NOx-Ox-HC-aerosol

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Methyl chloroform lifetime

Chulkyu Lee (cklee79@gmail.com) wrote:

I'm trying to compare the methyl-chloroform (CH3CCl3) lifetime in two GEOS-Chem simulations.
I've been going through the GEOS-Chem diagnostics listed on the GEOS-Chem website (Appendix 5) and noticed that there is a diagnostic for the lifetime of Methyl-chloroform: ND23. It doesn't appear in the input.geos file, and from looking at the code itself, I'm not certain if it is still active. I was wondering if this diagnostic is functional for Methyl-chloroform.

Bob Yantosca (yantosca@seas.harvard.edu) wrote:

We have since removed the CH3CCl3 lifetime from the code as a diagnostic of how "hot" the chemistry is. That has since been replaced with the mean OH concentration diagnostic (which is automatic when you run the code).
I also discovered that the CH3CCl3 diagnostic was mislabled in the Appendix 5 -- that should have been "Mean OH". Sorry for the confusion.

Jingqiu Mao (jmao@seas.harvard.edu) wrote:

The CH3CCl3 lifetime is supposed to be the destruction rate of CH3CCl3 by OH. For the reaction OH+CH3CCl3, you can get the reaction rate from JPL2006:
   k = 1.64e-12 * exp( -1520 / T ).
The lifetime of CH3CCl3 is
   1 / ( k * [OH] ). 
So if the global mean OH is 1e6 molecule/cm3, and the temperature is 298K, the lifetime of CH3CCl3 would be 1e8 seconds, which equals to 3.2 years.
Also you can calculate the lifetime of CH3CCl3 at each grid box, which you need to read the temperature and OH concentration. Then you can average globally to get a mean lifetime of CH3CCl3.

Loretta Mickley (mickley@fas.harvard.edu) wrote:

Historically, we calculated the lifetime of CH3CCL3 in a different way. In Bey et al., 2001, this is the procedure we used.
First the value you want to calculate is: the lifetime of global CH3CCl3 against the tropospheric sink of OH, Toh.
   Toh = sum ( burden CH3CCl3 ) / sum (loss rate due to tropospheric OH),
where sum is the sum over all gridboxes, including the stratosphere, and the units are molecules / (molecules/s). For the stratsopheric gridboxes, the loss rate will be zero by definition. Note you need to do the sums first, then the ratio.
Another way to write this is:
   Toh =  sum ( [CH3CCl3] * air dens * volume)
       / sum (k * conc OH * [CH3CCl3] * air dens * volume)
where the brackets signify mixing ratio, conc OH is OH molecules/cm3, air dens is the density in molecules/cm3, and volume is the volume of each grid box in cm3. Now assume that the mixing ratio of CH3CCl3 is uniform throughout the atmosphere.
   Toh =  sum ( air dens * volume )
       / sum (k * conc OH * air dens * volume)
To apply this to GEOS-Chem, you need these quantities for each gridbox, either as monthly means or more finely time-resolved:
  • air density
  • volume
  • temperature
  • OH concentration in molecules /cm3
Then calculate this:
   Toh =  sum ( air dens * volume )
       / sum (k as a function of temperature * conc OH * air dens * volume)
Sum first over all gridboxes and timesteps. Then calculate the ratio.
You can see vestiges of this approach in old versions of GEOS-Chem, though that code didn't take into account the stratospheric boxes. I believe there exists IDL code kicking around Harvard that can calculate this in the proper way, once you have saved the necessary inputs. I will ask. Please let me know if you use IDL and would be interested in this.
By the way, the global lifetime of CH3CCl3, Ttot, is calculated in this way:
   1/Ttot = 1/Toh + 1/Tstrat + 1/Tocean
In Bey et al., we assumed Tstrat (the lifetime of CH3CCl3 against chemical loss in the stratosphere) was 43 years. We assumed that Tocean (the lifetime against loss to oceans) was 80 years. Since we had already calculated Toh, following the method above, we could calculate Ttot. This number is useful to compare with published values such as Spivakovsky 2000 and some of the Prinn papers.
In my view, however you decide to calculate the lifetime, you should make clear in your paper exactly what method you used.

--Bob Y. 10:19, 22 July 2008 (EDT)

CH4 concentrations

Chris Buten (cbuten@pdx.edu) wrote:

When running the NOxOxHC simulation, I presume that CH4 concentration is specified as an input and not determined by emissions and transport (and therefore not a tracer). If so, is there a way to easily visualize or extract these input concentrations?

Bob Yantosca (yantosca@seas.harvard.edu) wrote:

Yes, in the full chemistry mechanism, CH4 is a constant. The values for years and latitude bands are set by routine ,tt>get_global_ch4.f</tt>.

--Bob Y. 09:52, 14 July 2008 (EDT)

FAST-J Photolysis

NOTE: This discussion has now been moved to the Photolysis mechanism page.

Variable Tropopause

25-Sep-2007

  1. Note that the implementation of the variable tropopause is buggy in versions prior to GEOS-Chem v7-04-12. If you are using versions prior to v7-04-12, you should turn the variable tropopause OFF.
  2. Jennifer Logan (see correspondence below) suggested that we should cap the variable tropopause at 200hPa in near-polar regions (90-60S and 60-90N), to avoid the problem with anomalously high tropopause heights at high latitudes. This fix was not in v7-04-12, but will be implemented into GEOS-Chem internal version v7-04-13.


Jennifer Logan (jlogan@seas.harvard.edu) wrote:
After looking at the two papers I sent, I think we should restrict the tropopause at latitudes > 60 deg. to pressures greater than 200 mb (about 11 km). From Fig. 3 in Seidel and Randel, there are tropopause (TP) heights as high as 13.5 km in the Antarctic (median height is ~9.8 km, 250 mb), but I don't think we want to be doing trop. chem there. The median TP pressure at ~80 N is ~300 mb, compared to ~250 mb at 70-85 S. The extratropical TP heights are higher (lower pressure) in the SH than in the NH according to Fig. 3.
This approach is also very easy to explain in a paper.
Jennifer

Other errors in SMVGEAR

  • Click HERE for a description of the SMVGEAR bug that caused concentrations of certain tracers in STT to go to zero. This bug was fixed by May Fu and Philippe Le Sager.
  • Lok Lamsal reported a bug with NaN's in SMVGEAR. Bob Yantosca recommended a fix for this error. Click HERE to visit the discussion in the Bug Fixes Forum.