From: parson_r@cubldr.colorado.edu (Robert Parson)
Newsgroups: sci.environment
Subject: Re: Where's the "F" in CFCs?
Date: 17 Apr 92 06:10:50 GMT
Distribution: usa
Organization: University of Colorado, Boulder


 Ligon@macgw1.ge.com (Woody Ligon) writes:

>>Contrary to other comments on this thread, the HF/HCl ratio is a very 
>>important number to know. 

 Indeed. But not easy to interpret.

>>I think we should all be quite interested to 
>>realize that in spite of the fact that  HF is known to be removed more 
>>slowly, nevertheless there is much less of it than there is Cl--even 
>>though Freons deliver F and Cl in near equal quantities. Clearly there must be
>>a very large natural contribution of chlorine to explain this situation. 

 Such a natural source would have to have steadily increased since 1957, in
 order to explain the increase in total stratospheric Cl by a factor of 4
 since then. No such source has ever been identified. CHCl3 in the troposphere
 has stayed nearly constant over this period. On the other hand, CFC's and
 other manmade sources _have_ increased.

 It is a mistake to think of the stratosphere as a steady-state system. Only a
 small fraction of the CFC's have dissociated.  The amounts of various species
 therefore depend sensitively upon the _rates_ of dissociation and of the many
 subsequent reaction steps. Most of these rates have been measured in the lab
 but not all, which gives rise to uncertainties in the models. (Another source
 of uncertainty is the amount of CFC emissions from the East Bloc.) As I said 
 previously, CHCl3 and CCl4 have shorter lifetimes than CF2Cl2. (I neglected
 to mention CH3CCl3 - this is produced in larger quantities than any of the
 freons, but its lifetime is very short - about 6 years - so a lot of it 
 never makes it to the stratosphere.  Nevertheless it is estimated to 
 contribute 7% to ozone depletion; it will clearly skew the Cl/F ratio.)

 As David Siskind mentioned, the most recent kinetic models actually predict
 _less_ HF than is observed. (The same model predicts amounts of HCl, CF2O,
 and CFClO that are in reasonably good agreement with observations.) There
 is clearly a need for more work, esp. laboratory measurements of the
 rate constants. Still, the situation is orders of magnitude better
 than is the case with global warming.

 The following papers contain the most recent HF and HCl measurements,
 and also summarize the large body of previous work in this area:
 
 L. Wallace and W. Livingston, J. Geophys. Res. _96_,15513, 1991.
 C. Rinsland et al., ibid. p. 15523.
__________________________________________________________

 Carl Lydick writes:
> 
> This conclusion rests on the premise that F and Cl are photodissociated from
> the HFCs in equal quantities.  Is this premise valid?
> --------------------------------------------------------------------------------

 Not really. The process involves several steps. First, UV radiation pops off
 a Cl atom. (The C-Cl bond is weaker). The remaining atoms are removed in
 subsequent reactions with O2, OH, NO, etc. The intermediate compounds
 CCl2O, CF2O, and CClFO are fairly stable and hang around for a while - they
 have been measured in the stratosphere.

 Robert
From: alanm@hpindda.cup.hp.com (Alan McGowen)
Date: Sat, 4 Apr 1992 02:06:37 GMT
Subject: Re: AIDS in Africa (was: environment and animals)
Organization: HP Information Networks, Cupertino, CA