Usenet 1994 January/usenetsourcesnewsgroupsinfomagicjanuary1994.iso/answers/ozone-depletion/antarctic

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From: [e-mail] (Robert Parson)
Subject: Ozone Depletion FAQ Part III: The Antarctic Ozone Hole
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Summary: This is the third of four files dealing with stratospheric
         ozone depletion. It describes the massive losses measured in
         the Antarctic spring, and the smaller losses seen in the Arctic.
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* Copyright 1993 Robert Parson                                        *
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 This part deals specifically with springtime antarctic ozone
depletion (and with the similar but smaller effects seen in the
Arctic spring). More general questions about ozone and ozone
depletion, including the definitions of many of the terms used
here, are dealt with in parts I and II. Biological effects of the
ozone hole are dealt with in part IV.


| Caveat: I am not a specialist. In fact, I am not an atmospheric
| chemist at all - I am a physical chemist who talks to atmospheric
| chemists. These files are an outgrowth of my own efforts to educate
| myself over the past two years. I have discussed some of these
| issues with specialists but I am solely responsible for everything
| written here, including any errors. This document should not be
| cited in publications off the net; rather, it should be used as a
| pointer to the published literature.

*** Corrections and comments are welcomed.


- Robert Parson
  Associate Professor
  Department of Chemistry and Biochemistry,
  University of Colorado  (for which I do not speak)

  [e-mail]
  [e-mail]


CONTENTS

1.   What is the antarctic ozone hole?

2.   How big is the hole, and is it getting bigger?

3.   When did the hole first appear?

4.   How far back do Antarctic ozone measurements go?

5.   But I heard that Dobson saw an ozone hole in 1956-58...

6.   Why is the hole in the antarctic?

7.   What is the evidence for the present theory?

8.   Will the ozone hole keep growing?

9.   Why be concerned about an ozone hole over antarctica?
     Nobody lives down there.

10.  Is there an ozone hole in the arctic? if not, why not?

11.  Can the hole be "plugged"?


1. What is the Antarctic ozone hole?

 For the past decade or so, ozone levels over Antarctica have fallen
to abnormally low values between late August and late November. At
the beginning of this period, ozone levels are already low, about
300 Dobson units (DU), but instead of slowly increasing as the
light comes back in the spring, they drop to 150 DU and below. In
the lower stratosphere, between 15 and 20 km, about 95% of the
ozone is destroyed. Above 25 km the decreases are small and the net
result is a thinning of the ozone layer by about 50%.  In the late
spring ozone levels return to more normal values, as warm,
ozone-rich air rushes in from lower latitudes. The precise duration
varies considerably from year to year; in 1990 the hole lasted well
into December.

 In some of the popular newsmedia, as well as many books, the
term "ozone hole" is being used far too loosely.  It seems that
any episode of ozone depletion, no matter how minor, now gets
called an ozone hole (e.g. 'ozone hole over Hamburg - but only for
one day'). This sloppy language trivializes the problem and blurs
the important scientific distinction between the massive ozone
losses in polar regions and the much smaller, but nonetheless
significant, ozone losses in middle latitudes. It is akin to
using "gridlock" to describe a routine traffic jam.


2. How big is the hole, and is it getting bigger?

 During the years 1978-1987 the hole grew, both in depth (total ozone
loss in a column) and in area. This growth was not monotonic but
seemed to oscillate with a two-year period (perhaps connected with
the "quasibiennial oscillation" of the stratospheric winds.) The
hole shrank dramatically in 1988 but in 1989-1991 was as large as in
1987, and in 1992-93 was larger still. In 1987 and 1989-93 it
covered the entire Antarctic continent and
part of the surrounding ocean. The exact size is determined
primarily by meteorological conditions, such as the strength of
the polar vortex in any given year. The boundary is fairly steep,
with decreases of 100-150 DU taking place in 10 degrees of
latitude, but fluctuates from day to day. On occasion, the
nominal boundary of the hole has passed over the tip of S. America,
(55 degrees S. Latitude). Australia and New Zealand are far outside
the hole, although they do experience ozone depletion, more than
is seen at comparable latitudes in the Northern hemisphere.  After
the 1987 hole broke up, December ozone levels over Australia and
New Zealand were 10% below normal.
[WMO 1991] [Atkinson et al.] [Roy et al.].


3. When did the hole first appear?

 It was first observed by ground-based measurements from Halley Bay
on the Antarctic coast, during the years 1980-84. [Farman, Gardiner
and Shanklin.] At about the same time, ozone decreases were seen at
the Japanese antarctic station of Syowa; these were less dramatic than
those seen at Halley (Syowa is about 1000 km further north) and did not
receive as much attention.  It has since been confirmed
by satellite measurements as well as ground-based measurements
elsewhere on the continent, on islands in the Antarctic ocean, and at
Ushaia, at the tip of Patagonia. With hindsight, one can see the hole
beginning to appear in the data around 1976, but it grew much more
rapidly in the 1980's. [Stolarski et al. 1992]


4. How far back do antarctic ozone measurements go?

 Ground-based measurements began in 1956, at Halley Bay. A few years
later these were supplemented by measurements at the South Pole and
elsewhere on the continent.  Satellite measurements began in the
early 70's, but the first really comprehensive satellite data came
in 1978, with the TOMS (total ozone mapping spectrometer) and SBUV
(solar backscatter UV) instruments on Nimbus-7.  The TOMS, which
finally broke down on May 7 1993, is the source for most of the
pretty pictures that one sees in review articles and the
popular press. Today there are several satellites monitoring ozone
and other atmospheric gases; the Russian Meteor-3 carries a new
TOMS, while instrument on NASA's UARS (Upper Atmosphere Research
Satellite) simultaneously measure ozone, chlorine monoxide (ClO),
and stratospheric pressure and temperature.


5. But I heard that Dobson saw an ozone hole in 1956-58...

 This is a myth, arising from a misinterpretation of an out-of-
context quotation from Dobson's paper. A glance at the original
suffices to refute it.
In his historical account [Dobson], Dobson mentioned that
when springtime ozone levels over Halley Bay were first measured,
he was surprised to find that they were about 150 DU below
corresponding levels (displaced by six months) in the Arctic.
Springtime arctic ozone levels are very high, ~450 DU; in the
Antarctic spring, however, Dobson's coworkers found ~320 DU, close
to winter levels. This was the first observation of the _normal_,
pre-1980 behavior of the Antarctic ozone layer: because of the
tight polar vortex (see below) ozone levels remain low until late
spring. In the Antarctic ozone hole, on the other hand, ozone
levels _decrease_ from these already low values. What Dobson
describes is essentially the _baseline_ from which the ozone hole
is measured. [Dobson] [WMO 1989]

 For those interested, here is how springtime antarctic
 ozone has developed from 1956 to 1991:

-------------------------------------------------------------
   Halley Bay Antarctic Ozone Data

   Mean October ozone column thickness, Dobson Units

   From J. D. Shanklin, personal communication, 1993.
   See also [Dobson], [Farman et al.], [Hamill and Toon],
   [Solomon], and [WMO 1991], p. 4.6

    1956      321                    1975      308
    1957      330                    1976      283
    1958      314                    1977      251
    1959      311                    1978      284
    1960      301                    1979      261
    1961      317                    1980      227
    1962      332                    1981      237
    1963      309                    1982      234
    1964      318                    1983      210
    1965      281                    1984      201
    1966      316                    1985      196
    1967      323                    1986      248
    1968      301                    1987      163
    1969      282                    1988      232
    1970      282                    1989      164
    1971      299                    1990      179
    1972      304                    1991      155
    1973      289                    1992      142
    1974      274                    1993      117


6. Why is the hole in the Antarctic?

 This was a mystery when the hole was first observed, but
it is now well understood. I shall limit myself to a
brief survey of the present theory, and refer the reader to two
excellent nontechnical articles [Toon and Turco] [Hamill and Toon]
for a more comprehensive discussion. Briefly, the unusual
physics and chemistry of the Antarctic stratosphere allows the
inactive chlorine "reservoir" compounds to be converted into ozone-
destroying chlorine radicals. While there is no more chlorine over
antarctica than anywhere else, in the antarctic spring most of
the chlorine is in a form that can destroy ozone.

The story takes place in six acts, some of them occurring
simultaneously on parallel stages:

i. The Polar Vortex

 As the air in the antarctic stratosphere cools and descends during
the winter, the Coriolis effect sets up a strong westerly
circulation around the pole. When the sun returns in the spring the
winds weaken, but the vortex remains stable until November. The air
over antarctica is largely isolated from the rest of the
atmosphere, forming a gigantic reaction vessel. The vortex is not
circular, it has an oblong shape with the long axis extending out
over Patagonia.

(For further information about the dynamics of the polar vortex see
[Schoeberl and Hartmann], [Tuck 1989], [AASE], [Randel], [Plumb],
and [Waugh]). There is currently some controversy over just how isolated
the air in the vortex is. According to Tuck, the vortex is better
thought of as a flow reactor than as a containment vessel; ozone-rich
air enters the vortex from above while ozone-poor and ClO-rich air is
stripped off the sides. Recent tracer measurements lend some support
to this view, but the issue is unresolved. See [Randel] and [Plumb].)

ii. Polar Stratospheric Clouds ("PSC")

 The Polar vortex is extremely cold; temperatures in the lower
stratosphere drop below -80 C. Under these conditions large numbers
of clouds appear in the stratosphere. These clouds are composed
largely of nitric acid and water, probably in the form of crystals
of nitric acid trihydrate ("NAT"), HNO3.3(H2O). Stratospheric
clouds also form from ordinary water ice (so-called "Type II PSC"),
but these are much less common; the stratosphere is very dry and
water-ice clouds only form at the lowest temperatures.

iii. Reactions Catalyzed by Stratospheric Clouds

 Most of the chlorine in the stratosphere ends up in one of the
reservoir compounds, Chlorine Nitrate (ClONO2) or Hydrogen Chloride
(HCl). Laboratory experiments have shown, however, that these
compounds, ordinarily inert in the stratosphere, do react on the
surfaces of polar stratospheric cloud particles. HCl dissolves into
the particles as they grow, and when a ClONO2 molecule becomes
adsorbed the following reactions take place:

 ClONO2 + HCl -> Cl2 + HNO3

 ClONO2 + H2O -> HOCl + HNO3

The Nitric acid, HNO3, stays in the cloud particle.

In addition, stratospheric clouds catalyze the removal of Nitrogen
Oxides ("NOx"), through the reactions:

 N2O5 + H2O -> 2 HNO3

 N2O5 + HCl -> ClNO2 + HNO3

Since N2O5 is in (gas-phase) equilibrium  with NO2:

 2 N2O5 <-> 4 NO2 + O2

this has the effect of removing NO2 from the gas phase and
sequestering it in the clouds in the form of nitric acid, a process
called "denoxification" (removal of "NOx").

iv. Sedimentation and Denitrification

 The clouds may eventually grow big enough so that they settle out
of the stratosphere, carrying the nitric acid with them
("denitrification"). Denitrification enhances denoxification.
If, on the other hand, the cloud decomposes while in the
stratosphere, nitrogen oxides are returned to the gas phase.
Presumably this should be called "renoxification", but
I have not heard anyone use this term :-).

v. Photolysis of active chlorine compounds

 The Cl2 and HOCl produced by the heterogeneous reactions are
easily photolyzed, even in the antarctic winter when there is
little UV present. The sun is always very low in the polar winter,
so the light takes a long path through the atmosphere and the
short-wave UV is selectively absorbed.  Molecular chlorine,
however, absorbs _visible_  and near-UV light:

     Cl2 + hv -> 2 Cl

     Cl + O3 -> ClO + O2

The effect is to produce large amounts of ClO. This ClO would
ordinarily be captured by NO2 and returned to the ClONO2 reservoir,
but "denoxification" and "denitrification" prevent this by removing
NO2.

vi. The chlorine peroxide mechanism

 As discussed in Part I, Cl and ClO can form a catalytic cycle that
efficiently destroys ozone. This cycle uses free oxygen atoms,
however, which are only abundant in the upper stratosphere, whereas
the ozone hole forms in the lower stratosphere. Instead, the
principal mechanism involves chlorine peroxide, ClOOCl (often
referred to as the "ClO dimer"):

     ClO + ClO -> ClOOCl
     ClOOCl + hv -> Cl + ClO2
     ClO2 -> Cl + O2
     2 Cl + 2 O3 -> 2 ClO + 2 O2
    -------------------------------
    Net: 2 O3 -> 3 O2

 At polar stratospheric temperatures this sequence is extremely fast
and it dominates the ozone-destruction process. The second step,
photolysis of chlorine peroxide, requires UV light which only
becomes abundant in the lower stratosphere in the spring. Thus one
has a long buildup of ClO and ClOOCl during the winter, followed by
massive ozone destruction in the spring. This mechanism is believed
to be responsible for about 70% of the antarctic ozone loss.

Another mechanism that has been identified involves chlorine and
bromine:

     ClO + BrO -> Br + Cl + O2
     Br + O3 -> BrO + O2
     Cl + O3 -> ClO + O2
     -----------------------
    Net: 2 O3 -> 3 O2

 This is believed to be responsible for ~25% of the antarctic
ozone depletion. Additional mechanisms have been suggested, but
they seem to be less important. [WMO 1991]

(For further information on the "perturbed chemistry" of the
antarctic stratosphere, see [Solomon], [McElroy and Salawich],
and [WMO 1989, 1991]).


7. What is the evidence for the present theory?

The evidence is overwhelming - the results from a single 1987
expedition (albeit a crucial one) fill two entire issues of the
Journal of Geophysical Research. What follows is a very sketchy
summary; for more information the reader is directed to [Solomon]
and to [Anderson et al.].

 The theory described above (which is often called the
"PSC theory") was developed during the years 1985-87. At the same
time, others proposed completely different mechanisms, making no
use of chlorine chemistry. The two most prominent alternative
explanations were one that postulated large increases in nitrogen
oxides arising from enhanced solar activity, and one that
postulated an upwelling of ozone-poor air from the troposphere into
the cold stratospheric vortex.  Each hypothesis made definite
predictions, and a program of measurements was carried out to test
these. The solar activity hypothesis predicted enhanced NOx, whereas
the measurements show unusually _low_ NOx ("denoxification), in
accordance with the PSC hypothesis.  The "upwelling" hypothesis
predicted upward air motion in the lower stratosphere, which is
inconsistent with measurements of atmospheric tracers such as
N2O which show that the motion is primarily downwards.

 Positive evidence for the PSC theory comes from ground-based and
airborne observations of the various chlorine-containing compounds.
These show that the reservoir species HCl and ClONO2 are extensively
depleted in the antarctic winter and spring, while the concentration
of the active, ozone-depleting species ClO is strongly enhanced.
Measurements also show enormously enhanced concentrations of the
molecule OClO. This is formed by a side-reaction in the BrO/ClO
mechanism described above.

 Further evidence comes from laboratory studies. The gas-phase
reactions have been reproduced in the laboratory, and shown to
proceed at the rates required in order for them to be important in
the polar stratosphere. [Molina et al. 1990] [Sander et al.]
[Trolier et al.] [Anderson et al.]. The production of active
chlorine from reservoir chlorine on ice and sulfuric acid surfaces
has also been demonstrated in the laboratory [Tolbert et al.
1987,1988] [Molina et al. 1987]. (Recently evidence for these
reactions has been found in the arctic stratosphere as well: air
parcels that had passed through regions where the temperature
was low enough to form PSC's were found to have anomalously
low concentrations of HCl and anomalously high concentrations
of ClO [AASE].)

 The "smoking gun", however, is usually considered to be the
simultaneous in-situ measurements of a variety of trace gases from
an ER-2 stratospheric aircraft (a converted U2 spy plane) in
August-October 1987. [Tuck et al.] These measurements demonstrated a
striking "anticorrelation" between local ozone concentrations and ClO
concentrations. Upon entering the "hole", ClO concentrations
suddenly jump by a factor of 20 or more, while ozone concentrations
drop by more than 50%. Even the local fluctuations in the
concentrations of the two species are anticorrelated. [Anderson et al.]

 In summary, the PSC theory explains the following observations:

1. The ozone hole occupies the region of the polar vortex where
   temperatures are below -80 C and where polar stratospheric clouds
   are abundant.

2. The ozone hole is confined to the lower stratosphere.

3. The ozone hole appears when sunlight illuminates the vortex, and
   disappears soon after temperatures rise past -80 C, destroying PSC's.

4. The hole is associated with extremely low concentrations of NOx.

5. The hole is associated with very low concentrations of the chlorine
   "reservoirs", HCl and ClONO2, and very high concentrations of active
   chlorine compounds, ClO, and byproducts such as OClO.

6. Inside the hole, the concentrations of ClO and ozone are precisely
    anticorrelated, high ClO being accompanied by low ozone.

7. Laboratory experiments demonstrate that chlorine reservoir compounds
   do react to give active chlorine on the surfaces of ice particles.

8. Airborne measurements in the arctic stratosphere show that air
   which has passed through regions containing PSC's is low in
   reservoir chlorine and high in active chlorine.

The antarctic ozone hole, once a complete mystery, is now
one of the best understood aspects of the entire subject; it is
much better understood than the small but steadily growing ozone
depletion at mid latitudes, for example.


8. Will the ozone hole keep growing?

 To answer this, we need to consider separately the lateral
dimensions (the "area" of the hole), the vertical dimension (its
"depth") and the temporal dimension (how long the hole lasts.)

a.) Lateral Extent

 Let us define the "hole" to be the
region where the total ozone column is less than 200 DU,
i.e. where total ozone has fallen to less than 2/3 of normal
springtime antarctic values. Defined thus, the hole is always
confined to the south polar vortex, south of ~55 degrees. At
present it does not fill the whole vortex, only the central core
where stratospheric temperatures are less than ~-80 C. Typically
this region is south of ~65 degrees, although there is a great deal
of variation - in some years the center of the vortex is displaced
well away from the pole, and the nominal boundary of the hole has
on a few occasions passed over the tip of Chile. As stratospheric
chlorine continues to rise, the hole might "fill out" the vortex;
this could as much as double its area. [Schoeberl and Hartmann]. So
far this does not seem to be happening. The 1992 hole was 15-25%
larger than previous years, and the 1993 hole appears to be almost
as large. This increase is probably due to the
stratospheric sulfate aerosols from the July 1991 eruption of Mt.
Pinatubo, which behave in some respects like polar stratospheric
clouds. [Solomon et al. 1993] These aerosols settle out of the
stratosphere after 2-3 years, so the increases seen in 1992 are
expected to be temporary. In any case, it cannot grow beyond
~55 degrees without a major change in the antarctic wind patterns
that  would allow the vortex to grow. Such a change could
conceivably accompany global warming: the greenhouse effect warms
the earth's surface, but _cools_ the stratosphere. There is no
reason to expect the hole to expand out over Australia, S. Africa,
etc., although these regions could experience further ozone
depletion after the hole breaks up and the ozone-poor air drifts
north.

b. Vertical Depth

 The hole is confined to the lower stratosphere, where the
clouds are abundant. In this region the ozone is essentially
gone. The upper stratosphere is much less affected, however, so
that overall column depletion comes to ~50%. As stratospheric
chlorine concentrations continue to increase over the next 10
years or so, some penetration to higher altitudes may take place,
but large increases in depth are not expected.  (Once again,
aerosols from Mt. Pinatubo have allowed the 1992 and 1993 holes
to extend over a larger altitude range than usual, both higher
and lower, but this is probably a temporary effect.)

c. Duration of the hole

 Here we might see major effects. The hole is destroyed in late
spring/early summer when the vortex breaks up and warm, ozone-rich
air rushes in. If the stratosphere cools, the vortex becomes more
stable and lasts longer. As mentioned above, the greenhouse effect
actually cools the stratosphere. There is a more direct cooling
mechanism, however - remember that the absorption of solar UV by
ozone is the major source of heat in the stratosphere, and is the
reason that the temperature of the stratosphere increases with
altitude. Depletion of the ozone layer therefore cools the
stratosphere, and in this sense the hole is self-stabilizing. In
future years we might see more long-lived holes like that in 1990,
which survived into early December.

 (The relationship between ozone depletion and climate change is
 complicated, and best dealt with in a separate FAQ, preferably
 written by someone other than myself :-) )


9. Why be concerned about an ozone hole over antarctica?
   Nobody lives down there.

 First of all, even though the ozone hole is confined to the
antarctic, its effects are not. After the hole breaks up in the
spring, ozone-poor air drifts north and mixes with the air there,
resulting in a transient decrease at middle and high latitudes.
This has been seen as far north as Australia [WMO 1991][Roy et al.]
[Atkinson et al.] On a time scale of months short-wave UV
regenerates the ozone, but it is believed that this "dilution" may
be a major cause of the much smaller _global_ ozone depletion, ~3%
per decade, that has been observed. Moreover, the air from the
ozone hole is also rich in ClO and can destroy more ozone as it
mixes with ozone-rich air. Even during the spring, the air in
the vortex is not _completely_ isolated, although there is some
controversy over the extent to which the ozone hole acts as
a "chemical processor" for the earth's atmosphere.
([Tuck 1989] [Schoeberl and Hartmann] [AASE] [Randel] [Waugh].)

 From a broader standpoint, the ozone hole is a distant early
warning message. Because of its unusual meteorological properties
the antarctic stratosphere is especially sensitive to chemical
perturbations; the natural mechanisms by which chlorine is
sequestered in reservoirs fail when total stratospheric chlorine
reaches about 2 parts per billion. This suggests that allowing
CFC emissions to increase by 3% per year, as was occurring during
the 1980's, is unwise, to say the least. The emission reduction
schedules negotiated under the Montreal Protocol (as revised in
1990 and 1992) lead to a projected maximum of ~4 ppb total strat.
chlorine in the first decade of the 21st century, followed by a
gradual decrease. Letting emissions increase at 3%/year would have
led to >16 ppb total stratospheric chlorine by 2040, and even a
freeze at 1980 rates would have led to >10 ppb. [Prather et al.].


10. Is there an ozone hole in the arctic? if not, why not?

 There is no _massive_ ozone loss in the arctic, although there _is_
unusually large springtime ozone depletion, so the word "hole" is
not appropriate. I like the expression "arctic ozone dimple" but
this is not canonical :-). The arctic polar vortex is much weaker
than the antarctic, arctic temperatures are several degrees higher,
and polar stratospheric clouds are much less common (and they tend
to break up earlier in the spring.) Thus even though wintertime ClO
gets very high, as high as antarctic ClO in 1991-2, it does not
remain high through the spring, when it counts. [AASE] (Recent UARS
measurements, however, indicate that in 1993 arctic stratosphere
temperatures stayed low enough to retain PSC's until late February,
and ClO remained high into March. Large ozone depletions, ~10-20%,
have now (spring 1993) been reported for high latitudes in the
Northern Hemisphere; these still do not qualify as an "ozone hole"
but they do seem to indicate that the same physics and chemistry
are operating, albeit with much less efficiency. [Waters et al.]
[Gleason et al.])

 If "global warming" does indeed take place during the first
few decades of the next century, we may see a dramatic change in
arctic ozone. The greenhouse effect warms the surface of the
earth, but at the same time _cools_ the stratosphere. Since there
is much less air in the stratosphere,  2-3 degrees of surface
warming corresponds to a much larger decrease in stratospheric
temperatures, as much as 10 degrees. This could lead to a true
ozone hole in the arctic, although it would still probably be
smaller and weaker than the antarctic hole. [Austin et al.]

The 27 August issue of _Science_ magazine contains 8 papers devoted
to arctic ozone depletion in the winter of 1991-92. [AASE]


11. Can the hole be "plugged"?

The present ozone hole, while serious, is not in itself
catastrophic. UV radiation is always low in polar regions since the
sun takes a long path through the atmosphere and hence through the
ozone layer. There may be serious consequences for marine life in
the antarctic ocean, which is adapted to the normally low UV
levels. When the hole breaks up in summer, there may be temporary
increases in UV-b at high latitudes of the southern hemisphere as
air that is poor in ozone and rich in "active", ozone-destroying
forms of chlorine mixes with the air outside.

 Nevertheless it looks like we are stuck with the hole for the
next 50 years at least, and we don't know what new surprises the
atmosphere has in store for us. Thus, some atmospheric scientists
have been exploring the possibility of "fixing" the hole by
technological means. All such schemes proposed so far are highly
controversial, and there are no plans to carry any of them out
until the chemistry and dynamics of the stratosphere are much
better understood than they are at present.

 It should be made clear at the beginning that there is no
point in trying to replace the ozone directly. The amounts are far
too large to be transported to the stratosphere, and the antarctic
mechanisms are so fiendishly efficient that they will easily
destroy added ozone (recall that where the catalytic cycles
operate, ~95% of the ozone is gone, in spite of the fact that the
sun is generating it all the time.) It is far better to try to
remove the halogen catalysts.  One suggestion made a few years ago
was to release sodium metal into the stratosphere, in hopes that it
would form sodium chloride crystals which would settle out. The
problem is that the microcrystals remain suspended as long as they
are small, and can play the same role as clouds and aerosols in
converting reservoir chlorine to active chlorine.

 A second suggestion is to destroy the CFC's while they are
still in the troposphere, by photolyzing them with high-powered
infrared lasers installed on mountainsides. (CFC's and similar
molecules can absorb as many as 30 infrared photons
from a single laser pulse, a phenomonon known as infrared
multiphoton dissociation). The chlorine atoms released would
quickly be converted to HCl and rained out. The power requirements
of such a project are daunting, however, and it appears that much
of the laser radiation would be shifted out of the desired
frequency range by stimulated raman scattering. [Stix]

 A more serious possibility is being explored by one of the
discoverers of chlorine-catalyzed ozone depletion, Ralph Cicerone,
together with Scott Elliot and Richard Turco [Cicerone et al.
1991,1992]. They considered the effects of dumping ~50,000 tons of
ethane or propane, several hundred planeloads, into the antarctic
stratosphere every spring. The hydrocarbons would react rapidly
with the Cl-containing radicals to give back the reservoir HCl. The
hydrocarbons themselves are fairly reactive and would decompose by
the end of a year, so the treatment would have to be repeated
annually. The chlorine would not actually be removed from the
stratosphere, but it would be bound up in an inert form - in other
words, the catalyst would be "poisoned". There are
no plans to carry this or any other scheme out in the near future;
to quote from Cicerone et al. (1991), "Before any actual injection
experiment is undertaken there are many scientific, technical,
legal and ethical questions to be faced, not the least of which is
the issue of unintended side effects."


REFERENCES FOR PART III

A remark on references: they are neither representative nor
comprehensive. There are _hundreds_ of people working on these
problems. For the most part I have limited myself to papers that
are (1) widely available (if possible, _Science_ or _Nature_ rather
than archival sources such as _J. Geophys. Res._) and (2) directly
related to the "frequently asked questions". This gives very short
shrift to much important work; for example, I say very little about
stratospheric NOx, even though a detailed accounting of chemistry
and transport of the nitrogen oxides is one of the major goals
of current research. Readers who want to see "who did what" should
consult the review articles listed below, or, if they can get them,
the extensively documented WMO reports.


Introductory Reading:

[Graedel and Crutzen] T. Graedel and P. Crutzen, _Atmospheric
 Change: an Earth System Perspective_, Freeman, 1993.

[Hamill and Toon] P. Hamill and O. Toon, "Polar stratospheric
clouds and the ozone hole", _Physics Today_ December 1991.

[Stolarski] Richard Stolarski, "The Antarctic Ozone Hole", _Sci.
American_ 1 Jan. 1988. (this article is now seriously out of date,
but it is still a good place to start).

[Toon and Turco] O. Toon and R. Turco, "Polar Stratospheric Clouds
and Ozone Depletion", _Sci. Am._ June 1991

[Zurer] P. S. Zurer, "Ozone Depletion's Recurring Surprises
Challenge Atmospheric Scientists", _Chemical and Engineering News_,
24 May 1993, pp. 9-18.

-----------------------------------------
Books and Review Articles:

[Anderson, Toohey and Brune] J.G. Anderson, D. W. Toohey, and W. H.
Brune,  "Free Radicals within the Antarctic vortex: the role of
CFC's in Antarctic Ozone Loss", _Science_ _251_, 39 (4 Jan. 1991).

[McElroy and Salawich] M. McElroy and R. Salawich, "Changing
Composition of the Global Stratosphere", _Science_ _243, 763, 1989.

[Solomon] S. Solomon, "Progress towards a quantitative
understanding of Antarctic ozone depletion",
_Nature_ _347_, 347, 1990.

[Wayne] R. P. Wayne, _Chemistry of Atmospheres_, 2nd.  Ed.,
Oxford, 1991, Ch. 4.

[WMO 1989] World Meteorological Organization Global Ozone Research
and Monitoring Project - Report #20, "Scientific Assessment of
Stratospheric Ozone: 1989".

[WMO 1991] World Meteorological Organization Global Ozone Research
and Monitoring Project - Report #25, "Scientific Assessment of
Ozone Depletion: 1991".

-------------------------
More Specialized:

[AASE] Papers resulting from the Second Airborne Arctic Stratosphere
 Expedition, published in _Science_ _261_, 1128-1157, 27 Aug. 1993.

[Atkinson et al.] R. J. Atkinson, W. A. Matthews, P. A. Newman,
 and R. A. Plumb, "Evidence of the mid-latitude impact of Antarctic
 ozone depletion", _Nature_ _340_, 290, 1989.

[Austin et al.] J. Austin, N. Butchart, and K. P. Shine,
"Possibility of an Arctic ozone hole in a doubled-CO2 climate",
_Nature_ _360_, 221, 1992.

[Cicerone et al. 1991] R. Cicerone, S. Elliot, and R. Turco,
"Reduced Antarctic Ozone Depletions in a Model with Hydrocarbon
Injections", _Science_ _254_, 1191, 1991.

[Cicerone et al. 1992] R. Cicerone, S. Elliot, and R. Turco,
"Global Environmental Engineering", _Nature_ _356_, 472, 1992.

[Dobson] G. M. B. Dobson, "Forty Years' research on atmospheric
ozone at Oxford", _Applied Optics_, _7_, 387, 1968.

[Farman et al.] J. C. Farman, B. G. Gardiner, and J. D. Shanklin,
"Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx
interaction", _Nature_ _315_, 207, 1985.

[Frederick and Alberts] J. Frederick and A. Alberts, "Prolonged
 enhancement in surface ultraviolet radiation during the Antarctic
 spring of 1990", _Geophys. Res. Lett._ _18_, 1869, 1991.

[Gleason et al.] J. Gleason, P. Bhatia, J. Herman, R. McPeters, P.
Newman, R. Stolarski, L. Flynn, G. Labow, D. Larko, C. Seftor, C.
Wellemeyer, W. Komhyr, A. Miller, and W. Planet, "Record Low Global
Ozone in 1992", _Science_ _260_, 523, 1993.

[Molina et al. 1987] M. J. Molina, T.-L. Tso, L. T. Molina, and
F.C.-Y. Yang, "Antarctic stratospheric chemistry of chlorine
nitrate, hydrogen chloride, and ice: Release of active chlorine",
_Science_ _238_, 1253, 1987.

[Molina et al. 1990] M. Molina, A. Colussi, L. Molina, R.
Schindler, and T.-L. Tso, "Quantum yield of chlorine atom formation
in the photodissociation of chlorine peroxide (ClOOCl) at 308 nm",
_Chem. Phys. Lett._ _173_, 310, 1990.

[Plumb] A. Plumb, "Mixing and Matching",
_Nature_ _365_, 489-90, 1993. (News and Views)

[Prather et al.] M.J. Prather, M.B. McElroy, and S.C. Wofsy,
"Reductions in ozone at high concentrations of stratospheric
 halogens", _Nature_ _312_, 227, 1984.

[Randel] W. Randel, "Ideas flow on Antarctic vortex",
_Nature_ _364_, 105, 1993 (News and Views)

[Roy et al.] C. Roy, H. Gies, and G. Elliott, "Ozone Depletion",
 _Nature_ _347_, 235, 1990. (Scientific Correspondence)

[Sander et al.] S.P. Sander, R.J. Friedl, and Y.K. Yung, "Role of
the ClO dimer in polar stratospheric chemistry: rate of formation
and implications for ozone loss", _Science_ _245_, 1095, 1989.

[Schoeberl and Hartmann] M. Schoeberl and D. Hartmann, "The
dynamics of the stratospheric polar vortex and its relation to
springtime ozone depletions", _Science_ _251_, 46, 1991.

[Solomon et al. 1993] S. Solomon, R. Sanders, R. Garcia, and J.
Keys, "Increased chlorine dioxide over Antarctica caused by
volcanic aerosols from Mt. Pinatubo", _Nature_ _363_, 245, 1993.

[Stix] T. H. Stix, "Removal of Chlorofluorocarbons from the
 earth's atmosphere", _J. Appl. Physics_ _60_, 5622, 1989.

[Stolarski et al. 1992]  R. Stolarski, R. Bojkov, L. Bishop, C.
Zerefos, J. Staehelin, and J. Zawodny, "Measured Trends in
Stratospheric Ozone",  Science _256_, 342 (17 April 1992)

[Tolbert et al. 1987] M.A. Tolbert, M.J. Rossi, R. Malhotra, and
D.M. Golden, "Reaction of chlorine nitrate with hydrogen chloride
and water at Antarctic stratospheric temperatures", _Science_
_238_, 1258, 1987.

[Tolbert et al. 1988] M.A. Tolbert, M.J. Rossi, and D.M. Golden,
"Antarctic ozone depletion chemistry: reactions of N2O5 with H2O
and HCl on ice surfaces", _Science_ _240_, 1018, 1988.

[Trolier et al.] M. Trolier, R.L. Mauldin III, and A. Ravishankara,
"Rate coefficient for the termolecular channel of the self-reaction
of ClO", _J. Phys. Chem._ _94_, 4896, 1990.

[Tuck 1989] A. F. Tuck, "Synoptic and Chemical Evolution of the
Antarctic Vortex in late winter and early spring, 1987: An ozone
processor", J. Geophys. Res. _94_, 11687, 1989.

[Tuck et al.] A. F. Tuck, R. T. Watson, E. P. Condon, and J. J.
Margitan, "The planning and execution of ER-2 and DC-8 aircraft
flights over Antarctica, August and September, 1987"
J. Geophys. Res. _94_, 11182, 1989.

[Waters et al.] J. Waters, L. Froidevaux, W. Read, G. Manney, L.
Elson, D. Flower, R. Jarnot, and R. Harwood, "Stratospheric ClO and
ozone from the Microwave Limb Sounder on the Upper Atmosphere
Research Satellite", _Nature_ _362_, 597, 1993.

[Waugh] D. W. Waugh, "Subtropical stratospheric mixing linked to
disturbances in the polar vortices", _Nature_ _365_, 535, 1993.