Methyl chloroform lifetime

On this page we list information about how to compute the lifetime of methyl chloroform (chemical formula = CH3CCl3, also informally denoted as "MCF"). This is used as a measure of atmospheric OH concentration, and thus can provide insights into how well GEOS-Chem's chemical mechanism is performing.

Overview

Jingqiu Mao 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 OH is 1e6 molecule/cm3, and temperature is 298K, the lifetime of CH3CCl3 would be 1e8 seconds, which equals to 3.2 years.

For the global mean OH, the right way to do this is to calculate the lifetime of CH3CCl3 in each [GEOS-Chem] grid box and then weigh it globally with air masses in each box, to get a mean lifetime of CH3CCl3.

--Bob Y. 15:23, 8 June 2011 (EDT)

Historical method of computing CH3CCl3 lifetime w/r/t OH

Loretta Mickley 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 of 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:

1. air density
2. grid box volume
3. temperature
4. 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.

--Bob Y. 15:21, 8 June 2011 (EDT)

Total atmospheric CH3CCl3 lifetime

Loretta Mickley wrote:

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., (2001), 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.

Jennifer Logan wrote:

I was checking the new WMO/UNEP Scientific Assessment of Ozone Depletion: 2010 for any recent reference on the MCF lifetime, and saw that they now have a section devoted to the definition of lifetimes. I extracted the relevant pages (the report comes with a CD of the text, figures etc). It is attached. This report gives 5 years for Ttot for MCF, citing Prinn et al., (2005). It also says the lifetime with respect to ocean loss is 94 years (range is 81-145 y).

--Bob Y. 16:06, 8 June 2011 (EDT)

Implementation into GEOS-Chem

The methyl chloroform lifetime was originally computed on-the-fly in older versions of GEOS-Chem. At some point (maybe version 5), this online diagnostic was removed and replaced with the mean OH concentration diagnostic.

As of 08 Jun 2011, the GAMAP routine mcf_lifetime.pro can be used to compute the methyl chloroform lifetime w/r/t tropospheric OH from diagnostic fields saved to the GEOS-Chem output files.

Chulkyu Lee 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 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.

--Bob Y. 15:28, 8 June 2011 (EDT)

References

1. Bey I., D. J. Jacob, R. M. Yantosca, J. A. Logan, B. Field, A. M. Fiore, Q. Li, H. Liu, L. J. Mickley, and M. Schultz, Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation, J. Geophys. Res., 106, 23,073-23,096, 2001. PDF
2. Lawrence, M.G, Joeckel, P, and von Kuhlmann, R., What does the global mean OH concentraton tell us?, Atm. Chem. Phys, 1, 37-49, 2001.
3. Prather, M. and C. Spivakovsky, "Tropospheric OH and the lifetimes of hydrochlorofluorocarbons", J. Geophys. Res., 95, D11, 18723-18729, 1990.
4. Spivakovsky, C. M., J. A. Logan, S. A. Montzka, Y. J. Balkanski, M. Foremen-Fowler, D. B. A. Jones, L. W. Horowitz, C. A. M. Brenninkmeijer, M. J. Prather, S. C. Wofsy, and M. B. McElroy, Three dimensional climatological distribution of tropospheric OH: update and evaluation, J. Geophys. Res., 105, 8931-8980, 2000. PDF
5. WMO/UNEP Scientific Assessment of Ozone Depletion: 2010.

--Bob Y. 15:40, 8 June 2011 (EDT)