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4 Stratospheric Ozone Depletion: Antarctic Processes ROBERT T. WATSON National Aeronautics and Space Administration This paper describes the current understanding of the antarctic ozone hole phenomenon. A later speaker, James Anderson, will expand on the role of chlorine in causing the hole to occur. This talk addresses two questions: (1) What are the observations of ozone that define how large and ~deep" the antarctic hole is? and (2) What is our present understanding of cause and effect? As Daniel Albritton noted, the Montreal Protocol did not explicitly take the appearance of the hole into account. However, the occurrence of the hole at about that time served as a major driving force to get the Europeans to view ozone as a serious issue and to get them to the table. The U.S. position at the Montreal meeting was bred on the assumption that we did not know the cause of the antarctic ozone hole, although by then we recognized that an ozone hole existed. It is worth noting that the appearance of the ozone hole was an unexpected event in the sense that the models referred to by Albritton did not predict the hole. The ozone hole has made modelers realize that stratospheric modeling needs further work and has rekindled scientific interest in the problem of stratospheric ozone. The ozone over Antarctica had, by October 1987, been reduced by more than 50 percent of its 1979 value (Watson et al., 1988; Figure 4-~. I.ocally, depletion was as great as 95 percent between 15 and 20 km altitude (Figure 4-2~. Not only was the ozone level the lowest on record in 1987, but the seasonal period of depletion also lasted 19

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22 ROBERT T. WATSON the longest. The ozone hole itself did not fully disappear until late November or early December, 1987. This long-lasting minimum may have had significant consequences for the ecosystems in the antarctic region. The solar elevation angle is comparatively low by October, when the hole was at its deepest, but is much higher in November, when the ultraviolet (UV) effect might be stronger at the surface. The ozone amount was also the lowest on record at all latitudes south of 60S latitude in 1987. E urthermore, the occurrence of strong depletion was a year-Ion" phenomenon south of 60S and was not confined to the spring season as in preceding years, although the greatest depletion occurred during the Southern Hemisphere spring. Therefore it is no longer a relatively local, isolated effect (Figure 4-3~. For 1987, the measurements show little in the way of an ozone hole as late as August 17. After that, the ozone hole developed rapidly, especially after September 5, so that by October 5, the ozone over the middle of Antarctica had dropped from 320 Dob- son units (DU) to 120 DU. The monthly average October mean of ozone decreased from about 300 DU in 1979 to 120 DU in 1987 over the middle of Antarctica. Additionally, the amount of ozone in the horseshoe-shaped maximum that extends out to at least 60S decreased by around 100 DU in 1987, compared to 1979. Two ozonesondes were obtained on October 6 and 9, 1987, at the U.S. Palmer Peninsula station (64S latitude). This station is located near the edge of the region of low ozone. On October 6, when the edge of the strongly depleted region was poleward of the Paimer station, the ozone showed a fairly normal vertical profile. Three days later, the edge of the chemically disturbed and depleted region moved northward past the station, and the profile then showed a decrease of around 95 percent between 15 and 20 km. Other ozonesonde data from the South Pole, McMurdo, and Halley Bay stations, stations that were continually in the polar vortex region of depletion, show an almost complete disappearance of ozone after October 5. Hence, the ozone hole was a continent-wide phenomenon extending out to around the latitude of the Palmer station, where a steep horizontal gradient of ozone exited. A series of Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) satellite images shows some recovery of ozone in the antarc- tic hole by November 15, 1987, but the ozone amount remained below 175 DU over most of the continent (Figure 4-4a,b). By November 29, the minimum had moved from the polar region to over the Weddell Sea, surrounded by a large region of less than 200 DU. By about

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24 90 70 50 30 llJ ~ 1 0 - 6 -10 -30 -50 -70 -90 ROBERT T. WATSON v&~"2 ;;'i';'';';'''-;';1 -- ~7 1' '. :'.. '.... 1. ,.'. ' :. ', it'. : ~ ~ 1 ~ ~ ~ o 1 ...... .: .~ I ~ ~ l ~7~ -~47( Q _- ~ -6 J 03- ~o~ i ( / --4 ~\~ ~-it/ 1 ,/-S ~~ 1 . .- :::::::::::: ::::::::::: :::::::::::::::::: ~ ~ ~ {ADA=_ ~ \ \ ~ ~ /2~) t;~, 1- 1 - ~1\~1~1111 J F M A M J J A S O N D MONTH FIGURE 4-3 Percentage total column ozone changes, (1986-1987) minus (1979- 1980), from the TOMS instrument as a function of latitude and month. Shaded areas are regions of no data during the polar night. December 5, the hole had filled considerably, with a minimum of about 250 DU located well out over the Weddell Sea not far from the southern tip of South America. The relatively high sun angle of November and December coupled with this shift in the location of the ozone minimum likely resulted in a significant UV impact on the aquatic ecosystem of that region. Further comparison of 1987 with earlier years indicates a pro- gressively more rapid decrease of ozone during September in the later years. This fact is in agreement with a chemical hypothesis that, as more chlorine is- added to the system, the rate of seasonal ozone decrease becomes greater.

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OZONE DEPLETION: ANTARCTIC PROCESSES 25 A comparison of 1986 plus 1987 TOMS data minus 1979 plus 1980 data (2-year averages are used to remove possible effects of the quasi-biennial oscilIation) shows that at least a 5 percent decrease in ozone occurred at all seasons south of about 50S latitude. The satellite data have been corrected for drift relative to the ground- based stations and are believed to be correct to within ~ percent, but possibly 2 percent in the immediate South Polar region. TOMS data from the Northern Hemisphere show a decrease in ozone from 1979 through 1985. This is consistent with an increase in trace gases, primarily chIorofluorocarbons (CFCs), and a decline in solar output (solar maximum in 197~1980, minimum in 1985-1986~. From 1985 to 1987, the ozone curve flattened or increased slightly, consistent with increased solar output after 1985 that countered the effects of increasing trace gases. In the Southern Hemisphere, however, the data show a continued decrease after the 1985 solar minimum (Figure 4-5~. This is consistent with the concept that the antarctic ozone hole phenomenon causes a dilution effect throughout much of the Southern Hemisphere. We now turn to the question of what is causing the antarctic ozone hole. The unique meteorology of the antarctic region in the winter and spring seasons results in the development of a strong polar vortex that consists of an air mass that ~ largely isolated from air farther north. Within the vortex, temperatures become cold enough to form stratospheric ice crystals. The ice crystab then allow unusual chemical interactions among nitrogen, hydrogen, and chlorine atoms. The weight of observational evidence indicates that the chlorine is primarily responsible for the ozone hole. Without chlorine in the antarctic stratosphere, there would be no ozone hole. (Here "hole" refers to a substantial reduction below the naturally occurring concentration of ozone over Antarctica.) The relevant chemical reactions occur within the polar vortex. The vortex is not a uniform cylinder but has a shape that varies with altitude and is strongest and most isolated above the 40~K isentropic surface, around 15 km and above. Below 15 km, there is considerably more exchange of air between the midIatitudes and the polar regions. Hence, it is easier to explain behavior above 15 km in terms of chemistry than below 15 km, where atmospheric dynamics has an important role. Behavior below 15 km is still largely unexplained and is a matter of active research. The National Aeronautics and Space Administration (NASA), the National Oceanic ant] Atmospheric Administration (NOAA),

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26 ROBERT T. WATSON 8/1 7/87 . 9/ 5/87 , - ~ _ "I'''" 10/ 5/87 , - , _ - , , I---- ~ ,"'~/~''\'t'. i.... )QO~ I,, FIGURE 4-4a Daily Southern Hemisphere maps (polar orthographic projec tion) of total ozone measured by TOMS for the indicated dates. Period shown includes the formation of the ozone minimum in 1987. the Chemical Manufacturers Association, and the National Science Foundation collaborated on ground-based expeditions to McMurdo in both 1986 and 1987 and provided extremely important results. These included observations of chlorine dioxide by Susan Solomon, which served as a very good indicator of perturbed chlorine chemistry, as well as measurements of very low nitrogen dioxide, showing that the atmosphere appears to be denitrified. ~ participated in an aircraft expedition that was based in Puntas Arenas, Chile. We flew a DC-8 all over the antarctic continent during a 6-week period. We used remote sensors looking upward to obtain total column measurements of a wide variety of gases. We had a UV

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OZONE DEPLETION: ANTARCTIC PROCESSES 1 1/15/87 , ;-, - ~ I, 1 o/ 1 s/87 I'' 1 1/29/87 1 2/ 5/87 , . . - . _ ~ --- ! _ --' 27 FIGURE 4-4b Daily Southern Hemisphere maps (polar orthographic projec- tion) of total ozone measured by TOMS for the indicated dates. Period shown includes the breakup of the ozone minimum in 1987. spectrometer similar to the one that Susan Solomon took to the ice to look at chlorine dioxide and bromine monoxide (BrO). We had a lidar instrument to look at both ozone and the aerosols. The ER-2 aircraft flew as-high as 18.5 km in geometric altitude and measured several quantities: bromine and chlorine monoxide (James Anderson, Harvard University) to determine if the chIorine-bromine chemistry was perturbed, ozone (Walt Starr, NASA, and Mike Proffit, NOAA), total amount of nitrogen compounds (NOy) in the atmosphere to determine whether they were enhanced or depleted (David Fahey, NOAA), whole air samples for CFCs, methane (CH4), and nitrous oxide (N2O) (Leroy Heidt, National Center for Atmospheric Research

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28 6 4 A ~2 L,J it_ A o 2 c:, ROBERT T. WATSON 1 1 1 1 1 1 1 1 l . ,,; I., A.!, by. '; _ ~ an' .. a. . ~ I1. '' ,'; '`~;'':; ijIl'' 1 : -2 _ - 4 NTD ad.: 6 An-_ - - ~ I;: Or ~ I\,' _.~ ,., . ~ ~ I: SLOPE 7 YEARS' -0.40 % per year SLOPE 9 YEARS- -0.32 ~ per year _ , : ,. , 1 1 1 1 1 1 1 - _ ~; t~ ~ or,, ~ w.-.;: _ . ; I`.; .u . . . ~ . 7 B 79 80 81 82 83 YEAR 84 85 86 87 88 FIGURE 4-5 Total column ozone integrated from 53S to 53N latitude, from 1979 to 1987, as determined from TOMS data, shown as percent deviation from an arbitrary reference level. (NCAR)) to determine rising or sinking motions in the air mass, and a large number of aerosol measurements, as well as a system to measure the temperature lapse rate (Bruce Gary, Jet Propulsion Laboratory (IMPLY. The data from this ensemble of instruments are being used to test the various hypotheses that have been proposed. These theories include the solar theory, whereby periodically the amount of nitrogen compounds is enhanced. These enhanced levels can catalytically destroy ozone in the lower stratosphere. This theory, if correct, implies that levels of oxides of nitrogen should be elevated; besides, the ozone hole should occur cyclically. The data showed that this theory is completely and utterly wrong. The oxides of nitrogen were measured as being unusually low. Some other theories that require an increase in nitrogen compounds are likewise incorrect. The fluorocarbon-halon theory suggests that there should be a

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OZONE DEPLETION: ANTARCTIC PROCESSES 29 change in the partitioning of chlorine from the inactive forms of chlorine, namely hydrochloric acid and chlorine nitrate, into the active forms of chlorine, namely chlorine atoms and chlorine oxide radicals. Therefore, Anderson's tests were critical for deterrn~ning whether the chlorine oxide (and bromine oxide) abundances were enhanced. Another theory, advanced by K.-K. Tung, requires a change from downward to upward motions over Antarctica in association with other circulation changes. If this is correct, one should see enhanced levels of tropospheric trace gases such as nitrous oxide and methane in the lower and middle stratosphere. Therefore, the Heidt measurements were critical for this purpose. The ER-2 aircraft could not climb higher than 18.5 km because of the very cold, dense atmosphere and the need to carry a lot of fuel for safety reasons. Also, it did not range farther south than 72S latitude, about midway down the Palmer Peninsula. Therefore, many of the measurements were made close to the inside edge of the polar vortex. It would have been scientifically desirable to have flown higher and farther southward in the vortex. In any event, our flights from Puntas Arenas to 72S and back were useful for comparing conditions just inside the vortex to those outside. One of our first flights was made on August 23, 1987. Water vapor dropped from about 3 ppm outside the vortex to about half this value inside, indicating that the atmosphere inside the vortex was dehydrated. Ozone changes were only slight across the vortex boundary. However, the abundance of the chlorine monoxide radical (ClO) increased from about 10 parts per trillion by volume to about 500 parts per trillion. The nitrogen compounds (except for nitrous oxide) dropped from 8 to 10 parts per billion by volume (ppbv) to only 1.5 to 2 ppbv. Thus, the vortex atmosphere on this date was dehydrated, denitrified, and highly enriched in chlorine oxides, but with little effect on ozone levels. By the end of our mission on September 22, the polar vortex atmosphere was still dehydrated and denitrified, the chlorine oxides had increased to about 1 ppbv, and the ozone concentration had dropped to less than half of its value outside the vortex. Therefore, it appears that a significant amount of time, approximately 1 month, is required for the chlorine oxides to destroy ozone. The concentrations of bromine oxides within the vortex were in the range of 3 to 5 parts per trillion on all flights. These small concentrations, compared to the chlorine oxide concentrations, imply that whereas chlorine oxides

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30 ROBERT T. WATSON play a major role in destroying stratospheric ozone by the ClO-ClO mechanism, the proposed ClO-BrO mechanism for destroying ozone is of relatively minor importance, accounting for less than 10 percent of the total ozone depletion. The DC-8 flew nearly to the South Pole and obtained many bulk column measurements. These show that, in crossing inside the po- lar vortex, nitrogen dioxide drops off significantly, nitric acid peaks around 70S and then drops, chlorine nitrate reaches a maximum at the edge of the vortex, and hydrochloric acid fails off significantly within the vortex. The JPL and NCAR measurements are in excel- lent agreement with each other. The chlorine nitrate results from the presence of both ClO and nitrogen dioxide; hence it maximizes near the vortex boundary, where ClO is increasing rapidly but nitrogen dioxide is decreasing rapidly. Calculations by Anderson show that ozone depletion at the 410- and 420-K isentropic surfaces between August 23 and September 22 can be almost entirely explained by the amount of ClO present if one assumes that the ClO-ClO mechanism is effective. At the 36~K surface, the calculated ozone loss is somewhat less than the observed loss. At least we can say that above about the 40~K level, there does seem to be enough ClO to explain the observed ozone loss. A number of measurements were obtained of particles, ranging from sulfuric acid particles (less than 0.1 micron) through nitric acid tribydrate (0.5 micron) to relatively large ice crystals (2 to 3 mi- crons). These measurements tend to support the current hypothesis of how chlorine oxide concentrations become enhanced in the polar stratosphere. Measurements of nitrous oxide and methane obtained at an al- titude of about 18 km and near the inner edge of the vortex did not give any evidence of upward vertical motions. Since the ER-2 flights did not penetrate poleward of 72S, one cannot make a blanket state- ment that upward motions did not occur anywhere within the polar vortex region. However, these data, in conjunction with the other data, suggest that upward vertical motions do not play an important role in the ozone depletion process. In summary, chlorine ~ intimately involved in the depletion of ozone, and most of the ozone lo" can be explained quantitatively on a chemical bash. All theories, especially the solar theory, that require elevated concentrations of oxides of nitrogen are incorrect, and the apparent absence of large-scale upward motions suggests that the K.-K. Tong type of theory is wrong as well.

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OZONE DEPLETION: ANTARCTIC PROCESSES 31 (In answer to a question about ice crystals and temperature): Since 1984, there has been an increase in the persistence of polar stratospheric clouds (PSCs) at 16, 18, and 20 km, with the PSCs persisting through October. This is possibly consistent with the temperature getting colder through October. We have looked at the temperature record, and there is no evidence for a change in strato- spheric temperature from 1979 to 1987 in August or September, when the hole forms, but the stratosphere appears to be about 8C colder in October and November at the 100 mb level over Antarctica. This implies that the temperature change is a result of ozone depletion rather than a cause of it. Since it is colder in October than formerly, PSCs seem to be persisting longer now. Question: What would be the first signs of damage to the biota in Antarctica from increased UV radiation? Answer: Presumably, the first sign would be a die-off of the phytoplankton and then the krill in the surrounding waters. Un- fortunately, there are no long-term records of the phytoplankton population over the last 20 to 40 years, so a good comparison cannot be made. Laboratory studies suggest that enhanced leveb of UV would be quite catastrophic to the phytoplankton and krill life in the region, but such measurements may not properly represent how the natural system works. Question: How effective will the Montreal Protocol be in reduc- ing the severity of the antarctic ozone hole? Answer: There was no antarctic ozone hole from 1965 to 1970 with chlorine at a concentration of 2 ppbv. There is a huge antarctic ozone hole today with chlorine at 3 ppbv, and there is evidence that the ozone hole is enlarging and spreading. Under the Montreal Protocol, the concentration of chlorine will certainly rise to at least 5 ppbv and possibly to as high as 8 or 9 ppbv. Therefore, ~ believe that the protocol will do absolutely nothing to protect the antarctic region. The ozone hole may get worse, and there will be more hemispheric, and possibly global, ramifications. If policymakers believe that we should protect ozone over Antarctica, then it is quite clear that the Montreal Protsco! will have to be revised and the measures made much more stringent. Question: How is the edge of the southern polar vortex defined? Answer: ~ have heard two different definitions of the vortex. They are based on the location of the steepest potential vorticity gradient and the location of the jet of maximum winds, which is about 5 to 8 of latitude wide. Some define the inner edge of the

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l 32 ROBERT T. WATSON wind jet an the limit of the vortex air. This appeared to correspond closely to the boundary of the chemically perturbed region, and we usually encountered it around 68 to 70S latitude. The alternate definition is the equatorward side of the belt of maximum winds, which extends to about 60S. This definition is not consistent with our measurements. A related question is: How fast does air move across the polar vortex boundary? Several scientists are looking at this question. Question: At lower latitudes where there are no stratospheric ice crystals, is the role of ice mimicked by other aerosols such as volcanic dust? Answer: The next two papers address that question. The evi- dence that ~ have seen from laboratory studies indicates that liquid sulfuric acid particles will not provide such an efficient surface for heterogeneous chemistry, partly because the rate of reaction proceeds more slowly compared to that with ice crystals, and partly because the typical density of the sulfuric acid aerosols is less than that for ice crystals over Antarctica. Therefore, the likelihood of significant heterogeneous chemistry appears to be less at lower latitudes. REFERENCE Watson, R.T., M.J. Prather, and M.J. Kurylo. 1988. Present State of Knowl- edge of the Upper Atmosphere 1988: An Assessment Report. NASA Reference Publication No. 1208. National Aeronautics and Space Admin- istration, Washington, D.C.