NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/bit/listserv/biosphl/1247

Text File | 1992-12-22 | 9.0 KB | 156 lines

Comments: Gated by [e-mail]
Path: sparky!uunet!gatech!paladin.american.edu!auvm!CUBLDR.COLORADO.EDU!PARSON_R
X-VMS-To: IN::"[e-mail]"
MIME-version: 1.0
Content-transfer-encoding: 7BIT
Message-ID: <[e-mail]>
Newsgroups: bit.listserv.biosph-l
Date:         Mon, 21 Dec 1992 14:56:42 -0700
Reply-To:     [e-mail]
Sender:       List Owner <[e-mail]>
From:         [e-mail]
Subject:      Re: ozone layerSEND/EDIT
Lines: 141

 First of all I would like to thank Jeremy Whitlock for pointing out the
 statement in Van Nostrand's Scientific Encyclopedia (1989 edition,  article on
 volcanoes) that Mt. St. Augustine  put 289 billion kg (289 Mt) into the
 stratosphere. I've seen this impossible  figure (it's hundreds of times larger
 than the total amount of HCl _in_ the  stratosphere!) quoted in several places
(Dixy Lee Ray's book, for example)  but always without citation.

 This appears to be a horrible mistake on the part of the compilers of the
 encyclopedia. The article gives an accurate summary of Johnston's
 claim ("Volcanic Contribution of Chlorine to the Stratosphere: More Significant
 to Ozone that Previously Estimated?", _Science_ _209_, 491, 1980) that
 St. Augustine put *0.082-0.175* Mt of chlorine into the stratosphere (17-36%
 of the 1975 world production of chlorine in fluorocarbons). As I said in
 my previous post, these numbers are now considered to be too large. (In any
 case, they are at most _comparable_ with CFC emissions, not far in excess of
 them.)
        In the next paragraph, however, the encyclopedia states the impossible
 figure given above, apparently without seeing any discrepancy! I looked up
 Johnston's paper again and found the following (p. 492):

 "eruption of the Bishop Tuff from Long Valley Caldera, 700,000 years ago,
 generated 100 km^3 of air fall ash. If the magma degassed 0.25% chlorine
 (equivalent to Augustine Volcano), this eruption may have injected 289x10^9
 kg of HCl into the stratosphere..."

 Yup, there it is - it's for the Long Valley Caldera, _not_ for St. Augustine!
 This was one of the so-called "giant caldera eruptions" that occur every
 100,000 years or so. These are unimaginably violent events, probably the
 most destructive weapons in Nature's arsenal (excepting asteroid collisions).
 For example, the most recent such eruption, Lake Toba in Indonesia (~70,000
 years ago) dumped ash on India and Madagascar and may have triggered an ice
 age. Another example is Yellowstone Park, which blows out every 600,000 years
or so (numbers from memory). If Long Valley were to erupt  today it would kill
millions. 289 Mt HCl is quite conceivable for such an event (I have seen
similar estimates for Toba).

 Conceivably, a typesetter accidentally substituted Augustine for Long Valley;
 it's still an inexcusable mistake on the part of the Encyclopedia.

 More recently Jeremy Whitlock wrote:

>I disagree that the baseline is as important as the relative change.  It
>may be a fact of nature that the ozone layer is thinner over the antarctic,
>and indeed polar plantlife is adapted to a higher UV-B flux.  The relative
>change, which is what will affect lifeforms, is on the same order as
>relative changes with latitude and season elsewhere on the globe.

 Records going back to 1956 show a thick ozone layer over the antarctic
(although the seasonal variation is somewhat different from that in  the arctic
- the winter low of ~275 persisted until November, because  of the polar
vortex. The ozone hole consists of a _decrease_ from this  naturally low
springtime baseline).  UV-B levels in polar regions are always very low,
because the sun is never high and light takes a long path through  the
atmosphere. For this reason, the ozone hole is _not_ directly  dangerous to
humans or other species that have evolved closer to the  equator. There were
reports last year of blind sheep, etc. in Chile  and Patagonia, but it is very
unlikely that this was due to the ozone  hole, since these same sheep do just
fine in the Andean highlands where  they get much more UV-B.
        The real danger is to organisms that have evolved in the normally  low
UV-B, such as marine phytoplankton. These get exposed to springtime  UV-B
levels that approach, and on occasion exceed, those characteristic of  an
Antarctic midsummer (Frederick and Alberts, _Geophys. Res. Lett._ _18_,  1869,
1991.)  In other words, summer has suddenly become twice as long, as  far as
UV-V goes. The effect is exacerbated because the antarctic sea ice is  much
more transparent in spring than in summer. There is one recent study  (_Smith
et al., _Science_ _255_, 952, 1992) which finds ~10% reduction in
"phytoplankton productivity" upon passing into the hole - this is well  within
natural year-to-year fluctuations (~25%) but could be serious if  it represents
a cumulative effect. I will let the regular subscribers to  this list discuss
the significance of this sort of effect since my knowledge  of biology (let
alone ecology) is minimal.

>This tells me that ClO and ozone concentrations are anticorrelated.  I'd
>still like to know if cause-and-effect has been demonstrated for man-made
>sources.

 I addressed this in an earlier post, which appears not to have gotten out
 (Jan Schloerer tells me he didn't see it). The observation of anticorrelation,
 together with laboratory experiments that reproduce it, is convincing evidence
 that the ozone hole is due to Cl (and Br) catalyzed reactions. It remains to
 show that the chlorine is manmade. The evidence for this consists of
 measurements of sources and sinks, a laborious task that has been going on
 for ~20 years. The concentrations of the various source gases have been
 measured _as a function of altitude_, and this provides us with a way to
 estimate the flux of these gases into the stratosphere.

 The total amount of chlorine in the stratosphere has increased by a factor
 of 4 since ~1950 (S. Solomon, _Nature_ _347_, 347, 1990). The major natural
 source is CH3Cl; concentrations of CH3Cl in the troposphere have fluctuated
 but have not shown any steady increase over this period. As discussed
 previously, volanoes don't add up to enough - and in any case the frequency
 of stratosphere-piercing eruptions (about 1 per year, most of which only
 go a little way into the strat) has not increased. CFC emissions _did_
 increase over this period, in about the right amounts.
 To pin things down a little more firmly, consider the altitude dependence
 of the CFC concentrations. These are typically constant all the way through
 the troposphere (showing that there is no chemical sink for them in the
 troposphere, just as the popular discussions say) and then drop off rapidly
 starting at about 15 km, where there begins to be enough UV-C around to
 break them up. For example, the concentration of CFCl3 in 1980-1983 was
 measured at 0.2 parts per billion from 0-14 km, drops to 0.1 ppb by 20km,
 and is down to 0.02 ppb at 25 km. (For comparison, total stratospheric
 chlorine was about 2.0 ppb in 1980, and remember that each CFCl3 gives
 3 Cl atoms, so that 0.1 ppb CFCl3 destroyed between 14 and 20km gives
 0.3 ppb Cl, a sizable fraction of the total amount that had been accumulating
 over 30 years - and that's just _one_ CFC (the second most widely-produced
 one). The calculation is a good deal more complicated than this (the CFC's
 don't decay all at once, UV-C pops off one Cl and the others come off
 after subsequent reactions which can take years at the low concentrations
 in the strat., - and all the time stuff is drifting up and down, north and
 south) but that's the basic idea.

 Ref. for the CFCl3 data: R. P. Wayne, _Chemistry of Atmospheres_, 2nd
 edition, Oxford, 1991, p. 162 - I'm sorry I don't have a _Science_ or
 _Nature_ reference, but this book is pretty readable.)

 Additional evidence:

 1. _Fluorine_ concentrations in the strat. have also been rising - the
 concentration of HF (the final destination for F in the strat) has increased
 by a factor of 10 since 1978 (Zander et al., _J. Geophys. Res._ _95_,
 20519, 1990.)

 2. Some of the decay products of CFC's (radicals like CF2O, formed when
 the CF2Cl radical reacts with oxygen) have also been observed.


 One final remark: it is important to distinguish between _ozone depletion_,
 which so far is a small effect (5% for the US over the past decade, more
 in southern Australia, much less (essentially none) in the tropics), from
 the Antarctic ozone hole, which is a huge, but so far localized, effect.
 Although there is a strong case linking CFC emissions to the Antarctic hole
 (and to the much smaller springtime "dimple" over the north pole), there is
 still argument about the causes of the depletion at mid-latitudes - there is
 a substantial majority view, but I would not call it a consensus. As an
 outsider, though, I reason as follows: ClO wipes out 3% of the world's
 ozone every spring. Most of it comes back, but we see a small decrease -
 ~3% _per decade_ - over the earth as a whole. Seems reasonable that effect #2
 is linked to effect #1, although this is not proof.

 Robert