Guest commentary by Drew Shindell
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.