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 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 [GEOS-Chem] grid box, which you need to read the temperature and OH concentration. Then you can average globally to get a mean lifetime of CH3CCl3.

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.

You can see vestiges of this approach in old versions of GEOS-Chem, though that code didn't take into account the stratospheric boxes.

NOTE: In GEOS-Chem, chemically-produced OH is only available in the tropopshere (because the full-chemistry mechanism is not applied to stratospheric boxes. Therefore, the atmospheric burden of methyl chloroform can be computed by summing all grid boxes in the tropopshere + stratosphere. However, the sum of the loss rate of methyl chloroform by OH can only be summed over tropospheric grid boxes.

--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.

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

Notes

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.