NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/talk/environm/4561

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From: [e-mail] (Robert Parson)
Subject: Re: Ozone Depletion
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Date: Wed, 11 Nov 1992 04:00:11 GMT
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In article <[e-mail]>, [e-mail] (Larry London) writes:
> In article <[e-mail]> [e-mail] writes:
>>Could someone tell me the name of the chemical used in the agricultural industry
>>that is said to cause ozone depletion at a rate of three times more than CFCs are
>>said to.   A description of its use, manufacturer and any alternatives would also
>>be appreciated.  My email address is [e-mail]    Thank-you
>
> Methyl bromide, a soil fumigant (to kill weeds, nematodes, etc.) usually
> dispensed from aerosol containers underneath plastic sheets covering
> planting beds, typically tobacco seedlings; a highly toxic gas.
>
 I don't know what precisely is meant by "a rate three times more than CFC's".
 _Molecule for molecule_, Bromine is 10-50 times more efficient as a catalyst
for ozone depletion than Chlorine. However, there is much less Br than Cl in the
 stratosphere, and anthropogenic sources comprise a smaller proportion of the
 whole. (Total Stratospheric chlorine is 3.5 parts per billion, of which
 ~2.9 comes from CFC's and related compounds; total Br is 20 parts per trillion,
 of which ~8 are _estimated_ to be anthropogenic.) Last I heard (this spring)
 it was not clear how much of the methyl bromide was anthropogenic and how much
 was natural, although natural sources were believed to dominate. (I don't have
 any figures handy for the contribution of Halons to the total strat. Br. I
 believe it is less than methyl bromide but not by a huge margin.)

 In any event, the _total_ contribution of Br compounds to ozone depletion is
 not three times that of chlorine compounds. Standard references (e.g.
 Wayne's _Chemistry of Atmospheres_, 1991) give 5-20%, which is the sort of
 thing you'd get by playing with the above numbers. The April NASA press
 release asserts that Br and Cl were contributing about equally to the
 ozone depletion over the Northern Hemisphere, which suggests that the
 older figures should be revised upwards. (One must keep in mind that
 there are many distinct mechanisms for halogen-catalyzed ozone depletion,
 and their relative importance depends on temperature and other local factors.)
 The major Bromine-catalyzed pathways actually include both Bromine and
 Chlorine, so reducing stratospheric chlorine will slow them down as well.

 Robert