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12 What is the evidence for the present theory?




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This article is from the Ozone Depletion: The Antarctic Ozone Hole FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.

12 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.

 

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