From: parson_r@cubldr.colorado.edu (Robert Parson)
Newsgroups: bit.listserv.biosph-l
Subject: Re:      Re: Ozone Layer
Date: 18 Dec 92 02:20:16 GMT
Organization: University of Colorado, Boulder

In article <01GSF3BIZTWI8WVZMQ@SSCVAX.CIS.MCMASTER.CA>,         Jeremy Whitlock <WHITLOCK@SSCVAX.CIS.MCMASTER.CA> writes:
> In response to requests, private and posted, for more info. on volcanic
> sources of ozone-depleting gases, here is a table from _21st Century_, Fall
> 1991:

 "21st Century" is put out by Lyndon Larouche's organization. It is utterly
 unreliable. Some of what they say about ozone is distortion, some is 100%
 wrong.
> 
> -------------------------------
> SOURCE OF ATMOSPHERIC CHLORINE (millions of tons)
> 
> Seawater                600

  Wrong - 300 Mt, most of which never gets far above the ocean.
 (Ref: Symonds et al, _Nature_ _334_, 415 (1988); R. Cadle, _Rev.
 Geophys. Space Phys. _18_, 746 (1980).)

> Volcanoes               36

 Wrong - total emissions are estimated at 0.3 - 10.0 Mt, most of which does
 _not_ get into the stratosphere (see below).

> Biomass Burning         8.4

 This could be correct - but only the methyl chloride (which is a small fraction
 of the above) gets into the strat. (and only a fraction of that).

> Ocean Biota             5
 
 This isn't too far off - Symonds et al. give 1.4-3.5 Mt. for the _total_
 methyl chloride from both ocean biota and biomass burning. Most of this is
 destroyed in the troposphere, but some gets into the strat, and this is the
 principal natural source.
> 
> 
> --------------------------------
> 
> It must be remembered that this is the atmospheric source, not what gets to
> the stratosphere.  Much of the Cl from sea water, for example, is rained
> out.  Volcanoes, however, inject their gases directly to the stratosphere.

 No, only the occasional _explosive_ eruption with a Volcanoc Explosivity
 Index (VEI) greater than 4 (this is a sort of Richter scale for volcanic
 eruptions) injects its gases into the stratosphere. These occur about
 once every 10 years. Jan Schloerer posted my figures for the two most
 recent such eruptions, El Cichon and Pinatubo, last night. To repeat,
 El Cichon put out 0.04 Mt, and Pinatubo < 0.02 Mt.  Low grade sizzlers
 like Kilauea (VEI 1) or Erebus (VEI 1-2) don't amount to much.

 Symonds et. al. collected geophysical estimates of the _total_ amount
 of chlorine put out by historic eruptions, and using this estimated that
 explosive eruptions put 0.1 - 1.0 Mt of chlorine into the stratosphere
 each year. The _upper_ end of this is _comparable_ to the amount of chlorine
 that reaches the stratosphere from CFC's (>0.9 Mt during the 1980's).
 However, subsequent work (Pinto et al., J. Geophys. Res. _94_, 11165 (1989))
 showed that these  estimates are too large, by a factor of as much as 100. 
The present consensus is that volcanoes don't contribute much chlorine to
 the stratosphere, in agreement with the _direct_ measurements for El Cichon,
 the largest eruption of the 80's, and Pinatubo, the largest since 1912.

 An _enormous_ amount of work has been done since 1974 to assess the 
 sources of stratospheric chlorine. The evidence that CFC's produce 
 most of it can be summarized as follows:
 
 1. Stratospheric chlorine has risen by a factor of 4 since 1958. The
 principal natural source (biologically produced methyl chloride) has
 stayed constant. The frequency of stratosphere-piercine eruptions has
 stayed constant. 

 2. The concentrations of CFC's have been measured _as a function of altitude_
 (see R. P. Wayne, _Chemistry of Atmospheres_. Oxford, 1991, p. 162). They
 are nearly constant all the way up to the stratosphere, then they drop off
 rapidly (when the CFC's get high enough up to be photolyzed). 

 3. The decay products of CFC photolysis (radicals like CF2Cl, CFCl2, and
 products of reactions of these with oxygen and nitrogen) have been measured
 in the stratosphere.

 When this is all put together, the numbers work out - the measured 
 flux of CFC's into the stratosphere accounts for the measured increase in
 chlorine in the stratosphere.

 The present consensus is:
 CFC's and related compounds : ~75-85%
 Methyl Chloride: ~10 - 20%
 Everything else: ~10% or less.

 Robert
 
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