Some aspects of the interaction between chemical and dynamic processes relating to the antarctic ozone hole

Adv. Space Res. Vol. 13, No. 1, pp. (l)311—(l)319, 1993 Printed inGreat Britain.

02731177/93 $15.00 1992 COSPAR

SOME ASPECTS OF THE INTERACTION BETWEEN CHEMICAL AND DYNAMIC PROCESSES RELATING TO THE ANTARCTIC OZONE HOLE R. S. Eckman,* R. E. Turner,* W. T. Blackshear,* T. D. A. Fairlie** and W. L. Grose* *

Atmospheric Sciences Division, NASA Langley Research Center, Mail Stop

401B, Hampton, VA 23665-5225, U S. A. ** Science and Technology Corporation, 101 Research Drive, Hampton, VA 23666, U S. A.

ABSTRACT Observational and modeling studies have been conducted to examine the interaction between the chemical and dynamical processes that occur during springtime in the lower stratosphere of the Southern Hemisphere. The temporal evolution of the ozone distribution and the circulation during 1987 is contrasted with that for 1988 as an illustrative example of how dynamical processes and the resulting meteorological conditions modulate the ozone depletion. Concurrently with the observational analysis, an effort was initiated to simulate the ozone depletion during austral spring using a three-dimensional chemical/transport model. The model includes a parameterized representation of the heterogeneous processes thought to be important in this region. The simulation indicates that the inclusion of this additional chemistry, which results in the release of free chlorine and the redistribution of odd nitrogen into reservoir species, reproduces many aspects of the observations. While significant uncertainties and difficultiesremain in order to include heterogeneous chemistry in stratospheric models in a self-consistent manner, the preliminaiy results are encouraging and provide the impetus for improving current models. INTRODUCTION Observations from ground-based instruments of significant and unexpected depletions of column ozone above Antarctica during spring /1/ have caused an unprecedented effort to understand this phenomenon. Substantial data /2,3,4/ from ground-based and airborne expeditions in the polar regions of both hemispheres have led to a consensus that the observed ozone reductions are predominantly the result of chlorine-catalyzed chemistry taking place on cloud surfaces in regions of the lower stratosphere. Previously, such reactions were considered of minimal importance in relation to stratospheric chemistry. Although the primary cause of the Antarctic ozone depletion is thought to be chemical, it is apparent that the variability of dynamical processes act to modulate the phenomenon /5,6,7/. Furthermore, radiative/dynamical coupling may be important /8/. Thus, a general understanding exists, but considerable uncertainty remains in achieving a quantitative understanding of the phenomenon and the interactions between chemical, radiative, and dynamical processes /9/. This paper has two purposes: (1) to illustrate some aspects of the interaction between chemistry and dynamics in modulating ozone depletion by contrasting the evolution of the ozone distribution and the circulation for 1987 with that of 1988 using observations from satellite-borne instruments and (2) to present the results of a simulation conducted with a three-dimensional chemical/transport model which incorporates parameterized heterogeneous chemical processes believed responsible for the ozone depletion. Results from the simulation will be discussed and compared with observations. There are still considerable uncertainties and difficulties in parameterizing heterogeneous chemical processes in stratospheric models in a self-consistent manner. The parameterization presented here is an initial attempt to characterize the general processes in a computationally efficient manner, while retaining much of the relevant physics. Further, it should be emphasized that the model simulation reported on here is not intended to be representative of ozone depletion in a specific year. POLAR OZONE IN THE SOUTHERN HEMISPHERE DURING 1987 AND 1988 The ozone data used in this study have been obtained from the Total Ozone Mapping Spectrometer (TOMS) aboard the Nimbus 7 satellite. Details of the TOMS data are given in /10/. Maps of temperature have been derived from daily analyses of geopotential height produced routinely at the U.K. Meteorological Office. The analyses are based on radiosonde data and radiance measurements made by stratospheric sounding units (SSU’s) aboard NOAA satellites/ill.

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The evolution of polar ozone in the stratosphere of the Southern Hemisphere during spring 1987 was strikingly different from that in 1988, as shown by time series of TOMS data in Figure 1. In 1987, the ozone “hole” was the deepest and most persistent on record /12/. Minimum values of column ozone fell below 150 DU in the middle of September and remained so until the end ofOctober. Values as low as 109 DU were recorded in early October. Not until late November were values more typical of springtime (240 DU or so) recovered (data not shown). In early September 1988, indications were that ozone was being destroyed at a rate similar to that which was observed at the same time in 1987. However, minimum values stabilized between 170 and 180 DU in middle to late September and recovered to 240 DU by the end of October, almost a month earlier than was the case in 1987. Figure 2 shows synoptic maps of total column ozone near the times in each year when ozone was most depleted. They indicate that the 1987 ozone hole was not only deeper, but much more extensive than that of 1988. The relative shallowness of the 1988 ozone hole did not indicate that springtime values of polar ozone were on the road to long-term recovery. The 1989 ozone hole was almost as deep as that of 1987 /13/. Instead, different meteorological conditions in 1987 and 1988 appear to have had a crucial influence on the extent of ozone depletion. During winter and spring 1987, the polar lower stratosphere was very cold and relatively undisturbed dynamically /14/. A synoptic map of temperature for 5 September 1987 [Figure 3(a)] shows conditions representative of late winter/early spring in the lower stratosphere in that year. A large, cold pool of air with temperatures as low as 187°Kwas centered over the polar cap. The stratospheric polar vortex which coincided with the cold air was surrounded by relatively strong gradients of potential vorticity, PV (not shown). Such conditions favor the presence of polar stratospheric clouds (PSC’s), which exist at temperatures below about 197°Kfor pressures typical ofthe lower polar stratosphere and upon which the anomalous chemical reactions that precondition the air for ozone destruction take place /15/. The SAM II instrument recorded an unusually large number of sightings of PS C’s at 16-km--over 200 in September 1987 and almost 80 in October /16/. Strong PV gradients that surrounded the vortex in 1987 tend to resist transport into the polar region of air rich in ozone and odd nitrogen from lower latitudes /5/; this suggests that chemical reactions within the vortex occurred in relative isolation. When sunlight returned to polar latitudes in September, total ozone values fell sharply as described above and shown in Figure 1. In late winter and spring 1988, planetary wave disturbances were more prevalent in the stratosphere of the Southern Hemisphere, the vortex was commonly displaced from the pole, and polar temperatures were higher than in 1987. At the end of August 1988, a planetary wave disturbance raised minimum temperatures in the lower stratosphere about 8°Kand weakened gradients of potential vorticity /5/. Typically, warmer conditions existed over the polar cap in early September 1988 [Figure 3(b)] than for 1987 [Figure 3(a)]. The polar vortex was smaller, and PV gradients were weaker (not shown). Fewer PSC’s were observed--sightings were down more than 50 percent in September and 75 percent in October compared with those made during the same months in 1987 /16/. Ozone and meteorological data from 1987 and 1988 suggest that dynamical variability was instrumental in modulating the depletion of ozone during springtime in the stratosphere of the Southern Hemisphere. Furthermore, it is plausible that reduced radiative heating due to reduced levels of ozone helped sustain low polar temperatures in 1987 and delay the transition to summer conditions until the end ofNovember, much later than usual /14/. Thus, a qualitative understanding of the interplay between dynamics, chemistry, and radiation in the polar stratosphere exists. However, a detailed quantitative analysis is precluded both by the uncertainties inherent in global satellite data sets (e.g. uncertainties arising from calibration, inversion, resolution, and sampling) and by the lack of a full complement of the necessary data (i.e., winds and other constituents). Simulation studies conducted with a model incorporating the relevant physical and chemical processes are a logical adjunct to the observational studies. In particular, a three-dimensional chemistry/transport model is ideally suited for enhancing interpretation of the ozone hole phenomena. In following sections, such a model is briefly described, and preliminary results from a simulation conducted with the model are presented. MODEL DESCRIPTION The Langley Research Center three-dimensional chemistry/transport model has been described previously /17,18/. For the purposes of this study, several modifications were made to account for chemistry occurring during the polar night. The number of vertical levels in the model has been increased from 12 to 24, providing for increased resolution. The model was configured to transport explicitly eight families or species: Ox, NOy, HNO3, Clx, N205, H202, HC1, and C1ONO2. The addition of the two chlorine species allows for their unambiguous determination at all times and locations. Prior to their inclusion, the concentrations of the members of the odd chlorine family remained indeterminant at night. In addition, the chemistry of the dimer, C1202, believed to be of prime importance in the lower stratospheric chlorine-catalyzed ozone destruction, is included /19/. In the current simulation, three heterogeneous reactions are included: N205 + H20(s) — 2HN03(s) C1ONO2 + H2O(s)—~HOCI + HNO3(s) C1ONO2 + HCl(s)—~C12

+

HNO3(s).

5 sec~/20,21/. This All three rate reactions are parameterized with anrate effective first-order rate constant of 4.6 x i0 effective is a simplification of the true of a gas-surface reaction which is characterized by a sticking

Antarctic Ozone Hole Studies

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coefficient or reaction probability which is a function of the particle surface area, itself critically dependent on temperature. For the purposes of this simulation, the reactions are included only polewards of 64°Sand between 128- to 23-mb when the temperature, as calculated by the GCM, is less than 200°K. As this parameterization does not consider the explicit formation of polar stratospheric clouds and the subsequent fate of condensedphase HNO3, either in the form of nitric acid trihydrate or its possible loss from the stratosphere through sedimentation /22/, a simplified mechanism was consideredto simulate the observed denithfication of the Antarctic stratosphere. For the purposes of this initial simulation, all HNO3 in the solid phase formed by the above reactions is considered lost to the system. The model-derived removal of odd nitrogen is, therefore, larger than observed /23/ and, hence, likely overestimates the odd oxygen destruction, as no active chlorine is converted to C1ONO2 in this region. MODEL RESULTS Calculated total ozone as a function of latitude and season from an annual simulation of the model with only gas -phase chemistry is presented in Figure 4. The model results are in reasonable accord with observations in many respects /24/, but differ in some details. For example, the Southern Hemisphere spring maximum shows a pronounced poleward progression in time. The model results display only a slight tendency in this respect. In addition, the maximum column amounts seen at high latitudes are larger than the equivalent values from the 20-year climatology, but not with observations for some individual years. The present model simulation was initialized at the summer solstice. The heterogeneous chemistry parameterization was enabledin early July. Figure 5(a) shows the calculated total ozone distribution above 128-mb (the first model level above the tropopause) in the Southern Hemisphere for late June. Near the pole, values near 280 DU predominate. A collar of higher levels ofcolumn ozone is present from 45°Sto 65°Swith values up to 410 DU. At low latitudes, a broad region of minimum column ozone is seen, in general agreement with climatological levels. During the polar night, temperatures in the model fall below 200K in a region of the lower and middle stratosphere poleward of 60°S. As a result, heterogeneous reactions are triggered in the model, liberating chlorine from reservoir species into forms which are photolyzed rapidly with the return of sunlight to produce active chlorine species. The HNO3 produced by these reactions plays a multiple role in the chemistry responsible for the enhanced destruction of odd oxygen. First, its presence inhibits the production of active forms of nitrogen, since HNO3 photolyzes relatively slowly. Active nitrogen, particularly in the form of NO2, converts active chlorine back to its reservoir form by three-body reaction with ClO. Second, condensed phase HNO3 participates in the formation of polar stratospheric clouds upon which these heterogeneous reactions take place. Finally, HNO3 may subsequently be lost from the stratosphere by sedimentation if the particles reach sufficiently large sizes as discussed above. With the return of sunlight in the southern polar region in late August, chlorine-catalyzed destruction of odd oxygen begins. Figure 5(b) shows the initial impact on polar column ozone for 10 September. Total ozone in the area poleward of about 70°has been reduced by approximately 30 DU from midwinter values, while ozone at mid- and low-latitudes show little change from the solstice values. At the end of the model simulation in mid-October, Figure 5(c) shows that the total ozone has been significantly depleted, with minimum total ozone values near 170 DU, a decrease of 100 DU from the model initialization. During the month of September, column ozone decreased in the polar region at a rate of 5 DU/day. This is somewhat larger than the 3.3 DU/day decrease seen during September 1987 /12/. Zonal mean profiles of some of the constituents important in the chemistryof the ozone hole are displayed in Figure 6 for 10 September of the simulation. In Figure 6(a), odd oxygen is similar to other modeling efforts /25/ except for a small depletion relative to unperturbed conditions poleward of 70°S. Below about 1-mb, odd oxygen is predominantly in the form of ozone so that odd oxygen may be used as a proxy for ozone. Maximum mixing ratios near the equator of 11.5 ppmv are in good accord with our previous simulation /18/, but the vertical structure is much improved and in better agreement with observations as a result of the doubling of the number of vertical levels. Figure 6(b) shows zonal mean odd nitrogen, defined here as all nitrogen-containing species. Maximum levels of 22 ppbv are in accord with other modeling studies /25/, but somewhat below the 24 ppbv maximum estimates derived by summing LIMS nighttime N02 and HNO3 measurements /26/. The impact of heterogeneous reactions is seen clearly poleward of 60°S between 100-mb and 10-mb where the deniirification process is almost complete. Chlorine nitrate (C1ONO2), another key constituent involved in the polar ozone depletion, is shown in Figure 6(c), Its distribution is in agreement with other two-dimensional modeling studies /25/, but the redistribution of chlorine from its reservoir to active forms is evident in southern polar regions where CIONO2 is nearly depleted. Vertical distributions of calculated ozone for 2 September and 16 October at 8l°S are presented in Figure 7. The majority of the ozone loss occurs between 120-mb and 15-mb. This range is somewhat higher in the polar stratosphere than observed by balloon-borne instruments in 1986, which showed a maximum depletion between 200-mb and 30-mb /27/. This is not entirely surprising, as the temperatures calculated by the GCM reach their minima above 100-mb (not shown) and, hence, the heterogeneous chemistry which is driven by the temperature -dependent parameterization would be most effective at these levels.

z z

0 (I, 0

0 0

1 50 1987

11

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21 OCTOBEP

Fig. 1. Minimum column ozone south of 60°Smeasured by the TOMS instrument for September and October 1987 (full line) and 1988 (dashedline). (Units: DU).

z 0 ()

i~

Fig. 2. Synoptic maps of column ozone measured by TOMS for (a) 2 October 1987 and (b) 23 September 1988. The maps extend from the South Pole (center) to 20°S. Blank (white) areas indicate where no data was available. In (a) ozone values below 175 DU (dark blue) cover over a third of the total area south of 60S. In (b), hardly any values below 175 DU appear. 5

(a)

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90 E

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Fig. 3. (a) Synoptic map of temperature at 90-mb (about 17-km) for 5 September 1987, derived from U.K. Meteorological Office analyses of geopotential height. (Contour interval: 5°K). (b) As for (a), but for 5 September 1988. (1)3 14

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Fig. 5. (a) Calculated total ozone (Dobson Units) above 128-mb for 26 June. (b) As in (a) except for 10 September. (c) As in (a), except for 16 October. (1)3 15

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Fig. 6. (a) Calculated zonal maan mixing ratio ofozone (ppmv) as a function of latitude and pressure for 10 September. (b) Same as (a), except for total odd nitrogen (,ppbv). (c) Same as (a), except for C1ONO2 (ppbv). (1)316

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Antarctic Ozone Hole Studies

90

60 30 0

0 —30

-J

—60 —90

Month

Fig. 4. Variation of zonal mean total column ozone (Dobson Units) from an annual control simulation.

.0

E 100

2 3 4 5 Ozone Mixing Ratio (ppmv)

Fig. 7. Vertical profile of the zonal mean ozone mixing ratio calculated by the 3-D model at 8l°Sfor 2 September (solid line) and for 16 October (dotted line).

0.1

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Fig. 8. Vertical profile of the calculated ClO mixing ratio for 10 September at 75°S.

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Corresponding to the ozone loss, Figure 8 shows the calculated vertical distribution of chlorine monoxide on 10 September at 75°S. The maximum at about 2-mb is due to ‘standard gas-phase chemistry. At 20-mb, a secondary maximum is evident with peak mixing ratios of 2 ppbv. The maximum is due to the enhanced levels of active chlorine formed by the heterogeneous chemistry. The maximum level is in reasonable accord with ClO levels measured in the Antarctic during 1987, where peak levels of 1.6 ppbv at 19-km were noted /28/. Again, the calculated peak occurs at somewhat lower pressures than observed due to the GCM-derived thermal structure. The results of this initial simulation show that several aspects of the springtime depletion of Antarctic ozone are reproduced. The redistribution of chlorine to its active form and nitrogen to its reservoir form, vital for odd oxygen destruction to proceed through the ClO dimer mechanism, occurs in the simulation, but with some quantitative differences with observed ClO and denitrification levels. The level ofmaximum calculated ozone destruction occurs higher in the stratosphere than observed. This is a manifestation of the vertical structure of the temperature minimum in the 0CM. The secondary maximum in C1O produced as a result of the heterogeneous chemistry is approximately 20 percent greater than observations, which is due to the highly denitrified state of this region of the stratosphere. Nonetheless, these initial three-dimensional chemisty/transport model results are sufficiently encouraging to validate the utility of this type of model in the examination of the ozone hole. SUMMARY We have examined the morphology of the springtime depletion in polar ozone in the Southern Hemisphere using observations of ozone and temperature derived from satellite-borne instruments for 1987 and 1988. The dramatic differences in the morphology of the ozone hole between the 2 years illustrated how dynamical variability played an important role in modulating the depletion. A three-dimensional chemical/transport model simulation of the formation of the Antarctic ozone hole was conducted. The model included a parameterized representation of the heterogeneous chemical reactions important in redistributing odd chlorine into active forms, while sequestering odd nitrogen in reservoir species. The model simulated many aspects of the observed depletion. The calculated ozone loss during the month of September in the polar vortex region is 5 DU/day compared to an observed rate of 3.3 DU/day in 1987. Calculated levels of the enhanced ClO resulting from chemistry occurring on cloud surfaces are in qualitative agreement with recent observations. The denitrification of the lower stratosphere is reproduced, but in amounts higher than those observed. Improvements to the parameterization of the heterogeneous chemistry are now under way to remedy some of the shortcomings of the simulation reported here. A more physically realistic model of polar stratospheric cloud formation is under development. Treatment of the condensedphase HNO3 is being enhanced to consider explicitly the removal of odd nitrogen from the stratosphere by sedimentation. These modifications should allow us to examine the subsequent fate of ozone and odd nitrogen poor air following the breakup of the polar vortex in late autumn. This “dilution” effect had been studied previously with this model /17/, but with an imposed ozone depletion and no pertubation to the nitrogen or chlorine chemistry. The more realistic treatment described here would allow for a more careful assessment of the possible impact of the dilution on the global ozone budget. ACKNOWLEDGMENTS We would like to thank Arlin Krueger for providing the TOMS ozone data. We are also grateful to Gretchen Lingenfelser for producing the TOMS color 03 maps and to Mary Kellerman for the production of the model color maps. Thanks are also due to Sheila Johnson for preparation ofthe manuscript. REFERENCES 1. J. C. Fannan, B. 0. Gardiner, and J. D. Shanklin, Large losses of total ozone in Antarctica reveal seasonal ClOxJNOx interaction, Nature 315, 207-210 (1985). 2. J. Geophys. Res. 94, # 9 (1989). 3. J. Geophys. Res. 94, # 14 (1989). 4. Geophys. Res. Lett. 17, # 4 (1990). 5. World Meteorological Organization, ScientWc Assessment of Stratospheric Ozone: 1989, Report # 20, Geneva (1990). 6. R. R. Garcia and S. Solomon , A possible relationship between interannual variabilityin Antarctic ozone and the quasi-biennial oscillation, Geophys. Res. Lett. 14, 848-851 (1987). 7. L. R. Lair, M. R. Schoeberl, and P. A. Newman, Quasi-biennial modulation of the Antarctic ozone depletion, J. Geophys. Res. 94, 11559-11571 (1990). 8. J. T. Kiehi, B. A. Boville, and B. P. Briegleb, Response of a general circulation model to a prescribed Antarctic ozone hole, Nature 332, 501-504 (1988).

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9. S. Solomon, Antarctic ozone: Progress towards a quantitative understanding, Nature, in press (1990). 10. A. J. Fleig, K. B. Pawan, C. G. Wellemeyer, and D. S. Silberstein, Seven years of total ozone from the TOMS instrument -- a report on data quality, Geophys. Res. Lert. 13, 1355-1358 (1986). 11. 5. A. Clough, N. S. Grahame, and A. O’Neill, Potential vorticity in the stratosphere derived from satellites, Q. J. Roy. Met. Soc. lii, 335-358 (1985). 12. A. J. Krueger, M. R. Schoeberl, R. S. Stolarski, and F. S. Sechrist, The 1987 Antarctic ozone hole: A new record low, Geophys. Res. Lett. 15, 1365-1368 (1988). 13. R. S. Stolarski, M. R. Schoeberl, P. A. Newman, R. D. McPeters, and A. J. Krueger, The 1989 Antarctic ozone hole as observed by TOMS, Ge’ophys. Res. Lerr. 17, 1267-1270 (1990). 14. W. J. Randel, The anomalous circulation in the Southern Hemisphere stratosphere during spring 1987, Geophys. Res. Lett. 15, 911-915 (1988). 15. S. Solomon, The mystery of the Antarctic ozone “hole”, Rev. Geophys. 26, 131-148 (1988). 16. L. R. Poole, S. Solomon, M. P. McCormick, and M. C. Pitts, Interannual variability ofpolar stratospheric clouds and related parameters in Antarctica during September and October, Geophys. Res. Lett. 16, 1157-1160 (1989). 17. W. L. Grose, R. S. Eckman, R. E. Turner, and W. T. Blackshear, Antarctic ozone depletion and potential effects on the global ozone budget, in: Dyna.’nics, Chemistry and Photochemistry in the Middle Atmosphere of the Southern Hemisphere, ed. A. O’Neill and C. R. Mechoso, Kluwer, Dordrecht, Holland 1990. 18. W. L. Grose, J. E. Nealy, R. E. Turner, and W. T. Blackshear, Modeling the transport of chemically active constituents in the stratosphere, in: Transport Processes in the Middle Atmosphere, ed. T. Schneider et al., D. Reidel and Co., Hingham, MA 1987, pp. 229-250. 19. L. T. Molina and M. J. Molina, Production of Cl202 from the self reaction of the ClO radical, J. Phys. Chem. 91, 433-436 (1990). 20. R. L. Jones, private communication, 1990. 21. M. P. Chipperfield and J. A. Pyle, Two-dimensional modeling of the Antarctic lower stratosphere, Geophys. Res. Lett. 15, 875-878 (1988). 22. 0. B. Toon, P. Hamill, R. P. Turco, and J. Pinto, Condensation of HNO3 and HCI in the winter polar stratospheres, Geophys. Res. Lett. 13, 1284-1287 (1986). 23. D. W. Fahey, K. K. Kelley, G. V. Ferry, L. R. Poole, J. C. Wilson, D. M. Murphy, M. Loewenstein, and K. R. Chan, In situ measurements of total reactive nitrogen, total water, and aerosol in a polar stratospheric cloud in the Antarctic, J. Geophys. Res. 94, 11299-113 15 (1989). 24. J. London, The observed distribution and variations of total ozone, in: Proceeding of the NATO Advanced Studies Institute on Atmospheric Ozone, ed. M. Nicolet and A. Aikin, U.S. Department ofTransportation, Washington 1979, pp. 31-44. 25. C. H. Jackman, R. K. Seals, Jr., and M. J. Prather, Two-dimensional intercomparison of stratospheric models, NASA Conference Publication 3042, Washington, DC (1989). 26. L. B. Callus, M. Natarajan, and J. M. Russell III, Estimates of the stratospheric distribution of odd nitrogen from the LIMS data, Geophys. Res. Letr. 12, 259-262 (1985). 27. D. J. Hofmann, J. W. Harder, S. R. Roif, and J. M. Rosen, Balloon-borne observations of the development and vertical structure ofthe Antarctic ozone hole in 1986, Nature 326, 59-62 (1987). 28. R. L. deZafra, M. Jaramillo, L. K. Emmons, P. M. Solomon, and A. Parrish, New observations of a large concentration of ClO in the springtime lower stratosphere over Antarcticaand its implications for ozonedepleting chemistry, J. Geophys. Res. 94, 11423-11428 (1989).