[CF-metadata] wiki for standard_names for aerosols and chemistry updated

[Philip J. Cameron-smith]

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
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