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5 The Role of Halocarbons in Stratospheric Ozone Depletion F. SHERWOOD ROWLAND University of California, Irvine This presentation will cover two topics: (1) halocarbons in the atmosphere and (2) the measurement of ozone. Starting in 1978, my research group made gas chromatographic measurements of trichiorofluoromethane (CCI3F, known as CFC-ll) with air samples from many locations in both hemispheres that were judged to be sufficiently remote from local emission sources. Other research groups had collected similar data beginning as early as 1970. A set of our measurements of CCI3F from 1979 shows only a small hemispheric difference in the lower atmosphere (Figure 5-1~. Although about 95 percent of the chiorofluorocarbons (CFCs) are released in the Northern Hemisphere, the redistribution between the hemispheres is rapid enough that the Southern Hemisphere lags behind the Northern by only about 10 percent. Measurements from later summers show an increase at all latitudes. As part of the Global Atmospheric Gas Experiment (GAGE), intensive measurements have been made with automatic gas chromatography operating at five stations most of the time since July 1978. These GAGE measure- ments through 1983 for dichIorodifluoromethane (CCI2F2, known as CFC-12) in Ireland and Tasmania (Cunnold et al., 1986) are shown in Figure 5-2, together with flask data, from the Oregon Graduate Center, for January measurements in Oregon and at the South Pole (Rasmussen and Khalil, 1986~. All of the data show the level of 33
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34 200 1 90 180 - _~ 170 160 1 50 F. SHERWOOD ROWLAND ' ' ' ' ' ' 1 ' l l , , , SUMMER 1979 8 ° ° o o 8 GO o o o o _ o °O o o ,, , I I I I 1 1 , , ~ ~ II 90 60 30 o 30 60 90 N LATITUDE S FIGURE 5-1 Tropospheric concentration (in pptv) of trichlorofluoromethane (CCl3F) as a function of latitude for summer 1979. (Note nonzero origin of ordinate.) CFCs rising rapidly during the 1980s, with the Southern Hemisphere lagging behind the Northern Hemisphere by about 10 percent. In order to estimate the average lifetime of these molecules in the atmosphere, one needs to know the amount being put into the atmosphere for comparison with the amount that is still there. For CCI2F2, the estimate of its mean life is more than 100 years, and this is believed to be a fairly typical value for CFCs. Since the average CFC molecule has been in the atmosphere only about 10 to 12 years, 90 percent of them are still there. The estimated mean life for CCI3F is given as 75 years. We ran a numerical calculation in which CCI3F release was as- sumed to grow exponentially until 1976, remain constant for 15 years until 1991, and then drop to zero after that. At 75 years after 1991, the amount of chlorine compound remaining in the stratosphere had dropped to 37 percent and at 150 years (i.e., 2160 A.D.) had declined to 13 percent of the 1991 amount. Most of the other CFCs have somewhat longer lifetimes, up to 140 years, so that an appreciable fraction may still remain in the atmosphere even after 200 years, given the above scenario. All indications are that the chlorine atoms
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l ROLE OF HALOCARBONS 400 360 320 CM IL 280 240 200 1 975 . 1~1 ,1~ ,' NORTH 0 1 l i u 1 ,1l ,l I SOUTH 18 11. . 11e ~1 . . . 1 980 1 985 YEAR 35 FIGURE 5-2 Concentration (in pptv) of dichloroditluoromethane (CC12F2) from 1978 to 1985 for GAGE measurements in Ireland (vertical lines labeled "North") and Tasmania (vertical lines labeled "South") (Cunnold et al., 1986), together with January flask data obtained in Oregon (upper dots) and at the South Pole (lower dots) (Rasmussen and Khalil, 1986~. (Note nonzero origin of ordinate.)
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36 1 .0 0.5 F. SHERWOOD ROWLAND CIX at '` 40 Km / \ I \~;75 YEARS AT- 19.6 YEARS - - - - - - - t~ /INPUT 0.0 1960 19~0 2000 2020 2040 2060 2080 2100 2120 2140 2160 YEAR I , 1 FIGURE 5^3 Concentrations (in ppbv) as a function of year of two typical stratospheric chlorinated molecules, one with a mean life of 75 years and the other with a mean life of 19.6 years. These concentrations are based on an assumed input of CFCs to the atmosphere (increasing to 1976, then constant to 1991, then no further emissions) as shown in the lower left of the figure. (Adapted from Rowland and Molina, 1976.) released from CFCs are going to be with us for a long time, even if release of CFCs is discontinued tomorrow. A related point concerns what the Montreal Protocol calls the "ozone depletion potential. Consider a chlorinated molecule with a mean life of 75 years and another with a mean life of 20 years (Fig- ure 5-3~. If their respective effects on ozone depletion are compared as a function of time, the difference between them does not become large until more than 50 years have passed (Rowland and Molina, 1976), by which time the 20-year compound will be largely gone. If CFC emissions are assumed to continue into the future at a constant rate, stratospheric chlorine compounds will continue to in- crease (as shown in Figure 5-3~. In 1974, the Northern Hemisphere contained chlorine compounds at about 1.8 ppbv; this has now in- creased to about 3.5 ppbv. Continued release at 1986 rates will result in increases to over 5.0 ppbv by the year 2000 (Figure 5-4~. If, in- stead, we assume a 20 percent reduction of CFC emissions in 1994
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ROLE OF HALOCARBONS 7.0 6.0 5.0 m 40 'in, c' 3.0 2.0 1 .0 0.0 _ 1950 , _ CON STA NT EM I SS I ON, NO CONTROLS MONTREAL PROTOCOL COMPLETE CFC P H A SEOUT // - 1960 1970 1980 1990 2000 YEAR 37 OTHERS CC12FCCI F2 (CFC- I 13) CC 12F2 (CFC -12) - CC 13F ( CF C - I I ) CH3CC13 CC14 CH3C I FIGURE 5-4 Increases in concentrations (in ppbv) of stratospheric chlorinated molecules assuming (1) continued release of CFCs at 1986 rates (solid curves), (2) a 20 percent reduction in release rates in 1994 and an additional reduction of 30 percent in 1999 (dashed curves), and (3) a complete phaseout of CFC emissions over a 10-year period beginning in 1989 (heavy solid curve). and a further 30 percent reduction in 1999, as the Montreal Protocol requires, the preclicted change in time will be different, assuming that all countries obey the protocol. Finally, if we assume a complete phaseout of CFC emissions over a lO~year period beginning in 1989,
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38 F. SHERWOOD ROWLAND TABLE 5-1 Trends in Total Ozone Percent Change Reference am_ 1970-1978 +0.28 ~ 0.67 Reinsel et al., 1981 1970-1979 +1.5 ~ 0.5 St. John et all, 1982 1970-1979 +0.1 i 0~55 Bloomfield et al., 1983 1970-1983 =0.003 ~ 1.12 per decade Reinsel et al., 1984 (-0.14 ~ 1.08) per decade with sunspot series in model Source: World Meteorological Organization-National Aeronautics and Space Administration, Washington, D.C. (1986~. the amount of stratospheric chlorine compounds will still continue to increase for some decades. The second topic ad^dressed in this presentation is ozone measure- ments. The WMO-NASA report Atmospheric Ozone ~ 985 (WMO- NASA, 1986) summarized recent calculations of trends in total global ozone. The trends (through 1983) are only slightly, if at all, down- ward (Table 5-1~. However, the trend estimates given in the WMO- NASA report are misleading, as explained below. The Arosa, Switzerland, station has the longest record of ground- based measurements of total ozone, made since 1931 with a Dobson spectrometer. Arosa is in the Swiss Alps at 47°N latitude. Neil Harris, University of California at Irvine, has examined the monthly averages of the data taken daily at Arosa. The amount of strato- spheric ozone at Arosa, and generally in the north temperate zone, varies seasonally, with a peak in March or April and a minimum in October or November. The standard deviation of the measurements is very large during the winter season and comparatively small in the summer (Figure ~5~. If one compares the data for each month for the periods 1931 to 1969 and 1970 to 1986, one sees that there is less ozone, on the average, during the winter months in the later period (Figure 5-6~. The greatest difference is observed for the month of De- cember, with a loss that substantially exceeds the standard deviation for the data. Measurements at Bismarck, North Dakota, which is also at 47°N latitude, began in 1963. We formed two sets of data, each 11 years long, one for 1965 through 1975 and the other for 1976 through 1986. A length of 11 years was chosen to permit comparison over two successive solar cycles. There ~ a wintertime loss of ozone during the
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ROLE OF HALOCARBONS 400 - ~n At O 350 m o - At o ~ 300 o 250 JAN MAR 1 1 MAY JUL SEP NOV MONTH 39 FIGURE 5-5 Mean monthly total column ozone measurements (Dobson units; includes data from Dutsch, 1984) at Arosa, Switzerland, for the period August 1931 through July 1986. Standard deviations from the mean of individual monthly data are also shown. second period at Bismarck, similar to the loss reflected in the Arosa results. The wintertime toss also shows up in data from Caribou, Maine, which is also at 47°N. Hence, a wintertime depletion of ozone in the last decade or so has been observed for numerous northern stations. Numerous factors are believed to affect the concentration of stratospheric ozone. The solar sunspot cycle affects the ozone con- centration because there is increased UV radiation at around 200 nm during the sunspot maximum. Radiation of this wavelength can split
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40 F. SHERWOOD ROWLAND o - ~n Z -5 o In m 0 -10 Cal - OCR for page 41
ROLE OF HALOCARBONS 41 stations, which correlate so well after 1977, cross one another ear- lier in a manner suggesting a problem with the continuing absolute calibrations at one or both stations. Long-term calibration problems such as these make the data at some stations not very useful for the determination of trends in ozone concentrations over a 20- or 30-year period. In order to determine long-term ozone trends, Rumen Bojkov from the Atmospheric Environment Service in Canada, Peter Bloom- field, a statistician from North Carolina State University, Neil Harris, and T have compiled data from all the Dobson stations and the So- viet stations by latitude bands. (The Soviet stations use a different instrument, the M-83, with somewhat different characteristics but qualitatively similar measurement techniques.) The data span the period 1965 to 1986 and are reported in terms of a "ramp" fit to the 22 years of data. The statistical analysis includes variations from the solar cycle and the QBO, plus an assumed linear change after 1969 from an otherwise constant value from 1965 to 1969. The data have been recorded as percentage changes over the 17-year period 1969 to 1986, from the linear ramp coefficients. The results, reported by the Ozone Trends Pane! (Watson et al., 1988), were as follows, on a monthly basis: 1. Between 53°N and 64°N (Figure 5-7a): not much change in July to September but very substantial decreases in December to March, similar to the Arosa data results. The QBO had a 2 per- cent variation in the statistical analysis, and the solar cycle showed 1.8 percent more ozone at the solar maximum than at the solar min- imum. (These variations were removed from the data in order to study the long-term trends.) 2. Between 40°N and 52°N (Figure 5-7b): again, a marked difference between summer and winter trends. The QBO and solar cycle are again apparent. 3. Between 30°N and 39°N (Figure 5-7c): less difference be- tween summer and winter trends, but all months show decreases in ozone, some of them large enough to be statistically significant, including decreases observed for July. Regression coefficients were also calculated including successive years of data from 1965 through 1980, 1981, 1982, and so on, up through 1986. The coefficients show some variation with the additional years of data for the 53°N to 64°N zone and for the 30°N to 39°N zone, but none of the changes appears to be statistically significant. A negative
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42 5; O ++tit t a t t - 5 -10 . _ - 1 5 _ 5 _ o 5 - 1 0 5 r O _5 - 1 0 - 1 5 F. SHERWOOD RO WLAND PERCENT | OZONE CHANGE t 53°N ~ 64°N 1969 ~ 1986 I 1 1 1 1 1 1 1 1 1 1 1 JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN b t , , f PERCENT t t OZONE CHANGE 40-N - 52°N 1969 - 1986 -15 + , ~ I I ~ ~ t JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN C t ~ " ' + t ~ I t t ~ PERCENT OZONE CHANGE 30- N - 39°N 1969 - 1986 JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN FIGURE 5-7 Percent change in total column ozone between 1969 and 1986 as a function of month for three Northern Hemisphere latitude bands. Estimates of uncertainty are shown by vertical lines. See text for additional explanation.
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ROLE OF HALOCARBONS 43 trend in the regression coefficients appears in several months for the 40°N to 52°N band (Figure ~8~. Detailed examination shows the coefficients tending to be more negative with added years of data, suggesting that the ozone decreases may not be quite linear but may tend toward steeper trends in later years. The great E! Chichon volcanic eruption in 1982 does not seem to have had any large effect on the regression coefficients (Dutsch, 1985~. In conclusion, the amount of measured ozone loss in the sum- mer is in reasonable agreement with theory, but the amount of loss poleward of 40°N latitude in winter is greater than that indicated by theoretical calculations. The statistical analysis suggests that there is something missing from the atmospheric models of ozone depletion that affects ozone levels in the Northern Hemisphere in winter. A speculation is that stratospheric ice clouds in the arctic region are having an effect somewhat similar to their effect in the Southern Hemisphere, even though the meteorological conditions are very dif- ferent in the two hemispheres. The fact that the wintertime ozone decrease in the Northern Hern~sphere diminishes gradually at lower latitudes, rather than abruptly as in the Southern Hemisphere, sug- gests that atmospheric circulation is causing dilution in the Northern Hemisphere. In the statistical analyses that were reported previously, e.g., the "no statistical change" result in the WMO-NASA report (WMO- NASA, 1986), the assumption was routinely made that the amount of Tong-term change was independent of month. Therefore, statis- tical analysts tried to fit all months with a trend having the same slope. Because the statistical reproducibility was much greater in the summer months than in the winter months (see Figure 5-5), the calculations tended to emphasize the summer months in the com- bined data and led to the conclusion that ozone concentrations were not changing much overall. Our studies show that, when the winter data are analyzed separately, a significant loss of ozone has occurred during the winter months. (In response to a question about the comparison of Dobson ground instrument data with satellite data): The basic problem with calibrating the satellite data is the known fact that the instrument's diffuser plate has been degracling under bombardment from the sun since launch in October 1978. Calibration is carried out periodically when the satellite passes over a ground-based Dobson instrument site. The ground-based instruments, in turn, are calibrated with the
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l 44 LL z 2 o (D -2 o lo 3 2 o U) O m o C) F. SHERWOOD ROWLAND LATITUDE BAND 40°N- 52°N MONTHLY L I NEAR REGRESSION COEFFICIENTS WITH SUCCESSIVE YEARS OF DATA, 1965 TO 1980 198 1 1982 1983 1984 1985 1986 :~ 11I o DEC o JAN o FEB o MAR ~O APR -2 ~ ~ ~ MAY 3 _ - 4 FIGURE 5-8 Computed regression coefficients for linear change in ozone con- centrations after 1969, using data for the periods 1965 to 1980, 1965 to 1981, and 1965 to 1982 through to 1986 for each calendar month for the latitude band 40°N to 52°N. Estimates of uncertainty are shown by vertical lines. See text for additional explanation.
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ROLE OF HALOCARBONS o -2 3 ~_4 llJ 2 en He 1 o at) O m o C) - 1 -2 3 4 LATITUDE BAND 40°N-52°N MONTHLY LINEAR REGRESSION COEFFICIENTS WITH SUCCESSIVE YEARS OF DATA, 1965 TO 1980 198 1 1982 1983 1984 1985 1986 11t Tll tll Ili ITI ~ T FIGURE 5-8 (continued). 45 o JUN O JUL o AUG o SEP O OCT o NOV ll Ti! ITT TIT
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46 F. SHERWOOD ROWLAND worId-standard instrument at Mauna Loa, and the satellite instru- ment is also compared directly with the Manna Loa instrument on some overpasses. from these comparisons, it is possible to estimate that the total degraciation in the satellite instrument since October 1978 has been about 3.5 percent. Comparing the average readings from 1979 to 1980 with those from 1986 to 1987 (two years are used to remove the ejects of the QBO) shows a loss of ozone in most parts of the world. However, because the solar cycle went from a maximum in the period 1979 to 1980 to a minimum in 1986, a general decline is predicted during this time period. The Dobson data, on the other hand, span 22 years or more, and the effect of the solar cycle can be statistically removed from the data. The solar cycle cannot be reliably removed from the satellite data. . (In answer to a question about the role of tropospheric ozone in the total ozone measurements): The fraction of ozone in the tropm sphere is approximately 10 percent of the total. There are indications that the amount of tropospheric ozone has been increasing at a rate of about 1 percent per year. Hence, the tropospheric contribution is increasing total ozone at a rate of about ~ percent per decade. If correct, this means that the stratospheric ozone losses are somewhat greater than the total ozone column measurements indicate because of the increase in tropospheric ozone. However, there is some un- certainty, because it Is not well known how uniform the amount and rate of increase of tropospheric ozone are globally. (In answer to a question about the Umkehr ozone data): The Ozone Mends Pane! (Watson et al., 1988) has analyzed the Umkehr data and finds a loss of ozone at 40 km of -9 percent, compared to a loss of -3 percent as measured by the Stratospheric Aerosol and Gas Experiment (SAGE) satellite instrument. Theory, depending on latitude and season, predicts a loss of from -5 to-12 percent. So, there is plausible agreement but also concern over whether the Umkehr and SAGE instruments are measuring the same thing. REFE:RENCE:S . Bloomfield, P., G. Oehlert, M.L. Thompson, and S. Zeger. 1983. A frequency domain analysis of trends in Dobson total ozone records. J. Geophys. Res. 88:8512-8522. Cunnold, D.M., R.G. Prinn, R.A. Rasmussen, P.G. Simmonds, F`.N. Alyea, C.A. Cardelino, A.J. Crawford, P.J. Freer, and R.D. Rosen. 1986. Atmospheric lifetime and annual release estimates for CFC4 and CF2CI2 from 5 years - of ALE data. J. Geophys. Res. 91:10797-10817. Dutsch, H.U. 1984. An update of the Arosa ozone series to the present using a statistical instrument calibration. Q. J. R. Meteorol. Soc. 110:1079-1096.
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ROLE OF HALOCARBONS 47 Dutsch, H.U. 1985. Total ozone in the light of ozone soundings, the impact of E1 Chichon. Pp. 263-268 in Atmospheric Ozone (Eds. C.S. Zerefos and E. Ghazi). D. Reidel Co., Dordrecht, The Netherlands. Rasmussen, R.A., and M.A.K. Khalil. 1986. Atmospheric trace gases: trends and distributions over the last decade. Science 232:1623-1624. Reinsel, G., G.C. Tiao, M.N. Wang, R. Lewis, and D. Nychka. 1981. Statistical analysis of stratospheric ozone data for the detection of trend. Atmos. Environ. 15:1569-1577. Reinsel, G., G.C. Tiao, J.L. DeLuisi, C.L. Mateer, A.J. Miller, and J.E. Frederick. 1984. Analysis of upper stratospheric Umkehr ozone profile data for trends and the effects of stratospheric aerosols. J. Geophys. Res. 89:4833-4840. Rowland, F.S., and M.J. Molina. 1976. Estimated future atmospheric concen- trations of CC13F (Fluorocarbon-11) for various hypothetical tropospheric removal rates. J. Phys. Chem. 80:2049-2056. St. John, D., W.H. Bailey, W.H. Fellner, J.M. Minor, and R.D. Sull. 1982. Time series analysis of stratospheric ozone. Commun. Stat., Part A 11:1293-1333. 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. World Meteorological Organization-National Aeronautics and Space Adminis- tration (WMO-NASA). 1986. Atmospheric Ozone 1985: Assessment of Our Understanding of the Processes Controlling Its Present Distribution and Change. Global Ozone Research and Monitoring Project, Report No. 16,3 vole., WMO, Geneva. l
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