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.bak...@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 <mt...@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. <r...@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.hallo...@exeter.ac.uk>, "cf-metadata@cgd.ucar.edu"
<cf-metadata@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-boun...@cgd.ucar.edu> on behalf of Halloran, Paul
<p.hallo...@exeter.ac.uk>
Sent: 20 September 2018 16:40
To: cf-metadata@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: μatm
Definition: The fugacity of CO2 is its effective partial pressure.
Thanks for your consideration of this,
Paul
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