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Ozone Depletion FAQ Part III: The Antarctic Ozone Hole |
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Archive-name: ozone-depletion/antarctic
Last-modified: 16 Dec 1997
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Subject: Copyright Notice
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* Copyright 1997 Robert Parson *
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-----------------------------
Subject: General Information about this part
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.
-----------------------------
Subject: Caveats, Disclaimers, and Contact Information
| 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. On the other hand, if you find
| this document in an online archive somewhere, I am not responsible for
| any *other* information that may happen to reside in that archive.
| In general 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)
rparson@spot.colorado.edu
Robert.Parson@colorado.edu
-----------------------------
Subject: TABLE OF CONTENTS
How to get this FAQ
Copyright Notice
General Information about this part
Caveats, Disclaimers, and Contact Information
TABLE OF CONTENTS
1.) What is the Antarctic ozone hole?
2.) Where can I find pictures of the ozone hole on the net?
3.) How big is the hole, and is it getting bigger?
4.) When did the hole first appear?
5.) How far back do antarctic ozone measurements go?
6.) But I heard that Dobson saw an ozone hole in 1956-58...
7.) Why is the hole in the Antarctic?
a.) The Polar Vortex
b.) Polar Stratospheric Clouds ("PSC")
c.) Reactions On Stratospheric Clouds
d.) Sedimentation and Denitrification
e.) Photolysis of active chlorine compounds
f.) Catalytic destruction of ozone by active chlorine
8.) What is the evidence for the present theory?
9.) Will the ozone hole keep growing?
a.) Lateral Extent
b.) Vertical Depth
c.) Duration of the hole
10.) Why be concerned about an ozone hole over antarctica?
11.) Is there an ozone hole in the arctic?
12.) Can the hole be "plugged"?
REFERENCES FOR PART III
Introductory Reading
Books and Review Articles
More Specialized References
-----------------------------
Subject: 1.) What is the Antarctic ozone hole?
For the past decade or so, ozone levels over Antarctica have fallen
to abnormally low values between 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.
-----------------------------
Subject: 2.) Where can I find pictures of the ozone hole on the net?
The best-known images are those from the TOMS instrument on the Nimbus-7
satellite. These are available at http://jwocky.gsfc.nasa.gov/multi/multi.html
Gifs, sound files, and a short movie (mpeg or quicktime) are included.
Some other Web sites that carry ozone hole images include the International
Centre for Antarctic Information and Research (ICAIR) in New Zealand:
http://icair.iac.org.nz/ozone/index.html
the JPL images archive:
http://www.jpl.nasa.gov/archive/images.html
(Look for the file ozone93b.gif. If you do not have web access you can get it
by anonymous ftp to ftp.jpl.nasa.gov in the directory pub/images/browse.)
and a gopher menu at the University of Michigan:
gopher://blueskies.sprl.umich.edu/11/
The vertical distribution of ozone in the hole is shown dramatically in
a series of images archived at:
http://www.awi-bremerhaven.de/MET/Neumayer/ozone.html
Some plots showing how the size and depth of the ozone hole has changed from
year to year can be found at the EASOE home page:
http://www.atm.ch.cam.ac.uk/images/easoe/
The Antarctic Ozone Hole was discovered by the British Antarctic Survey,
and more information can be found on their web page:
http://www.nbs.ac.uk/public/icd/ozone_pub/index.html
A movie illustrating the dynamics of the antarctic vortex can be found at
http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/UARS_project.html
Lenticular Press publishes a multimedia CD-ROM (for Apple Macintosh)
containing ozone data and images, as well as a hypertext document similar
to this FAQ. For sample images and information about ordering the CD,
see http://www.lenticular.com/ Note that these samples are copyrighted
and may not be further distributed.
-----------------------------
Subject: 3.) 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 1992-95 was larger still. In 1987 and 1989-95 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.].
-----------------------------
Subject: 4.) 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, an ozone decline was seen at
the Japanese antarctic station of Syowa; this was less dramatic than
those seen at Halley since 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]
-----------------------------
Subject: 5.) 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 Nimbus-7 TOMS,
which finally broke down on 7 May 1993, is the source for most of
the pretty pictures that one sees in review articles as well as the
popular press. (See http://jwocky.gsfc.nasa.gov/). Today
there are several satellites monitoring ozone and other atmospheric
gases; instruments on NASA's UARS (Upper Atmosphere Research Satellite)
simultaneously measure ozone, chlorine monoxide (ClO), and
stratospheric pressure and temperature.
-----------------------------
Subject: 6.) 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 a review article by Dobson.
In his historical account [Dobson 1968b], 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 1968b] [WMO 1989]
For those interested, here is how springtime antarctic
ozone has developed from 1956 to 1995:
..............................................................
Halley Bay Antarctic Ozone Data
Mean October ozone column thickness, Dobson Units,
as measured at the British Antarctic Survey station
at Halley Bay (Latitude 76 south, Longitude 26 west)
1956 321 1971 299 1986 248
1957 330 1972 304 1987 163
1958 314 1973 289 1988 232
1959 311 1974 274 1989 164
1960 301 1975 308 1990 179
1961 317 1976 283 1991 155
1962 332 1977 251 1992 142
1963 309 1978 284 1993 111
1964 318 1979 261 1994 124
1965 281 1980 227 1995 129
1966 316 1981 237 1996 139
1967 323 1982 234 1997 139
1968 301 1983 210
1969 282 1984 201
1970 282 1985 196
Data from J. D. Shanklin, British Antarctic Survey, personal
communications, 1993-95. For published graphs, see
[Jones and Shanklin], [Hamill and Toon], [Solomon], or
[WMO 1991], p. 4.6. The original Farman et al. figure, which
includes the years 1957-84, is available on the web at
http://www.atm.ch.cam.ac.uk/images/easoe/total_ozone.gif
An updated version can be found on the British Antarctic Survey
web site: http://www.nbs.ac.uk/public/icd/jds/ozone/
Bulletins showing daily ozone measurements at Halley Bay and
at Faraday Station can also be obtained through this web site.
-----------------------------
Subject: 7.) 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:
a.) 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]. For a rather short movie (mpeg format) illustrating it,
go to http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/UARS_project.html.
There is some controversy about just how isolated
the air in the vortex is. Some believe that 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].
b.) 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.
c.) Reactions On 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"). [Crutzen and Arnold]
[Hamill and Toon]
d.) 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 language :-).
e.) 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.
f.) Catalytic destruction of ozone by active chlorine
As discussed in Part I, Cl and ClO can form a catalytic cycle that
efficiently destroys ozone. That cycle used free oxygen atoms,
however, which are only abundant in the upper stratosphere; it
cannot explain the ozone hole which forms in the lower stratosphere.
Instead, the principal mechanism involves chlorine peroxide, ClOOCl
(often referred to as the "ClO dimer") [Molina and Molina]:
ClO + ClO -> ClOOCl
ClOOCl + hv -> Cl + ClOO
ClOO -> 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 [McElroy et al. 1986]:
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 ~20% of the antarctic
ozone depletion. [Anderson et al.] Additional mechanisms have
been suggested, but they seem to be less important. [WMO 1991]
Since the reactions above photochemical steps, ozone depletion
begins when sunlight returns to the vortex in late winter or
early spring. Indeed, the hole forms "from the outside in": ozone
destruction begins in midwinter at the outer edge of the vortex,
and works its way in towards the pole as the sun gets higher.
[Roscoe et al. 1997]
The above description is highly schematic. For a thorough presentation,
see [Solomon], [McElroy and Salawich], or [WMO 1989, 1991, 1994].
-----------------------------
Subject: 8.) 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 (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 levels of
Nitrogen oxides (NOx), whereas the measurements show unusually _low_
NOx, 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" 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 ozone hole, ClO concentrations
suddenly jump by a factor of 20 or more, while ozone concentrations
drop by more than 50%. Even local fluctuations in the concentrations
of the two species are tightly correlated. [Anderson et al.]
The correlation is quantitatively accurate: from the measured local
ClO concentrations together with reaction rate constants measured
in the laboratory, one can calculate ozone destruction rates which
agree well with the measured ozone concentrations.
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 of byproducts such as OClO.
6. Inside the hole, the concentrations of ClO and ozone are precisely
anticorrelated, high ClO being accompanied by low ozone. The
correlation is quantitatively accurate.
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 polar 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.
-----------------------------
Subject: 9.) 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.)
-----------------------------
Subject: 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. If stratospheric chlorine continued to rise, the
hole could fill the entire vortex, which could as much as double its
area. [Schoeberl and Hartmann] [Schoeberl et al. 1996]. However, while
stratospheric chlorine is still rising its rate of increase has slowed
dramatically and it is expected to peak before the end of the century.
In any case, it cannot grow beyond ~55 degrees without a major change in
the antarctic wind patterns that would allow the vortex itself to grow.
There is no reason to expect the hole to expand out over Australia,
or South Africa, although these regions could experience further ozone
depletion after the hole breaks up and the ozone-poor air drifts north.
-----------------------------
Subject: 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. (The sulfate
aerosols from the eruption of Mt. Pinatubo in 1991 allowed the
1992 and 1993 holes to extend over a larger altitude range than
usual, both higher and lower [Hofmann et al. 1994, 1995].)
-----------------------------
Subject: c.) Duration of the hole
Here we might see important 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.
[Jones and Shanklin] In future years we might see more long-lived
holes like that in 1990, which survived into early December. Also,
with more chlorine in the stratosphere the hole may open earlier
in the season, as has happened between 1993 and 1996.
(The relationship between ozone depletion and climate change is
complicated; for an introduction see [WMO 1991].)
-----------------------------
Subject: 10.) 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
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