Hi Christiane, (cc the CF community)
I was looking through your website for atmospheric chemistry CF names
(below), and I see you have some 'chemical gross production' and 'chemical
gross destruction' quantities (eg,
tendency_of_atmosphere_mole_concentration_of_ozone_due_to_chemical_gross_production).
I know that HTAP worked hard to define what was meant by gross production
and destruction of ozone (and did as good a job as I know of), but even
then there was some residual ambiguity between models. And, another
intercomparison may choose a different definition.
Is there a definition somewhere under CF of gross production and
destruction you want to use (especially for ozone)?
For non-atmospheric chemists: Defining the gross chemical production is
not as simple as it sounds. Nominally, the gross chemical production for
a species X can be found by summing together the rates of all the
reactions that produce X. But, there are some species, most notably
ozone, where the simple definition is unhelpful. The reason is that there
are fast 'null-cycles' in which ozone gets destroyed then almost instantly
recreated, for example:
O3 + light --> O2 + O (eqn 1)
O + O2 + M --> O3 + M (eqn 2)
The rates of destruction and production of ozone by these two reactions
are generally large compared to other rates of destruction and production.
But, because eqn 1 is almost instantly followed by eqn 2, these reactions
don't really change the amount of ozone, and what atmospheric chemists are
usually interested in are the reactions that produce and destroy ozone
which are NOT part of fast null-cycles.
The challenge is that there is really a complex network of reactions,
including slower null-cycles, and branches from the null-cycles, for
example the reaction
O + O3 --> 2 O2 (eqn 3)
which is generally considered to be a sink of TWO ozone molecules, since
the oxygen atom can no longer react to form ozone via eqn 2.
The challenge then is to decide on how fast a null-cycle must be for
it to be ignored, and what to do with branches that appear to destroy
ozone, but may recreate some ozone via some other branches.
For a given set of chemical reactions, the gross production and
destruction can be unambiguously defined by specifying the reactions that
contribute to each quantity (although different researchers may make
different choices). But, it gets really tricky when trying to
compare models with different sets of chemical equations.
Nonetheless, there is a lot of value in trying to understand the
importance of chemical gross production and destruction compared to other
quantities such as deposition and stratospheric flux.
This ambiguity does NOT apply to NET chemical tendency, which is the
difference between the gross production and destruction. The NET chemical
tendency is usually small compared to the gross production and destruction
terms, so it is a difference of two large numbers. The net chemical
tendency is interesting, but when the net chemical tendency changes we
want to know whether it is because of a change in chemical gross
production, chemical gross destruction, or some other process. Hence, we
are back wanting to know the gross production and destruction.
In the end I think that chemical gross production and destruction are too
important to ignore, but that thought should be given to defining it as
well as possible (as it was under HTAP), and specifying that definition in
CF.
Yours truly,
Philip
On Wed, 11 Jul 2007, Christiane Textor wrote:
> Dear all,
>
> I have updated the wiki page for the discussions on the CF standard
> names for aerosols and chemistry, mainly concerning their sources and
> sinks, and optical thickness. Please have a look at
>
> http://wiki.esipfed.org/index.php/CF_Standard_Names_-_CF_Standard_Names_-_Submitted_Atmospheric_Chemistry_and_Aerosol_Terms
>
> This page contains the names we already discussed in quite some detail,
> and I hope they can be accepted in autumn. I am looking forward to your
> comments.
>
> Best regards,
> Christiane
>
>
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> CF-metadata at cgd.ucar.edu
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>
------------------------------------------------------------------------
Dr Philip Cameron-Smith Energy & Environment Directorate
pjc at llnl.gov Lawrence Livermore National Laboratory
+1 925 4236634 7000 East Avenue, Livermore, CA94550, USA
------------------------------------------------------------------------
Received on Thu Jul 19 2007 - 18:12:14 BST
