Solomon, Susan

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1-71513
On the depletion of Antarctic ozone
Susan Solomon, NOAA Aeronomy Laboratory, Boulder, CO
NASA trends workshop, Boulder, CO, March 5-7, 1986
Farman et al. (1985) have shown that total ozone abundances in
Antarctica have decreased substantially (about 30-50%) in the spring in
recent years. Satellite observations (Krueger and Stolarski, 1985) have
revealed similar depletions over a very extended region in general
agreement with Farman et al.'s ground based data. These large depletions
seem to be confined to high latitudes during Antarctic spring; no such
dramatic depletions are observed in Antarctica during other seasons, nor
in the Northern hemisphere.
Further information regarding the vertical profile of the ozone
perturbation is essential to an understanding of the origin of the observed
total column changes. Antarctic balloonsonde data are available from the
Syowa (695, 39E) ozonesonde station for several years in the late 1960's
and early 1970's, and for 1982. These data show that the ozone
perturbation occurs during August and September, (but not during the polar
ni-ght period) and that the largest changes in the vertical profile occur
near 15-20 km, where the observed percentage changes are about 30-50%.
At these low altitudes, the ozone photochemical time scale is of the order
of several years using presently accepted homogeneous photochemistry. If
the observed depletions are the result of a chemical process, then the said
process must obey the following constraints: 1) it must be essentially
confined to the southern hemisphere, 2) it must be a fairly strong function
of latitude, 3) it must produce an acceleration of the ozone chemical loss
rate from a time scale of years to a few months near 15-20 km, and 4) it
must increase with time over the past twenty years or so. Constraint 4)
clearly suggests that we examine the chemistry of atmospheric chlorine.
At the low altitudes where the ozone perturbation is observed, chlorine
can (in principle) destroy ozone through the process
OH + 03 -> H02 + 02
Cl + 03 -> CIO + 02 .
H02 + CIO -> HOC1 + 02
HOC1 + h v -> OH * Cl
Net: 2 03 -> 302
and by (McElroy et al., 1986):
https://ntrs.nasa.gov/search.jsp?R=19910073958 2020-03-17T14:52:56+00:00Z
CfO + BrO -> Cl + Br + 02
In practice, however, these processes are not very effective near 20 km
for presently accepted photochemistry since very little of the available
chlorine is expected to be present in the form of CIO at these altitudes.
Rather, the bulk of the available chlorine resides in the form of the
reservoirs, HC1 and C10N02 for presently accepted chemistry. Indeed, each
of these reservoirs is predicted to be present in about 100 times greater
abundance than CIO near 20 km. To appreciably enhance the density of CIO,
the equilibrium between CIO and its reservoirs must be dramatically
disturbed. If chlorine chemistry is the cause of the Antarctic ozone
phenomenon, then the crux of the problem is the identification and
validation of a process that can perturb these equilibria sufficiently to
allow CIO to become the major chlorine species at 20 km. Under these
circumstances (le., CIO abundances of the order of ppbv; HC1 and C10N02
greatly reduced from typical levels), the calculated ozone changes would
be comparable to those observed.
It is likely that the reaction between HC1 and C10N02 takes place
rapidly on surfaces (Rowland, 1984):
HC1 + C10N02 -> HN03 + C12
If this reaction is heterogeneous, it presumably takes place in the
stratosphere only in the presence of stratospheric aerosol, and indeed, it
is well known that the stratospheric aerosol surface area increases
substantially in the vicinity of polar stratospheric clouds. Furthermore,
these clouds are very common in the cold environment of Antarctic winter
and early spring near 10-20 km, where their occurence frequency is far
greater than in the warmer Northern hemisphere (McCormick, 1983),
suggesting that this process should be enhanced specifically in Antarctic
spring. It will be shown that this reaction can lead to ozone depletions
that are similar to those observed insofar as their vertical, latitudinal,
and temporal characteristics are concerned. Furthermore, the
implications of this process for the behavior of NOo and HMO? are
^_ «•'
consistent with observations by McKenzie and Johnston (1984) and
Williams et al. (1982), providing an independent check on inferences drawn
from ozone observations. This and other mechanisms capable of producing
the observed changes in ozone will be discussed and contrasted.
Implications for other species in Antarctic spring will be described.
References
Farman, J. C., Gardiner, B. G., and J. D. Shanklin, Nature, 315, 207-210
1985.
Krueger, A. J., and R. 5. Stolarski, Trans. AGU, 66, 838,1985.
McCormick, M. P., Adv. Space Res., 2, 73-86,1983.
McElroy, M. B., Salawitch, R. J., Wofsy, 5. C., and J. A. Logan, submitted to
Nature, 1986.
McKenzie, R. L, and P. V. Johnston, Geophys. Res. Lett, 11, 73-75,1984
Rowland, F.S., paper presented at NASA meeting on the stratosphere,
Feldafing, 1984
Williams, W. J., Kosters, J. J, and D. G. Murcray, J. Geophys. Res, 87,
8976-8980, 1982.


On the depletion of Antarctic ozone

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