrium. The partial pressure of a gaseous constituent of air is the pressure which it alone would exert with unchanged temperature and number of moles per unit volume.
>>
>> 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 Halloran, Paul <P.Halloran at exeter.ac.uk>
>> Sent: 20 September 2018 16:40
>> To: cf-metadata at cgd.ucar.edu
>> Subject: [CF-metadata] New name: fugacity of CO2
>>
>> Dear mailing list,
>>
>> I would like to propose a new standard name: ?fugacity_of_carbon_dioxide_in_sea_water?
>> Variable name: fCO2
>> Units: ?atm
>> Definition: The fugacity of CO2 is its effective partial pressure.
>>
>> Thanks for your consideration of this,
>> Paul
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>>
>>
>>
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Received on Wed Sep 26 2018 - 03:31:54 BST
