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Contribution of solar UV radiation to the observed ozone variations during the 21st and 22nd solar cycles
Adv. Space Res. Vol. 27, No. 12, pp. 1949-1954.2001 0 ZOO1 COSPAR. Published by Elsevier Science Ltd. A11 rights reserved Printed in Great Britain 0273- 1177/01 $20.00 + 0.00
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CONTRIBUTION OF SOLAR UV RADIATION TO THE OBSERVED OZONE VARIATIONS DURING THE 21ST AND 22ND SOLAR CYCLES I. G. Dyominov Novosibirsk
and A. M. Zadorozhny
State University. Pirogova 2, Novosibirsk,
630090, Russia
ABSTRACT A two-dimensional dynamical radiative-photochemical model of the ozonosphere including aerosol physics is used to examine the changes of the Earth’s ozone layer occurred during the 2 1st and 22nd solar cycles. The calculated global total ozone changes in the latitude range 60’S_60”N caused by 1l-year variation of solar W radiation, volcanic eruptions, and anthropogenic atmospheric pollution containing CO*, CH4, N20, and chlorine and bromine species are in a rather good agreement with the observed global ozone trend. The calculations show that the anthropogenic pollution of the atmosphere is a main reason of the ozone depletion observed during the last two solar cycles. However, the 1l-year solar UV variation as well as volcanic eruptions of El Chichon and Mt. Pinatubo also gave a significant contribution to the observed global ozone changes. 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
INTRODUCTION Recent theoretical studies showed that the response of the Earth’s ozone layer to solar activity impacts such as solar proton events (SPEs) and changes of solar UV radiation depends significantly on stratospheric chlorine content as well as on stratospheric sulphate aerosol density (e.g., Dyominov, 1992; Brasseur, 1993; Haigh, 1994; Fleming et al., 1995; Jackman et al., 1996, 2000). The main source of the stratospheric chlorine species is believed to be anthropogenic discharges to the atmosphere (WMO, 1999). Major volcanic eruptions lead to the strong variability of the sulphate aerosol density in the stratosphere (e.g., Hofman and Rosen, 1984; Bluth et al., 1992; McCormic and Veiga, 1992). Temperature changes caused by pollution of the atmosphere, in particular, by COz can also impact the solar activity effects on the stratospheric ozone. To take into account all these and similar effects in the model investigations of the solar activity impacts to the ozone layer, we use 2-D self-consistent dynamical radiative-photochemical model including aerosol physics and consider in the calculations maximum possible number of factors simultaneously impacting chemical composition and temperature of the ozonosphere. In this paper, the model is used to estimate a contribution of solar W flux variation in the course of solar activity to the observed atmospheric ozone changes during the 21st and 22nd solar cycles. The feature of the model calculations is simultaneous consideration of an anthropogenic increase of greenhouse gases CO;?, CH.,, and N20 together with the pollution of the atmosphere by chlorine and bromine species. Changes of stratospheric content of sulphate aerosol due to observed increasing of sulphate gas constituents and due to volcanic eruptions are taken into consideration, too. DESCRIPTION OF THE MODEL The interactive numerical two-dimension zonally averaged model of the ozonosphere by Novosibirsk State University is used for investigation of the role of variation of solar UV radiation flux in atmospheric ozone changes observed during the 21st and 22nd solar cycles. The model allows calculating self-consistently diabatic circulation, temperature, gaseous composition of the troposphere and stratosphere at latitudes from the South to North Poles, as well as distribution of sulphate aerosol particles and polar stratospheric clouds (PSCs) of types I and II. The model calculates the distribution of 50 minor gas constituents of seven families: O,, HO,, NO,, Cl,, Brx, CHO,, and HSO,, and their source components, which interact in 179 gas phase photochemical reactions and in 10
1949
1950
I. G. Dyominov and A. M. Zadorozhny
heterogeneous reactions on surfaces of polar stratospheric clouds and sulphate aerosols Particles of sulphuric acid hydrate (sulphate aerosol) with radii of 0.0064 pm < r <5.2 pm, particles of nitric acid trihydrate (PSC-I) with radii of 0.1 pm I r 125.6 urn, and ice particles (PSC-II) with radii of 0.5 pm < r _<128 urn are considered in the model. Heterogeneous and homogenous nucleation, condensation, evaporation, coagulation, sedimentation, washout, transport by diabatic circulation and eddy diffusion are taken into account in calculations of their distribution. The temperature stratification of the atmosphere is determined in the model by the heat balance equation. Calculation of atmospheric heating and cooling rates takes into account heat fluxes due to convection, turbulent heat exchange, and radiative transfer in three spectral ranges: UV and visible (0.175-0.9 pm), near infrared (0.94.0 pm), and infrared (>4 pm). The radiative fluxes in the UV and visible part of the spectrum are computed with account taken for radiation scattering on air molecules and aerosol particles, reflection from clouds and the Earth’s surface, and absorption by 02, Oj, and NOz. In the near infrared, absorption by water vapour (in the 0.94, 1.1, 1.38, 1.87, 2.7, and 3.2 urn bands) and by carbon dioxide (in the 2.0 and 2.7 ,um bands) is considered. Absorption by COz, 01, HzO, CHI, NzO, CFC13 and CF&lz is considered for infrared radiation. The fine spectral structure of these constituents is taken into account in absorption bands centred around 15 urn for COz; 9.6 pm for 03; 6.3 pm for H?O; 7.6 pm for CH4; 4.5, 7.78, 8.57, and I7 pm for NjO; 9.22 and 11.82 pm for CFC&; and 8.68, 9.13, and 10.93.pm for CF2C12. For HIO, rotational bands are also taken into account. Dynamical processes in the atmosphere are represented in the model by the residual circulation and eddy diffusion. Detailed description of photochemical, radiative, dynamical and aerosol blocks of the model is given by Dyominov and Zadorozhny (1989); Dyominov et al. (1992, 1996, 1999); Weisenstein et al., (1998). SOURCES OF LONG-TERM OZONE VARIATIONS We considered in the calculations three sources of long-term ozone changes: variation of solar UV radiation in the course of the 1 l-year cycle of solar activity, anthropogenic pollution of the atmosphere and volcanic eruptions. Solar proton events and variations of galactic cosmic rays are not taken into account in the calculations. A noticeable decrease in ozone concentration of about 20-25% was detected only in the upper stratosphere during the SPE of October 1989, which was the largest SPE during the 21st and 22nd solar cycles (Zadorozhny et al., 1994). Changes of about 1% in total ozone after this gigantic SPE were predicted in the model simulations only at polar latitudes (Jackman et al., 1996, 2000). The variation in the solar UV flux between solar maximum and solar minimum conditions used in the model is taken from Huang and Brasseur (I 993) and is shown in Table 1. This variability of the incident solar irradiance as a function of wavelength is adopted from Rottman (1988). Temporal changes of solar UV flux in the model are accepted to follow a sine function with 1l-year period. Anthropogenic pollution of the atmosphere by CO*, CH4, N20, chlorine species (CFC- 1 I, CFC-12, CFC- 113, CFC-114, CFC-115, HCFC-22, HCFC-123, HCFC-l4lb, HCFC-142b, CC&, CH$I, CH3CC13), and bromine species (H-121 1, H-1301, CHxBr) is considered in the model calculations. The scenarios of anthropogenic trends of these species for the period from 1975 through 2000 are taken from WMO (1999). An increase of the annual average total stratospheric mass of sulphate aerosol particles from 300 kilotons in 1975 to 600 kilotons in 2000 is taken into account in the model by using the corresponding scenarios for gaseous sulphate constituents. Additionally, discharges to the atmosphere of sulphate species due to volcanic eruptions of El Chichon (April 4, 1982, Mexico) and Mt. Pinatubo Table 1. Percentage Change in Solar UV Flux from (June 15, 1991, Philippines), which were most powerful Solar Maximum to Solar Minimum Conditions Used during the 2 1st and 22nd solar cycles, are also considered in the Model (Huang and Brasseur, 1993) in the calculations. Formation of sulphate clouds during Change % Wavelength, nm a few days after the eruptions in the latitude range from 150-159 20 5”s to 15"N at the heights 19-26 km for El Chichon 14 159-170 eruption and at 16-34 km for Mt. Pinatubo eruption is 10 170-185 with the included in the model in accordance 9 185-190 experimental data (McCormic and Swissler, 1983; 7.6 190-200 Vcdder et al., 1983; Hofman and Rosen, 1984; Bluth et 6.6 200-208 al.. 1992; McCormic and Veiga, 1992). Total masses of 3 208-266 sulphate species in the clouds are accepted to be equal to 0.6 266-270 8.5 and 20 megatons for El Chichon and Mt. Pinatubo 2 270-271 eruptions accordingly. It is assumed that the clouds 6 277-282 consist of SO2 gas (65%) and sulphate aerosol particles 0.8 282-303 (35%).
Contribution
of Solar UV
Radiation
to Observed Ozone
Total Ozone Trend (%/Decade)
Total Ozone Change Solar Max - Solar Min
a
b
90
,
9or
_
F
30
8 2 z 3
0
.P
I
‘y_>----=-
.5>
60 _4u4c
60 G $
1951
GJ 4
30=-..---
y: ---yT=z
:: 5
-30 -60 -90
J
F
M
M
A
J
J
A
S
0
N
J
D
F
MAMJJASOND
Month
Month
Fig. 1. Calculated (a) percent change in total ozone due to UV flux variation from solar minimum to solar maximum and (b) total ozone trend during the 21 st and 22nd solar cycles due to anthropogenic pollution of the atmosphere by CH4, N20, and chlorine and bromine species.
RESULTS AND DISCUSSION Calculated changes of total ozone due to the solar W flux variation in the 1l-year cycle are shown in Figure la. Anthropogenic pollution of the atmosphere and volcanic eruptions are not considered in these calculations. Figure la shows the total ozone increase of -1% near equator and -1.7% in spring at polar latitudes when passing from solar minimum to solar maximum. Increasing of the solar W flux during solar activity maximum is responsible for this enhancement due to more intensive production of ozone through photolysis of molecular oxygen. The maximum changes at higher latitudes are caused by meridianal circulation of the atmosphere with downward motions there. Figure lb shows the total ozone trend due to anthropogenic pollution of the atmosphere by CH4, NzO, chlorine and bromine species calculated for the period of the 21st and 22nd solar cycles. Here, 1 l-year variation of the solar UV radiation and volcanic eruptions are not considered. Increasing of chlorine and bromine content in the atmosphere gives the main contribution to the calculated trend. Comparing Figures la and lb, we can see that the total ozone changes at low and middle latitudes caused by solar UV variation between solar minimum and solar maximum are comparable in absolute value with those due to anthropogenic pollution of the atmosphere during the same time period. At higher latitudes, the relative contribution of the UV radiation to the ozone changes is appreciably less than that of the pollution, especially in the Southern Hemisphere. Variation of solar W radiation in the course of the I l-year cycle leads to noticeable changes in ozone and the other minor gas constituents as well. Figure 2 shows altitude profiles of the changes in oxygen, chlorine, nitrogen, and hydrogen minor gas constituents caused by the UV variation calculated for March at 60”N. The main changes in the distribution of these constituents occur in the middle and upper stratosphere above about 20 km. The Solar Maximum/Solar Oxygen Components 50
7
40 -
-
2
+5 35B ,z Z
3025 20 -
-2
NitrogenComponents tt
Y t t
15 -
Minimum, March, 60”N
ChlorineComponents
0
2
4
6
tt
NO-
03 O(‘D)
‘4”s HN03
O(3P)
N2°5
8 10 -8 -6 -4 -2
1I
HydrogenComponents ‘kY
0 2 4-10 -8 -6 -4 -2 0 Percent Change
2
4
6 -2
0
2
4
6
8
Fig. 2. Calculated percent changes in oxygen, chlorine, nitrogen, and hydrogen constituents at 60”N in March caused by UV flux variation in the course of the 1 l-year cycle of solar activity.
19.52
I. G. Dyominov
and A. M. Zadorozhny
increasing of the oxygen constituents O(“D), O(3P), and 01 when passing to solar activity maximum is explained by increasing of photodissociation rate of molecular oxygen, which, in turn, is connected with increasing solar UV radiation flux. At altitudes near 45 km there is a distinct minimum in the ozone changes. This minimum is due to strong temperature dependence of the reaction of O3 with O(3P), which gives a substantial ozone loss near the stratopause, and hydrostatic coupling effect: the enhanced stratospheric temperature in the maximum of solar activity leads to a decrease of the ozone loss in this reaction at about 50 km, that is, an increase of the ozone changes here, and to a decrease of the ozone changes at the lower altitudes because of increased atmospheric density. The enhancement of Cl and Cl0 concentrations in the upper stratosphere in the maximum of solar activity is mainly due to their increased production through the photolysis of CFCs and reactions O(‘D) with CFCs. The character of the changes of the nitrogen constituents in the 1 l-year solar cycle is explained mostly by reaction between O(‘D) and NzO, which is the main source of the NO, in the stratosphere. The enhancement of O(rD) concentration leads, through this reaction, to increased NO, content in the maximum of solar activity. On the other hand, photodissociation of N20 is the main sink for nitrous oxide in the stratosphere, and enhancement of the photolysis due to increased solar UV radiation flux results in decrease of its content and consequently NO, content in the upper stratosphere in the maximum of solar activity. The enhancement of the content of the hydrogen constituents, OH, HOa and H202, is caused by a water vapour increase, which, in turn, is caused by more intense methane oxidation in the maximum of solar activity initiated by the increased solar UV radiation. Calculated time dependence of global total ozone changes between 60”s and 60”N due to 1l-year UV flux variation during the Zlst and 22nd solar cycles is shown in Figure 3a. Results of the calculations showed that when passing from minimum to maximum of solar activity, the global total ozone increases by about 1.15%. Figures 3b and 3c show the calculated changes in the global ozone due to the volcanic eruptions of El Chichon and Mt. Pinatubo, Figure 3 shows that the changes of the global ozone due to the solar ultraviolet flux variations in the course of the 1 l-year solar cycle are comparable to that caused by the volcanic eruptions. Duration of the volcanic disturbances does not exceed about two to three years. The changes of the chemical composition of the atmosphere due to anthropogenic pollution are not considered in these calculations. Figure 4 shows the calculated global ozone trend between 60”s and 60”N due to anthropogenic pollution of the atmosphere as well as the trend due to simultaneous impact of the different sources of long-term ozone variations considered in the model. Solid curve in Figure 4 shows calculated changes of the global total
Global Ozone Change
(60°S-60*N)
Solar UV flux variations
1975
1980
1985
1990
1995
”
2000
Year El Chichon
Pinatubo
0.0 -0.2 -0.4 -0.6 a, -0.8 ,” -1.0 : -1.2 : 5 $ a
-1.4 -1.6 -1.8 -2.0 -2.2 -2.4 -2.6 -2.8 Apr-82
Apr-83
Jun-91
Apr.84
Jun-92
Jun-93
Jun-94
Time
Time
Fig. 3. Calculated total ozone changes in the latitude range from 605 to 60”N caused by (a) UV flux variations in the course of 11-year solar cycle, (b) volcanic eruption of El Chichon, and (c) volcanic eruption of Mount Pinatubo. Panel (c) also includes the observational data (solid circles) by Gleason et al. (1993).
Global Ozone Trend (60°S-600N) 1.5 1.0 0.5 0.0
---_...__---___-_-___
-0.5 % -‘.O k -1.5 5
-2.0
E
-25
g
-3.0
a” -3.5 -4.0 -4.5 -5.0
- t
Pollution Pollution + WV Pollution + UV + Volcanoes
-5.5 -6.0 1975
1980
1985
1990
1995
2000
Year Fig. 4. Calculated percent changes of global total ozone in the latitude range from 605 to 6O*N caused by anthropogenic pollution of the atmosphere, UV flux variation in the course of the 1l-year solar cycle and volcanic eruptions.
Contribution
1953
of Solar UV Radiation to Observed Ozone
ozone in the latitude range from 60”s to 60% caused by Global Ozone Trend (60°S-60”N) 4 I I I I I I I I s I anthropogenic pollutants containing COZ, CH4, NZO, and We can see that the chlorine and bromine species. 2 Global Ozone Model anthropogenic pollution of the atmosphere leads to WMO (1999) zil 0 _____________________-______________. 5 decreasing of the global ozone by about 4.6% toward 1997 with respect to 1975. In this connection, the global if -2 ozone trend during the 2 1st and 22nd solar cycle due to r; 8 anthropogenic emission amounts to about -2.1% per ‘, -4 a decade which is approximately equal in absolute value to -6 the trend due to the solar UV variation when passing El Chichon Mt. Pinatubo from solar minimum to solar maximum or back. The dashed curve in Figure 4 shows the calculated global total ozone changes due to simultaneous impact of anthropogenic pollution of the atmosphere and 1 l-year solar UV variation. The solid curve with dots presents the global total ozone changes caused by simultaneous n impact of all considered factors: anthropogenic pollution 1978 1982 1986 1994 1990 1990 Year of the atmosphere, 1 l-year solar W flux variation and volcanic eruptions of El Chichon and Mt. Pinatubo. We Fig. 5. Observed global ozone trend (WMO, 1999) and can see that the main contribution to the global ozone calculated percent changes of the global total ozone in the trend is caused by the anthropogenic pollution of the latitude range from 605 to 60”N caused by simultaneous atmosphere. However, the 1 l-year UV variation as well influences of anthropogenic pollution of the atmosphere, as the eruptions of El Chichon and Mt. Pinatubo gave the UV flux variations in the course of the 1 l-year solar cycle and volcanic eruptions. Vertical arrows mark the important contribution to the changes of the global total moments of El Chichon and Mt. Pinatubo eruptions. ozone during the 2 1st and 22nd solar cycle. Thus, the Sunspot numbers are shown in the bottom panel. enhancements of the total ozone due to increases of solar UV radiation in 1976-1980 and 1986-1991 compensated the total ozone depletion caused by the anthropogenic pollution and led even to small global ozone rise during these periods. This effect together with El Chichon eruption happened during the fall of solar activity resulted in a period of relatively low ozone changes between 1983 and 199 1. Ozone depletion due to the Mt. Pinatubo eruption, which happened near the peak of solar activity, is more than twice greater than the ozone enhancement due to solar UV radiation. This led to the extremely low ozone content in 1993. It is important to note that anthropogenic changes of the chemical composition, temperature and dynamics of the atmosphere during the impact of volcanic sulphur species to the ozone layer prolong significantly the duration of the volcanic eruption influence to the global total ozone (cf. Figures 3 and 4). The reasons of this prolongation will be analysed in another paper. Figure 5 shows the calculated global total ozone trend in the latitude range from 60’S to 6OW caused by anthropogenic pollution of the atmosphere, volcanic eruptions, and solar W variation during the 2 1st and 22nd solar cycle in comparison with the observed trend (WMO, 1999). Comparing the data, we can see a rather good agreement between the measurements and the theoretical prediction. In particular, this is result of a self-consistent simultaneous consideration of the impacts of various sources of the long-term changes in the atmosphere. For example, excluding the effects of anthropogenic emission of CO1 from the self-consistent calculalions leads to significantly greater ozone depletion with maximum factor of about 1.2 at the end of 1990s.
.-d
I
I,
I
I
I
I
l
,
CONCLUSION The model calculations predict the increase of the total ozone content when passing from solar minimum to solar maximum due to variation of solar UV radiation in the course of 1 l-year cycle. The maximum increases of about 1.7% are predicted in spring at Polar Regions and the minimum increases of about 1% are expected near the equator. The solar induced increasing of the global total ozone in the latitude range from 60”s to 60% is about 1.15%. The changes at low and middle latitudes caused by solar W variation between solar minimum and solar maximum are comparable in absolute value with those due to anthropogenic pollution of the atmosphere during the same time period. At higher latitudes, the relative contribution of the UV radiation to the ozone changes is appreciably less than that of the pollution. The calculations show that the anthropogenic pollution of the atmosphere is a main reason of the ozone layer depletion observed during the 21 st and 22nd solar cycle. However, the 1 l-year solar W variation as well as volcanic eruptions of El Chichon and Mt. Pinatubo also gave the A self-consistent consideration of gaseous and important contribution to the observed global ozone changes. aerosol composition, temperature and circulation of the atmosphere used in the calculation of simultaneous impact
1954
I. G. Dyominov and A. M. Zadorozhny
of solar UV radiation, volcanic eruptions and anthropogenic pollution of the atmosphere to the ozone layer led to a rather good agreement between the observed global total ozone trend and the model prediction. ACKNOWLEDGEMENTS This work was supported under Grant No. 00-05-65 187 of the Russian Foundation Project No. 274 of the Russian Federal Program ‘Integration of Science and Education’.
for Basic Research and
REFERENCES Bluth, G. J., S. D. Doiron, C. C. Schnetzler, A. J. Krueger, and L. S. Walter, Global Tracking of the SO* Clouds from the June 1991 Mount Pinatubo Eruptions, Geophys. Res. Lett., 19, I5 I - 154, 1992. Brasseur, G., The Response of the Middle Atmosphere to Long-Term and Short-Term Solar Variability: A TwoDimensional Model, J. Geophys. Res., 98, 23,079-23,090, 1993. Dyominov, I. G., Solar Activity Impact onto the Anthropogenically Disturbed Atmosphere (in Russian), in Investigation of Atmospheric Ozone, pp. 98-104, Gidrometeoizdat, Moscow, 1992. Dyominov, I. G., and A. M. Zadorozhny, The Numerical Simulation of Anthropogenic Impacts on the Ozonosphere, in Atmospheric Ozone. Proceedings of the 1st American-Soviet Meeting on Atmospheric Ozone, pp. 186-2 17, Gidrometeoizdat, Moscow, 1989. Dyominov, I. G., N. F. Elansky, Yu. E. Ozolin, and V. K. Petukhov, The Estimation of Effects of Regular Rocket Launches “Energy” and”Shuttle” on the Ozone Layer and Earth’s Climate (in Russian), Preprint of Institute of Atmospheric Physics, Moscow, 1 IO pp., 1992. Dyominov, I. G., A. M. Zadorozhny, and N. F. Elansky, Effects of NO, and SO1 Injections by Supersonic Aviation on Sulfate Aerosol and Ozone in the Troposphere and Stratosphere, in Proceedings of the International Colloquium “Impact of Aircraft Emission Upon the Atmosphere”, 2, pp. 595-600, Paris, 1996. Dyominov, I. G., M. A. Merzliakov, and N. A. Vityugova, Investigation of Global Influence of Nitrogen Oxides and Sulfur Dioxide Emission From Supersonic Aircraft on Ozone Layer and Aerosol Composition of Atmosphere, Final Report on the NASA Research Grant NAG 5-3409, 81pp., Novosibirsk, 1999. Fleming, E. L., S. Chandra, C. H. Jackman, D. B. Considine, and A. R. Douglass, The Middle Atmospheric Response to Short and Long Term Solar UV Variations: Analysis of Observations and 2D Model Results, J. Atmos. Terr. Phys., 57, 333-36.5, 199.5. Gleason, J. F., P. K. Bhartia, J. R. Herman, R. McPeters, P. Newman, R. S. Stolarski, L. Flynn, G. Labow, D. Larko, C. Sefior, C. Wellemeyr, W. D. Komhyr, A. J. Miller, and W. Planet, Record Low Global Ozone in 1992, Science, 260, 523-526, 1993. Haigh, J. D., The Role of Stratospheric Ozone in Modulating the Solar Radiative Forcing of Climate, Nature, 370, 544-546, 1994. Hofman, D. J., and J. M. Rosen, On the Temporal Variation of Stratospheric Aerosol Size and Mass During the First 18 Month Following the 1982 Eruptions of El Chichon, J. Geophys. Res., 89,4883-4890, 1984. Huang, T. Y. W., and G. P. Brasseur, Effect of Long-Term Solar Variability in a Two-Dimensional Interactive Model of the Middle Atmosphere, J. Geophys. Res., 98, 20,413-20,427, 1993. Jackman, C. H., E. L. Fleming, S. Chandra, D. B. Considine, and J. E. Rosenfield, Past, Present, and Future Modeled Ozone Trends with Comparison to Observed Trends, J. Geophys. Res., 101,28,7.53-28,767, 1996. Jackman, C. H., E. L. Fleming, and F. M. Vitt, Influence of Extremely Large Solar Proton Events in a Changing Stratosphere, J. Geophys. Rex, 105, 11,659-l 1,670, 2000. McCormic, M. P., and T. J. Swissler, Stratospheric Aerosol Mass and Latitudinal Distribution of the El Chichon Eruption for October 1982, Geophys. Rex Lett., 10, 877-880, 1983. McCormic, M. P., and R. E. Veiga, SAGE II Measurements of Early Pinatubo Aerosols, Geophys. Res. Lett., 19, 155-158, 1992. Rottman, G. J., Observation of Solar UV and EUV Variability, Adv. Space Res., 8, 53-66, 1988. Vedder, J. F., E. P. Condon, E. C. Inn, K. D. Tabor, and M. A. Kritz, Measurements of Stratospheric SO2 after the El Chichon Eruptions, Geophys. Res. Lett., 10, 1045-1048, 1983. Weisenstein, D., M. K. W. Ko, I. G. Dyominov, G. Pitari, L. Ricciardulli, G. Visconti, and S. Bekki, The Effects of Sulfur Emissions from HSCT Aircraft: A 2-D Model Intercomparison, J. Geophys. Rex, 103, 1527-1547, 1998. WMO, Scient$c Assessment of Ozone Depletion: 1998, Word Meteorological Organization, Global Ozone Research and Monitoring Project, Report No. 44, Geneva, Switzerland, 1999. Zadorozhny, A. M., V. N. Kikhtenko, G. A. Kokin, G. A. Tuchkov, A. A. Tyutin, A. F. Chizhov, and 0. V. Shtirkov, Middle Atmosphere Response to the Solar Proton Events of October 1989 Using the Results of Rocket Measurements, J. Geophys. Res., 99, 2 I ,059-2 1,069, 1994.
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CONTRIBUTION OF SOLAR UV RADIATION TO THE OBSERVED OZONE VARIATIONS DURING THE 21ST AND 22ND SOLAR CYCLES I. G. Dyominov Novosibirsk
and A. M. Zadorozhny
State University. Pirogova 2, Novosibirsk,
630090, Russia
ABSTRACT A two-dimensional dynamical radiative-photochemical model of the ozonosphere including aerosol physics is used to examine the changes of the Earth’s ozone layer occurred during the 2 1st and 22nd solar cycles. The calculated global total ozone changes in the latitude range 60’S_60”N caused by 1l-year variation of solar W radiation, volcanic eruptions, and anthropogenic atmospheric pollution containing CO*, CH4, N20, and chlorine and bromine species are in a rather good agreement with the observed global ozone trend. The calculations show that the anthropogenic pollution of the atmosphere is a main reason of the ozone depletion observed during the last two solar cycles. However, the 1l-year solar UV variation as well as volcanic eruptions of El Chichon and Mt. Pinatubo also gave a significant contribution to the observed global ozone changes. 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
INTRODUCTION Recent theoretical studies showed that the response of the Earth’s ozone layer to solar activity impacts such as solar proton events (SPEs) and changes of solar UV radiation depends significantly on stratospheric chlorine content as well as on stratospheric sulphate aerosol density (e.g., Dyominov, 1992; Brasseur, 1993; Haigh, 1994; Fleming et al., 1995; Jackman et al., 1996, 2000). The main source of the stratospheric chlorine species is believed to be anthropogenic discharges to the atmosphere (WMO, 1999). Major volcanic eruptions lead to the strong variability of the sulphate aerosol density in the stratosphere (e.g., Hofman and Rosen, 1984; Bluth et al., 1992; McCormic and Veiga, 1992). Temperature changes caused by pollution of the atmosphere, in particular, by COz can also impact the solar activity effects on the stratospheric ozone. To take into account all these and similar effects in the model investigations of the solar activity impacts to the ozone layer, we use 2-D self-consistent dynamical radiative-photochemical model including aerosol physics and consider in the calculations maximum possible number of factors simultaneously impacting chemical composition and temperature of the ozonosphere. In this paper, the model is used to estimate a contribution of solar W flux variation in the course of solar activity to the observed atmospheric ozone changes during the 21st and 22nd solar cycles. The feature of the model calculations is simultaneous consideration of an anthropogenic increase of greenhouse gases CO;?, CH.,, and N20 together with the pollution of the atmosphere by chlorine and bromine species. Changes of stratospheric content of sulphate aerosol due to observed increasing of sulphate gas constituents and due to volcanic eruptions are taken into consideration, too. DESCRIPTION OF THE MODEL The interactive numerical two-dimension zonally averaged model of the ozonosphere by Novosibirsk State University is used for investigation of the role of variation of solar UV radiation flux in atmospheric ozone changes observed during the 21st and 22nd solar cycles. The model allows calculating self-consistently diabatic circulation, temperature, gaseous composition of the troposphere and stratosphere at latitudes from the South to North Poles, as well as distribution of sulphate aerosol particles and polar stratospheric clouds (PSCs) of types I and II. The model calculates the distribution of 50 minor gas constituents of seven families: O,, HO,, NO,, Cl,, Brx, CHO,, and HSO,, and their source components, which interact in 179 gas phase photochemical reactions and in 10
1949
1950
I. G. Dyominov and A. M. Zadorozhny
heterogeneous reactions on surfaces of polar stratospheric clouds and sulphate aerosols Particles of sulphuric acid hydrate (sulphate aerosol) with radii of 0.0064 pm < r <5.2 pm, particles of nitric acid trihydrate (PSC-I) with radii of 0.1 pm I r 125.6 urn, and ice particles (PSC-II) with radii of 0.5 pm < r _<128 urn are considered in the model. Heterogeneous and homogenous nucleation, condensation, evaporation, coagulation, sedimentation, washout, transport by diabatic circulation and eddy diffusion are taken into account in calculations of their distribution. The temperature stratification of the atmosphere is determined in the model by the heat balance equation. Calculation of atmospheric heating and cooling rates takes into account heat fluxes due to convection, turbulent heat exchange, and radiative transfer in three spectral ranges: UV and visible (0.175-0.9 pm), near infrared (0.94.0 pm), and infrared (>4 pm). The radiative fluxes in the UV and visible part of the spectrum are computed with account taken for radiation scattering on air molecules and aerosol particles, reflection from clouds and the Earth’s surface, and absorption by 02, Oj, and NOz. In the near infrared, absorption by water vapour (in the 0.94, 1.1, 1.38, 1.87, 2.7, and 3.2 urn bands) and by carbon dioxide (in the 2.0 and 2.7 ,um bands) is considered. Absorption by COz, 01, HzO, CHI, NzO, CFC13 and CF&lz is considered for infrared radiation. The fine spectral structure of these constituents is taken into account in absorption bands centred around 15 urn for COz; 9.6 pm for 03; 6.3 pm for H?O; 7.6 pm for CH4; 4.5, 7.78, 8.57, and I7 pm for NjO; 9.22 and 11.82 pm for CFC&; and 8.68, 9.13, and 10.93.pm for CF2C12. For HIO, rotational bands are also taken into account. Dynamical processes in the atmosphere are represented in the model by the residual circulation and eddy diffusion. Detailed description of photochemical, radiative, dynamical and aerosol blocks of the model is given by Dyominov and Zadorozhny (1989); Dyominov et al. (1992, 1996, 1999); Weisenstein et al., (1998). SOURCES OF LONG-TERM OZONE VARIATIONS We considered in the calculations three sources of long-term ozone changes: variation of solar UV radiation in the course of the 1 l-year cycle of solar activity, anthropogenic pollution of the atmosphere and volcanic eruptions. Solar proton events and variations of galactic cosmic rays are not taken into account in the calculations. A noticeable decrease in ozone concentration of about 20-25% was detected only in the upper stratosphere during the SPE of October 1989, which was the largest SPE during the 21st and 22nd solar cycles (Zadorozhny et al., 1994). Changes of about 1% in total ozone after this gigantic SPE were predicted in the model simulations only at polar latitudes (Jackman et al., 1996, 2000). The variation in the solar UV flux between solar maximum and solar minimum conditions used in the model is taken from Huang and Brasseur (I 993) and is shown in Table 1. This variability of the incident solar irradiance as a function of wavelength is adopted from Rottman (1988). Temporal changes of solar UV flux in the model are accepted to follow a sine function with 1l-year period. Anthropogenic pollution of the atmosphere by CO*, CH4, N20, chlorine species (CFC- 1 I, CFC-12, CFC- 113, CFC-114, CFC-115, HCFC-22, HCFC-123, HCFC-l4lb, HCFC-142b, CC&, CH$I, CH3CC13), and bromine species (H-121 1, H-1301, CHxBr) is considered in the model calculations. The scenarios of anthropogenic trends of these species for the period from 1975 through 2000 are taken from WMO (1999). An increase of the annual average total stratospheric mass of sulphate aerosol particles from 300 kilotons in 1975 to 600 kilotons in 2000 is taken into account in the model by using the corresponding scenarios for gaseous sulphate constituents. Additionally, discharges to the atmosphere of sulphate species due to volcanic eruptions of El Chichon (April 4, 1982, Mexico) and Mt. Pinatubo Table 1. Percentage Change in Solar UV Flux from (June 15, 1991, Philippines), which were most powerful Solar Maximum to Solar Minimum Conditions Used during the 2 1st and 22nd solar cycles, are also considered in the Model (Huang and Brasseur, 1993) in the calculations. Formation of sulphate clouds during Change % Wavelength, nm a few days after the eruptions in the latitude range from 150-159 20 5”s to 15"N at the heights 19-26 km for El Chichon 14 159-170 eruption and at 16-34 km for Mt. Pinatubo eruption is 10 170-185 with the included in the model in accordance 9 185-190 experimental data (McCormic and Swissler, 1983; 7.6 190-200 Vcdder et al., 1983; Hofman and Rosen, 1984; Bluth et 6.6 200-208 al.. 1992; McCormic and Veiga, 1992). Total masses of 3 208-266 sulphate species in the clouds are accepted to be equal to 0.6 266-270 8.5 and 20 megatons for El Chichon and Mt. Pinatubo 2 270-271 eruptions accordingly. It is assumed that the clouds 6 277-282 consist of SO2 gas (65%) and sulphate aerosol particles 0.8 282-303 (35%).
Contribution
of Solar UV
Radiation
to Observed Ozone
Total Ozone Trend (%/Decade)
Total Ozone Change Solar Max - Solar Min
a
b
90
,
9or
_
F
30
8 2 z 3
0
.P
I
‘y_>----=-
.5>
60 _4u4c
60 G $
1951
GJ 4
30=-..---
y: ---yT=z
:: 5
-30 -60 -90
J
F
M
M
A
J
J
A
S
0
N
J
D
F
MAMJJASOND
Month
Month
Fig. 1. Calculated (a) percent change in total ozone due to UV flux variation from solar minimum to solar maximum and (b) total ozone trend during the 21 st and 22nd solar cycles due to anthropogenic pollution of the atmosphere by CH4, N20, and chlorine and bromine species.
RESULTS AND DISCUSSION Calculated changes of total ozone due to the solar W flux variation in the 1l-year cycle are shown in Figure la. Anthropogenic pollution of the atmosphere and volcanic eruptions are not considered in these calculations. Figure la shows the total ozone increase of -1% near equator and -1.7% in spring at polar latitudes when passing from solar minimum to solar maximum. Increasing of the solar W flux during solar activity maximum is responsible for this enhancement due to more intensive production of ozone through photolysis of molecular oxygen. The maximum changes at higher latitudes are caused by meridianal circulation of the atmosphere with downward motions there. Figure lb shows the total ozone trend due to anthropogenic pollution of the atmosphere by CH4, NzO, chlorine and bromine species calculated for the period of the 21st and 22nd solar cycles. Here, 1 l-year variation of the solar UV radiation and volcanic eruptions are not considered. Increasing of chlorine and bromine content in the atmosphere gives the main contribution to the calculated trend. Comparing Figures la and lb, we can see that the total ozone changes at low and middle latitudes caused by solar UV variation between solar minimum and solar maximum are comparable in absolute value with those due to anthropogenic pollution of the atmosphere during the same time period. At higher latitudes, the relative contribution of the UV radiation to the ozone changes is appreciably less than that of the pollution, especially in the Southern Hemisphere. Variation of solar W radiation in the course of the I l-year cycle leads to noticeable changes in ozone and the other minor gas constituents as well. Figure 2 shows altitude profiles of the changes in oxygen, chlorine, nitrogen, and hydrogen minor gas constituents caused by the UV variation calculated for March at 60”N. The main changes in the distribution of these constituents occur in the middle and upper stratosphere above about 20 km. The Solar Maximum/Solar Oxygen Components 50
7
40 -
-
2
+5 35B ,z Z
3025 20 -
-2
NitrogenComponents tt
Y t t
15 -
Minimum, March, 60”N
ChlorineComponents
0
2
4
6
tt
NO-
03 O(‘D)
‘4”s HN03
O(3P)
N2°5
8 10 -8 -6 -4 -2
1I
HydrogenComponents ‘kY
0 2 4-10 -8 -6 -4 -2 0 Percent Change
2
4
6 -2
0
2
4
6
8
Fig. 2. Calculated percent changes in oxygen, chlorine, nitrogen, and hydrogen constituents at 60”N in March caused by UV flux variation in the course of the 1 l-year cycle of solar activity.
19.52
I. G. Dyominov
and A. M. Zadorozhny
increasing of the oxygen constituents O(“D), O(3P), and 01 when passing to solar activity maximum is explained by increasing of photodissociation rate of molecular oxygen, which, in turn, is connected with increasing solar UV radiation flux. At altitudes near 45 km there is a distinct minimum in the ozone changes. This minimum is due to strong temperature dependence of the reaction of O3 with O(3P), which gives a substantial ozone loss near the stratopause, and hydrostatic coupling effect: the enhanced stratospheric temperature in the maximum of solar activity leads to a decrease of the ozone loss in this reaction at about 50 km, that is, an increase of the ozone changes here, and to a decrease of the ozone changes at the lower altitudes because of increased atmospheric density. The enhancement of Cl and Cl0 concentrations in the upper stratosphere in the maximum of solar activity is mainly due to their increased production through the photolysis of CFCs and reactions O(‘D) with CFCs. The character of the changes of the nitrogen constituents in the 1 l-year solar cycle is explained mostly by reaction between O(‘D) and NzO, which is the main source of the NO, in the stratosphere. The enhancement of O(rD) concentration leads, through this reaction, to increased NO, content in the maximum of solar activity. On the other hand, photodissociation of N20 is the main sink for nitrous oxide in the stratosphere, and enhancement of the photolysis due to increased solar UV radiation flux results in decrease of its content and consequently NO, content in the upper stratosphere in the maximum of solar activity. The enhancement of the content of the hydrogen constituents, OH, HOa and H202, is caused by a water vapour increase, which, in turn, is caused by more intense methane oxidation in the maximum of solar activity initiated by the increased solar UV radiation. Calculated time dependence of global total ozone changes between 60”s and 60”N due to 1l-year UV flux variation during the Zlst and 22nd solar cycles is shown in Figure 3a. Results of the calculations showed that when passing from minimum to maximum of solar activity, the global total ozone increases by about 1.15%. Figures 3b and 3c show the calculated changes in the global ozone due to the volcanic eruptions of El Chichon and Mt. Pinatubo, Figure 3 shows that the changes of the global ozone due to the solar ultraviolet flux variations in the course of the 1 l-year solar cycle are comparable to that caused by the volcanic eruptions. Duration of the volcanic disturbances does not exceed about two to three years. The changes of the chemical composition of the atmosphere due to anthropogenic pollution are not considered in these calculations. Figure 4 shows the calculated global ozone trend between 60”s and 60”N due to anthropogenic pollution of the atmosphere as well as the trend due to simultaneous impact of the different sources of long-term ozone variations considered in the model. Solid curve in Figure 4 shows calculated changes of the global total
Global Ozone Change
(60°S-60*N)
Solar UV flux variations
1975
1980
1985
1990
1995
”
2000
Year El Chichon
Pinatubo
0.0 -0.2 -0.4 -0.6 a, -0.8 ,” -1.0 : -1.2 : 5 $ a
-1.4 -1.6 -1.8 -2.0 -2.2 -2.4 -2.6 -2.8 Apr-82
Apr-83
Jun-91
Apr.84
Jun-92
Jun-93
Jun-94
Time
Time
Fig. 3. Calculated total ozone changes in the latitude range from 605 to 60”N caused by (a) UV flux variations in the course of 11-year solar cycle, (b) volcanic eruption of El Chichon, and (c) volcanic eruption of Mount Pinatubo. Panel (c) also includes the observational data (solid circles) by Gleason et al. (1993).
Global Ozone Trend (60°S-600N) 1.5 1.0 0.5 0.0
---_...__---___-_-___
-0.5 % -‘.O k -1.5 5
-2.0
E
-25
g
-3.0
a” -3.5 -4.0 -4.5 -5.0
- t
Pollution Pollution + WV Pollution + UV + Volcanoes
-5.5 -6.0 1975
1980
1985
1990
1995
2000
Year Fig. 4. Calculated percent changes of global total ozone in the latitude range from 605 to 6O*N caused by anthropogenic pollution of the atmosphere, UV flux variation in the course of the 1l-year solar cycle and volcanic eruptions.
Contribution
1953
of Solar UV Radiation to Observed Ozone
ozone in the latitude range from 60”s to 60% caused by Global Ozone Trend (60°S-60”N) 4 I I I I I I I I s I anthropogenic pollutants containing COZ, CH4, NZO, and We can see that the chlorine and bromine species. 2 Global Ozone Model anthropogenic pollution of the atmosphere leads to WMO (1999) zil 0 _____________________-______________. 5 decreasing of the global ozone by about 4.6% toward 1997 with respect to 1975. In this connection, the global if -2 ozone trend during the 2 1st and 22nd solar cycle due to r; 8 anthropogenic emission amounts to about -2.1% per ‘, -4 a decade which is approximately equal in absolute value to -6 the trend due to the solar UV variation when passing El Chichon Mt. Pinatubo from solar minimum to solar maximum or back. The dashed curve in Figure 4 shows the calculated global total ozone changes due to simultaneous impact of anthropogenic pollution of the atmosphere and 1 l-year solar UV variation. The solid curve with dots presents the global total ozone changes caused by simultaneous n impact of all considered factors: anthropogenic pollution 1978 1982 1986 1994 1990 1990 Year of the atmosphere, 1 l-year solar W flux variation and volcanic eruptions of El Chichon and Mt. Pinatubo. We Fig. 5. Observed global ozone trend (WMO, 1999) and can see that the main contribution to the global ozone calculated percent changes of the global total ozone in the trend is caused by the anthropogenic pollution of the latitude range from 605 to 60”N caused by simultaneous atmosphere. However, the 1 l-year UV variation as well influences of anthropogenic pollution of the atmosphere, as the eruptions of El Chichon and Mt. Pinatubo gave the UV flux variations in the course of the 1 l-year solar cycle and volcanic eruptions. Vertical arrows mark the important contribution to the changes of the global total moments of El Chichon and Mt. Pinatubo eruptions. ozone during the 2 1st and 22nd solar cycle. Thus, the Sunspot numbers are shown in the bottom panel. enhancements of the total ozone due to increases of solar UV radiation in 1976-1980 and 1986-1991 compensated the total ozone depletion caused by the anthropogenic pollution and led even to small global ozone rise during these periods. This effect together with El Chichon eruption happened during the fall of solar activity resulted in a period of relatively low ozone changes between 1983 and 199 1. Ozone depletion due to the Mt. Pinatubo eruption, which happened near the peak of solar activity, is more than twice greater than the ozone enhancement due to solar UV radiation. This led to the extremely low ozone content in 1993. It is important to note that anthropogenic changes of the chemical composition, temperature and dynamics of the atmosphere during the impact of volcanic sulphur species to the ozone layer prolong significantly the duration of the volcanic eruption influence to the global total ozone (cf. Figures 3 and 4). The reasons of this prolongation will be analysed in another paper. Figure 5 shows the calculated global total ozone trend in the latitude range from 60’S to 6OW caused by anthropogenic pollution of the atmosphere, volcanic eruptions, and solar W variation during the 2 1st and 22nd solar cycle in comparison with the observed trend (WMO, 1999). Comparing the data, we can see a rather good agreement between the measurements and the theoretical prediction. In particular, this is result of a self-consistent simultaneous consideration of the impacts of various sources of the long-term changes in the atmosphere. For example, excluding the effects of anthropogenic emission of CO1 from the self-consistent calculalions leads to significantly greater ozone depletion with maximum factor of about 1.2 at the end of 1990s.
.-d
I
I,
I
I
I
I
l
,
CONCLUSION The model calculations predict the increase of the total ozone content when passing from solar minimum to solar maximum due to variation of solar UV radiation in the course of 1 l-year cycle. The maximum increases of about 1.7% are predicted in spring at Polar Regions and the minimum increases of about 1% are expected near the equator. The solar induced increasing of the global total ozone in the latitude range from 60”s to 60% is about 1.15%. The changes at low and middle latitudes caused by solar W variation between solar minimum and solar maximum are comparable in absolute value with those due to anthropogenic pollution of the atmosphere during the same time period. At higher latitudes, the relative contribution of the UV radiation to the ozone changes is appreciably less than that of the pollution. The calculations show that the anthropogenic pollution of the atmosphere is a main reason of the ozone layer depletion observed during the 21 st and 22nd solar cycle. However, the 1 l-year solar W variation as well as volcanic eruptions of El Chichon and Mt. Pinatubo also gave the A self-consistent consideration of gaseous and important contribution to the observed global ozone changes. aerosol composition, temperature and circulation of the atmosphere used in the calculation of simultaneous impact
1954
I. G. Dyominov and A. M. Zadorozhny
of solar UV radiation, volcanic eruptions and anthropogenic pollution of the atmosphere to the ozone layer led to a rather good agreement between the observed global total ozone trend and the model prediction. ACKNOWLEDGEMENTS This work was supported under Grant No. 00-05-65 187 of the Russian Foundation Project No. 274 of the Russian Federal Program ‘Integration of Science and Education’.
for Basic Research and
REFERENCES Bluth, G. J., S. D. Doiron, C. C. Schnetzler, A. J. Krueger, and L. S. Walter, Global Tracking of the SO* Clouds from the June 1991 Mount Pinatubo Eruptions, Geophys. Res. Lett., 19, I5 I - 154, 1992. Brasseur, G., The Response of the Middle Atmosphere to Long-Term and Short-Term Solar Variability: A TwoDimensional Model, J. Geophys. Res., 98, 23,079-23,090, 1993. Dyominov, I. G., Solar Activity Impact onto the Anthropogenically Disturbed Atmosphere (in Russian), in Investigation of Atmospheric Ozone, pp. 98-104, Gidrometeoizdat, Moscow, 1992. Dyominov, I. G., and A. M. Zadorozhny, The Numerical Simulation of Anthropogenic Impacts on the Ozonosphere, in Atmospheric Ozone. Proceedings of the 1st American-Soviet Meeting on Atmospheric Ozone, pp. 186-2 17, Gidrometeoizdat, Moscow, 1989. Dyominov, I. G., N. F. Elansky, Yu. E. Ozolin, and V. K. Petukhov, The Estimation of Effects of Regular Rocket Launches “Energy” and”Shuttle” on the Ozone Layer and Earth’s Climate (in Russian), Preprint of Institute of Atmospheric Physics, Moscow, 1 IO pp., 1992. Dyominov, I. G., A. M. Zadorozhny, and N. F. Elansky, Effects of NO, and SO1 Injections by Supersonic Aviation on Sulfate Aerosol and Ozone in the Troposphere and Stratosphere, in Proceedings of the International Colloquium “Impact of Aircraft Emission Upon the Atmosphere”, 2, pp. 595-600, Paris, 1996. Dyominov, I. G., M. A. Merzliakov, and N. A. Vityugova, Investigation of Global Influence of Nitrogen Oxides and Sulfur Dioxide Emission From Supersonic Aircraft on Ozone Layer and Aerosol Composition of Atmosphere, Final Report on the NASA Research Grant NAG 5-3409, 81pp., Novosibirsk, 1999. Fleming, E. L., S. Chandra, C. H. Jackman, D. B. Considine, and A. R. Douglass, The Middle Atmospheric Response to Short and Long Term Solar UV Variations: Analysis of Observations and 2D Model Results, J. Atmos. Terr. Phys., 57, 333-36.5, 199.5. Gleason, J. F., P. K. Bhartia, J. R. Herman, R. McPeters, P. Newman, R. S. Stolarski, L. Flynn, G. Labow, D. Larko, C. Sefior, C. Wellemeyr, W. D. Komhyr, A. J. Miller, and W. Planet, Record Low Global Ozone in 1992, Science, 260, 523-526, 1993. Haigh, J. D., The Role of Stratospheric Ozone in Modulating the Solar Radiative Forcing of Climate, Nature, 370, 544-546, 1994. Hofman, D. J., and J. M. Rosen, On the Temporal Variation of Stratospheric Aerosol Size and Mass During the First 18 Month Following the 1982 Eruptions of El Chichon, J. Geophys. Res., 89,4883-4890, 1984. Huang, T. Y. W., and G. P. Brasseur, Effect of Long-Term Solar Variability in a Two-Dimensional Interactive Model of the Middle Atmosphere, J. Geophys. Res., 98, 20,413-20,427, 1993. Jackman, C. H., E. L. Fleming, S. Chandra, D. B. Considine, and J. E. Rosenfield, Past, Present, and Future Modeled Ozone Trends with Comparison to Observed Trends, J. Geophys. Res., 101,28,7.53-28,767, 1996. Jackman, C. H., E. L. Fleming, and F. M. Vitt, Influence of Extremely Large Solar Proton Events in a Changing Stratosphere, J. Geophys. Rex, 105, 11,659-l 1,670, 2000. McCormic, M. P., and T. J. Swissler, Stratospheric Aerosol Mass and Latitudinal Distribution of the El Chichon Eruption for October 1982, Geophys. Rex Lett., 10, 877-880, 1983. McCormic, M. P., and R. E. Veiga, SAGE II Measurements of Early Pinatubo Aerosols, Geophys. Res. Lett., 19, 155-158, 1992. Rottman, G. J., Observation of Solar UV and EUV Variability, Adv. Space Res., 8, 53-66, 1988. Vedder, J. F., E. P. Condon, E. C. Inn, K. D. Tabor, and M. A. Kritz, Measurements of Stratospheric SO2 after the El Chichon Eruptions, Geophys. Res. Lett., 10, 1045-1048, 1983. Weisenstein, D., M. K. W. Ko, I. G. Dyominov, G. Pitari, L. Ricciardulli, G. Visconti, and S. Bekki, The Effects of Sulfur Emissions from HSCT Aircraft: A 2-D Model Intercomparison, J. Geophys. Rex, 103, 1527-1547, 1998. WMO, Scient$c Assessment of Ozone Depletion: 1998, Word Meteorological Organization, Global Ozone Research and Monitoring Project, Report No. 44, Geneva, Switzerland, 1999. Zadorozhny, A. M., V. N. Kikhtenko, G. A. Kokin, G. A. Tuchkov, A. A. Tyutin, A. F. Chizhov, and 0. V. Shtirkov, Middle Atmosphere Response to the Solar Proton Events of October 1989 Using the Results of Rocket Measurements, J. Geophys. Res., 99, 2 I ,059-2 1,069, 1994.