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Uncertainty
in polar ozone depletion?
28 September 2007 Real Climate http://www.realclimate.org
Guest commentary by Drew Shindell
NASA
The unique chemistry that causes
dramatic ozone depletion in the polar springtime lower stratosphere has been
studied intensely for the past 2-3 decades and much that was speculated about 30
years ago when the problem first emerged has been verified and made more
coherent. However, a new report concerning laboratory measurements of a key
molecule involved in this chemistry have raised questions about current
understanding. The results (Pope et al., J. Phys. Chem., 2007) suggest a reduced
ability for sunlight to break apart the chlorine monoxide dimer (Cl2O2) and have
already led to a great deal of debate about their implications. I'll try here to
help assess what these new measurements really mean.
The past decades of study have
developed a comprehensive understanding of how polar ozone depletion (“Ozone
Holes”) takes place. In brief, human-produced halocarbons (chlorofluorocarbons
(CFCs) and a few other molecules like methyl bromide) are broken down by
sunlight in the stratosphere, releasing chlorine and bromine. These highly
reactive atoms mostly go into fairly long-lived molecules that are not very
reactive and therefore act as ‘reservoirs’. There are two situations in
which a substantial amount of chlorine, the more important of the two, can come
out of the reservoirs in large enough amounts to destroy a substantial amount of
ozone. One is in the upper stratosphere around 40-50 km altitude, where strong
sunlight forms reactive molecules that frees the chlorine. The other is the
polar springtime lower stratosphere, where extremely cold temperatures lead to
unique chemistry on the surface of ice particles that again transforms chlorine
from its reservoirs into more reactive forms.
Atmospheric observations show
that in both these situations, there is indeed enhanced reactive chlorine and
simultaneous depletion of ozone. Measurements from satellites, aircraft, and
ground-based instruments all give independent, consistent information verifying
the links between cold temperatures in the polar springtime lower stratosphere
and chlorine, and between chlorine and ozone. It's important to note that none
of the laboratory data on the direct chemical reactions that destroy ozone have
been questioned. What has now been questioned is not the link between the
chlorine released from CFCs and ozone loss, but rather the rate at which the
chlorine atoms can destroy ozone via a particular cycle involving the Cl2O2
molecule.
Measurements of this molecule are
exceedingly difficult to make in the laboratory as it is highly unstable.
Several earlier measurements of the relevant rate have shown variations of a
factor of 3 or so, so that the uncertainty in the rate is not new. However, we
have substantial auxiliary evidence for what the rates must be i.e. observations
of chlorine in the atmosphere provide independent constraints on Cl2O2. Limited
direct observations of Cl2O2, as well as many measurements of total chlorine and
of chlorine monoxide (ClO), constrain the amount of Cl2O2 (which can’t be
greater than the total minus the amount in the ClO molecule). These observations
are inconsistent with both the new measurements and earlier reports of a reduced
ability of sunlight to break up Cl2O2 (Shindell and de Zafra, GRL, 1995, 1996;
Stachnik et al., GRL, 1999; Stimpfle et al., JGR, 2004). Thus although the
current state of knowledge is that the laboratory measurements on the stability
of the Cl2O2 molecule vary by roughly a factor of 10 (including the newly
reported values), the independent measurements suggest strongly that the upper
half of that range is more likely to be correct, not the lower.
Given the difficulty in making
the laboratory measurements, it is quite possible that these are wrong, and
confirmation of the new results is certainly needed. Should the results hold up,
the chemistry involved in polar ozone loss may need to be re-evaluated. As there
are other cycles that do not involve the Cl2O2 molecule but cause similar
dramatic ozone depletion, such as cycles including both ClO and BrO (its
bromine-containing analogue), any revision to current understanding would most
likely simply shift the relative importance of the various ozone-destroying
cycles. However, as noted, it is not clear how one would reconcile these
measurements with actual atmospheric observations, which are not consistent with
a more stable Cl2O2 molecule.
A wealth of observational data
supports the role of chlorine and bromine in polar ozone loss, and uncertainty
in a single step of the relevant chemistry does not undermine the Montreal
Protocol controlling substances that release these atoms into the stratosphere.
It is important, however, that the new results be tested so that we can be
confident we understand the potential effects of future changes in temperature
on polar ozone loss (as different chemical reactions have different
sensitivities to temperature). This will allow us to better understand the
effects of climate change on the stratospheric ozone layer, and to verify the
effectiveness of the Montreal Protocol, which has already shown signs of success
in reducing the growth of atmospheric concentrations of CFCs, and seems to have
lead to at least a leveling off of ozone depletion over most of the planet. Full
recovery is not expected for a few decades though.
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