Antarctic ozone loss

Anderson, J. G., D. W. Toohey, and W. H. Brune, Free Radicals within the Antarctic Vortex The Role of CFC s in Antarctic Ozone Loss, Science, 251, 39-46 (1991). 

F. S. Rowland, Chlorofluorocarbons and the depletion of stratospheric ozone Am. Sci. 77, 36-45 (1989) T.-L. Tso, L. T. Molina, and F. C.-Y. Wang, Antarctic stratospheric chemistry of chlorine nitrate, hydrogen chloride and ice release of active chlorine. Science 238, 1253-1260 (1987) J. G- Anderson, D. W. Toohey, and W. H. Brune, Free radicals within the Antarctic vortex the role of CFCs in Antarctic ozone loss. Science 251, 39-46 (1991) P. S. Zurer, Complexities of ozone loss continue to challenge scientists. Chem. Eng. News June 12, 20-23 (1995). 

Current wisdom can be summarized in the flow chart of Fig. 3. More specifically, the CIO radical is involved in several possible kinetic mechanisms linking global release of chlorofluorocarbons (CFCs) to the Antarctic ozone loss during each austral spring (18). An accurate characterization of the spectroscopic and other properties of CIO is therefore vital, not least as a prelude to tracing the crucial correlation between CFCs, CIO, and 0 j. CIO is short-lived at the high molecular concentrations characteristic of the condensed phases but can be generated at low pressures in the gas phase, for example by the action of... 

Atmospheric measurements of CIO, BrO, 03, and NzO confirm the importance of reactions (43a)-(45) in the destruction of 03. For example, Anderson et al. (1989) showed that this cycle is the largest contributor to ozone loss in the Antarctic vortex from 14-18 km. 

Evidence for the contribution of the CIO + BrO interaction is found in the detection and measurement of OCIO that is formed as a major product of this reaction, reaction (31a). This species has a very characteristic banded absorption structure in the UV and visible regions, which makes it an ideal candidate for measurement using differential optical absorption spectrometry (see Chapter 11). With this technique, enhanced levels of OCIO have been measured in both the Antarctic and the Arctic (e.g., Solomon et al., 1987, 1988 Wahner and Schiller, 1992 Sanders et al., 1993). From such measurements, it was estimated that about 20-30% of the total ozone loss observed at McMurdo during September 1987 and 1991 was due to the CIO + BrO cycle, with the remainder primarily due to the formation and photolysis of the CIO dimer (Sanders et al., 1993). The formation of OCIO from the CIO + BrO reaction has also been observed outside the polar vortex and attributed to enhanced contributions from bromine chemistry due to the heterogeneous activation of BrONOz on aerosol particles (e.g., Erie et al., 1998). 

Second, the northern polar vortex is much less stable and hence less isolated from mixing with external air masses compared to the Antarctic case events in January and February in which there was substantial mixing of air from midlatitudes into the vortex have been reported (e.g., see Browell et al., 1993 Plumb et al., 1994). This makes it particularly important to make both measurements and model predictions with sufficient resolution (Edouard et al., 1996). In addition, the Arctic polar vortex tends to break up earlier than the Southern Hemisphere polar vortex since ozone destruction is determined to a large degree by the extent of exposure to sunlight, the earlier breakup and mixing with air external to the vortex cuts the ozone loss short. 

Indeed, these reactions play an important role in the Antarctic ozone hole and they have important implications for control strategies, particularly of the bromi-nated compounds. For example, Danilin et al. (1996) examined the effects of ClO -BrO coupling on the cumulative loss of O-, in the Antarctic ozone hole from August 1 until the time of maximum ozone depletion. Increased bromine increased the rate of ozone loss under the denitrified conditions assumed in the calculations by converting CIO to Cl, primarily via reactions (31b) and (31c) (followed by photolysis of BrCl). Danilin et al. (1996) estimate that the efficiency of ozone destruction per bromine atom (a) is 33-55 times that per chlorine atom (the bromine enhancement factor ) under these conditions in the center of the Antarctic polar vortex, a 60 calculated as a global average over all latitudes, seasons, and altitudes (WMO, 1999). 

While the growth in stratospheric chlorine should clearly be slowed by the Montreal Protocol agreements, it was still substantial and expected to lead to quite large losses of ozone. This recognition, bolstered by the dramatic appearance of the Antarctic ozone hole, led to further major amendments to the Montreal Protocol. 

It is clear from the data presented in this chapter that the effects of control strategies developed for CFCs and halons are already measurable. Although loss of stratospheric ozone with accompanying increases in ultraviolet radiation in some locations have clearly occurred, the tropospheric concentrations of CFCs are not increasing nearly as fast as in the past. Indeed, the concentrations of CFC-11 and CFC-113 appear to have peaked and have started to decline. The equivalent effective stratospheric chlorine concentrations are predicted to have peaked about 1997 and to return to levels found around 1980 at about the year 2050 (World Meteorological Organization, 1995). The significance of the 1980 level is that these levels resulted in detectable Antarctic ozone depletion. 

There is already one excellent example of our failure to make such a predictive leap—the Antarctic ozone hole. The reason for the failure to anticipate the rapid loss of ozone in the lower stratosphere was a failure to appreciate the potential role of the subtle photochemistry, in particular, the heterogeneous chemistry. Nor did researchers have a full appreciation for the consequences of the air parcels inside the polar vortices being relatively isolated from midlatitude air. Some of these same issues are important in the Arctic region in wintertime, but researchers lack the predictive capability to determine how ozone will ultimately be affected. 

However, even if such measurements were possible, would the uncertainty of the result be small enough to establish that production does indeed balance observed loss of ozone The calculation of ozone loss in the Antarctic ozone hole was shown to have an uncertainty of 35 to 50%. The uncertainty for analyzing whether production balances loss in the midlatitude stratosphere is similarly 35 to 50%. About half of the uncertainty is in the measurements of stratospheric abundances, which are typically 5 to 35%, and half is in the kinetic rate constants, which are typically 10 to 20% for the rate constants near room temperature but are even larger for rate constants with temperature dependencies that must be extrapolated for stratospheric conditions below the range of laboratory measurements. In addition to uncertainties in the photochemical rate constants, there are those associated with possible missing chemistry, such as excited-state chemistry, and the effects of transport processes that operate on the same time scales as the photochemistry. Thus, simultaneous measurements, even with relatively large uncertainties, can be useful tests of our basic understanding but perhaps not of the details of photochemical processes. 

A third issue is the estimates of ozone loss associated with the polar vortex. Will the Antarctic ozone hole expand and will the Arctic ozone hole begin Can the loss of ozone be better quantified, and what are the zonal asymmetries in the CIO and BrO fields How much additional ozone is lost when the polar vortex breaks up ... 

The chemical processes involved in depletion of lower stratospheric ozone are now fairly well understood [8]. However, 3-dimensional chemical transport models still under-predict ozone loss in the Arctic, where the winterly polar vortex is less stable compared to its Antarctic counterpart, temperatures in the lower... 

Roscoe H. K., Kreher K., and Friess U. (2001) Ozone loss episodes in the free Antarctic troposphere, suggesting a possible climate feedback. Geophys. Res. Lett. 28,... 

In 1984, a remarkable and totally unpredicted phenomenon was discovered by the British Antarctic Survey, the so-called ozone hole. The discovery of ozone depletion over Antarctica during the spring period provided the first observational support for the possible effect of CFCs on the stratospheric ozone. However, the observation of ozone loss did not indicate its cause. From 1984 to 1988, several theories were postulated from CFC chemistry to atmospheric dynamics or even to cosmic electron fluxes. It was not until 1988, with the results of the 1987 Airborne Antarctic Ozone Expedition, that a probable link with CFCs was established. This prompted the Natural Resources Defense Council, an American pressure group, to sue the EPA to fulfill its 1980 promise to seek legislation to further control the manufacture and use of CFCs in the United States of America. 

Ozone loss over the Arctic has been less dramatic than that over the Antarctic, mainly because the different distribution of land and sea in the Northern Hemisphere allows for only a weak vortex over the Arctic. There is more mixing of air with that from lower latitudes and temperatures do not become low enough for routine formation of polar stratospheric clouds. In years when the Arctic has been cold enough for cloud formation similar ozone destruction has been observed, but for less prolonged periods than over Antarctica. Trends in ozone over the rest of the globe have been small compared to those of the Antarctic, or even Arctic, and are quantified in section 2.4.1. 

Based on measurements of the total column ozone content of the atmosphere from the ground as well as from satellites, a consistent picture of the current loss of stratospheric ozone can be derived. The most recent results are discussed in ref. [3]. Relative to the values in the 1970 s, the ozone loss at the end of the 1990 s is estimated to be about 50% in the Antarctic spring, where the ozone hole appears every year, and about 15% in the Arctic spring. In the mid-latitudes of the Southern hemisphere the loss is about 5% all the year round, while in the Northern hemisphere it is about 6% in winter/spring and about 3% in sum-mer/fall. No significant trend in ozone has been found in the Equatorial regions. In the second half of the 1990 s relatively little change in ozone has been observed in the mid-latitudes of both hemispheres. 

Figure 6.11 shows measurements of the seasonal cycle of ozone at Halley in historical and recent data, which show that the depletion occurs only over a limited portion of the year. These observations demonstrate that contemporary observations of ozone at Halley in late August (end of austral winter) are near historical levels, while the bulk of the ozone loss there occurs rapidly during the month of September. In recent years, measurable ozone depletion is also observed in Antarctic summer — in part the result of dilution of the extreme losses in spring. 

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