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Infrared active gases are likely to change the dynamics and the stability of the atmosphere
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Atmospheric Environment Vol. 32, No. 16, pp. 2731—2736, 1998 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1352–2310(98)00055–7 1352—2310/98 $19.00#0.00
INFRARED ACTIVE GASES ARE LIKELY TO CHANGE THE DYNAMICS AND THE STABILITY OF THE ATMOSPHERE S. HARTWIG Universita¨t Wuppertal, 42097 Wuppertal, Germany (First received 1 May 1997 and in final form 25 January 1998. Published June 1998) Abstract—Examination of the monthly ground-level air concentration of the spallation product Be7 from up to three decades at 8 stations in 4 European countries shows a twofold trend for some specific months of the year. This trend is related to the month with the lowest concentration of the year and to the ratio of the month with the highest to that with the lowest concentration. Discussion of the possible causes for that trend leads to the conclusion that a steady change of the dynamic of the atmosphere can explain these findings. The positive trend of some seasonal Be7 concentrations indicate a positive trend of the stratospheric/tropospheric exchange possibly connected with higher tropospheric vertical diffusion. The general finding is commensurate with the trend of exponentially growing concentration of climatic gases within the atmosphere during the last decades. ( 1998 Elsevier Science Ltd. All rights reserved Key word index: Climatic gases, change of atmospheric stability, spallation products, Be-7, fossil fuel risk, long-term evaluation
INTRODUCTION
For over 30 years a growing concern has existed among scientists that increases in CO concentrations 2 in the atmosphere may be changing our climate to a considerable extent (Hartwig, 1981). Following sophisticated discussions and increased knowledge, it has become obvious that not only CO but also quite 2 a number of anthropogenic infrared active gases (AIAG) (i.e. CH , N O, FC11/12, etc.) have the 4 2 potential to alter our climate (Georgii 1981; see also Hartwig, 1981). As a result of these deliberations, there is an increasing amount of activity to model climatic changes and to predict their possible consequences. Scientific study groups have been founded all over the world which devote their efforts to help improve our understanding of the impending changes. The international organisations, e.g. the WMO, have become involved in channelling the efforts of these research groups. Numerous papers have been published on this subject by scientists and organisations (e.g. Cubasch, 1992/1995, WMO, 1988/90). The attention to climatic changes was focused by an interest in the matter itself on the one hand and the necessity of dealing with atmospheric parameters and with atmospheric phenomena which had to be easily recognisable against any type of background noise and stochastic variations on the other. Climatic parameters are determined through observation over periods of 30 years or more (WMO) and are therefore less susceptible to stochastic deteriorations than are short time parameters. Nevertheless,
the AIAG will change not only these climatic parameters but also short time conditions, especially short time dynamics of the atmosphere because inter alia they will change the heat source distribution of and the heat flow within the atmosphere (Raymond, 1993). In the following some aspects of a possible change of short-time parameters will be discussed. The atmosphere has a highly structured mode of existence. As with any physical phenomena in nature there exist many ways to describe and model its structure with varying degrees of complexity. In previous years, the atmosphere has been conceived of as a composition of an altitude dependent number of more or less static departments. They were thought of as well-mixed black boxes. The two lowest are the troposphere and the stratosphere separated by the tropopause, which is a dynamic non-static separation layer between the former two. A first step of a more complex description includes the consideration of the non-uniform distribution of the natural heat sources, i.e. the ground and the ozone layer. The heat source for the troposphere being the Earth’s surface, the temperature above the ground decreases and cool air layers are situated above warm areas. The troposphere is therefore usually unstable especially at low altitudes. In the stratosphere, the heat source, i.e. the ozone layer, is situated in the upper part leading to high stability. For many decades it has been observed that in the midlatitudes an exchange of air between stratosphere and troposphere exists (Dutkiewicz, 1985). This causes a characteristic seasonal pattern in the ground-level concentration of
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chemical and radioactive tracers which have their major source in the stratosphere. It was believed that this exchange occurs mainly in the midlatitudes by the tropopause folding related to the jet streams. Experimental evidence with radioactive tracers with different half-lives has shown that this exchange is not a continuous process but occurs in batches mainly during late spring and early summer. The nature and the details of the exchange processes described above have been supported through experimental measurements with spallation and fission products (e.g. Mu¨h, 1966; Hartwig, 1969; Mu¨h, 1969; Dibb, 1989, 1992). A still more complex description of the system would include the internal spatial structure of the troposphere and stratosphere mainly near the region of the tropopause, the temporal variation of the governing parameters and the coupling to the timedependent heat flow including scales of global dimensions. And indeed, in a recent publication (Holton, 1995) it has been shown that the stratosphere—troposphere exchange (STE) can be explained better if it is placed in the frame work of the general atmospheric circulation. It was pointed out that for STE—depending on the timescales—large-scale and small-scale waves mainly in the troposphere play an important role for seasonal and longer durations. In contrast to earlier views at least partially the opinion is now favoured that it is not tropopause-folding events but the concept of non-local control with wave dissipation that leads to the springtime or early summer maximum of radioactive or chemical tracers (Holton, 1995). Nevertheless, despite our growing knowledge about the interdependencies within the troposphere and stratosphere and STE, simple model concepts as sketched above can still play an important role in gaining an insight into the modes of the atmosphere’s behaviour. This is especially true if experimentally established changes of a single parameter are discussed as done in these publications, because complex models often have the freedom of several adaptable parameters which are not matched by reliable experimental data sets. In this publication a change in the seasonal pattern of ground-level air concentration of Be7 over several decades is discussed and put into perspective with possible dynamical changes and the growing amount of climatic gases in the atmosphere. Due to the growing concentration of the AIAG during the last decades, the relative importance of the distribution of the above described heat sources will gradually change because the AIAG will be uniformly distributed throughout the atmosphere in contrast to ground and ozone layer thus diminishing the relative importance of the latter as temperature gradient generating heat sources. Consequently, one can expect that the stability, eddy diffusivity and exchange processes will change too with the consequence that the seasonal pattern of tracer concentrations could change likewise.
MODE OF EVALUATION
Source configuration Spallation products such as Be7, P32, P33, Na35, Na24, etc., are produced by cosmic rays. The target nuclei for spallation in the atmosphere are mainly those of nitrogen, oxygen and argon. Because the number per volume of those nuclei is a function of altitude (barometric gradient) the production rate of the spallation product is also altitude-dependent. Spallation is a different nuclear process than fission. If a cosmic ray particle hits a target nucleus an intermediate ‘‘fire ball’’ is formed from which nucleons evaporate. The energy for that evaporation must be provided by the incoming cosmic ray particle. Therefore, an energy threshold exists above which spallation is possible. The particles should have an energy of 200 MeV or more. Cascading primary cosmic ray particles therefore generate an altitude-dependent particle flux which grows initially with the penetration of the atmosphere but will be less effective at lower altitudes because of insufficient energy per particle due to the growing number of particles and the absorption of energy. Both effects, air density and cascading particle flux, give rise to a strong altitude dependency of spallation products per mass unit of target material with an increase of up to 3 orders of magnitude from the ground to up to 40 km near the pole and a subsequent decrease at higher altitudes and at all latitudes. As spallation implies evaporation of nucleons from the target nucleus—spallation products must have a lower mass number than the original target nucleus thus lower than the number of nitrogen, oxygen or argon. In addition to the altitude dependency, a latitude dependency of spallation product concentration exists because of the terrestrial magnetic field. The magnetic stiffness for protons of cosmogenic origin is highest at the equator and lowest at the poles. Consequently, the spallation product concentration at the pole is higher than at the equator. Alltogether the polar stratosphere experiences the highest production rate and the tropical lower troposphere the lowest (Lal, 1962/67). If the production rate per unit volume is considered, the maximum is at 12 km near the equator and at 16 km near the pole (Mu¨h, 1966). ¹he annual course of Be7 Because of the altitude and latitude dependency of spallation in the atmosphere air concentration measurements of isotopes produced by spallation have been used to investigate atmospheric transport patterns (e.g. Mu¨h, 1966/69; Hartwig, 1973; Dutkiewicz, 1985; Feely, 1989; Brost, 1991) such as STE. It has been shown that the annual pattern of groundlevel-air Be7-concentration in the midlatitude, with its maximum during late spring, for most of the stations can be explained by the flux of air from the stratosphere to the ground which accounts for up to 40% of
Dynamics and the stability of the atmosphere
the amount (Koch, 1996; Dutkiewicz, 1985) of the month with the highest concentration. The rest is of tropospheric origin. Because STE is a consequence inter alia of the dynamics and stability patterns of the troposphere and the stratosphere—and these in essence depend on the heat source distribution and heat flow—it is to be expected that with a gradual change of the heat source pattern over the years, a gradual change of STE and spallation product concentration will likewise occur.
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time span it is advisable to consider as short a period within the year as possible because the signal/noise ratio is getting increasingly worse with longer averaging periods such as annual means. In addition if various changes of dynamics appear at different times of the year their effect can cancel out completely in annual means. Therefore, the month with the lowest concentration of each year (Min1) has been used for the discussion. Sun spot activity
Radiation age Experimental investigations have shown (Lal, 1964; Mu¨h, 1966; Dutkiewicz, 1985) that the radiation age of stratospheric air parcels is much higher due to the low eddy diffusivity compared to the tropospheric situation. Thus, stratospheric air parcels will usually have a very high specific spallation-isotope activity because of the longer radiation age and higher cosmic ray flux (e.g. Lal, 1962/67; Mu¨h, 1966). If the stratosphere becomes less stable due to the increasing AIAG concentration, which mitigate the negative temperature gradient (Raymond, 1993) lower stability and a lower radiation age will result. Consequently, the specific air parcel radioactivity connected with increasing STE will decrease. Causes for deviations In order to obtain information about the dynamics of the atmosphere from ground-level air concentration measurements of spallation products one has to discuss and understand the annual course of the concentration. The concentration of spallation products, such as Be7, in the troposphere is caused primarily by the intensity of the cosmic radiation and by the irradiation period of the air parcel under consideration. Be7 concentration in ground-level air can vary for three groups of reasons. Despite (a) the varying production rates, (b) atmospheric removal processes such as dry and wet deposition can change. In addition, (c) dynamic processes such as stratosphere-troposphere exchange (STE), downward transfer (DT) or horizontal transfer (HT) can change too. The latter on the condition that the dynamic processes connect atmospheric areas with principally strongly different Be7 concentrations.
Figure 1 shows the annual-mean, ground-level Be7 concentrations at five stations in France over three decades (IPSN, 1996) and the average of the Min. 1 concentrations of the same stations. The average concentration at a German station (Braunschweig) is also shown (Kolb, 1995). This can be compared with the inverse sun spot activity over the same period of time which is shown in arbitrary units (Koch, 1996). A correlation can be seen and leads to correlation coefficients of r"0.59 (France), r"0.60 (Braunschweig) and r"0.48 (France Min. 1). The correlation is obviously obscured by atmospheric dynamics and removal processes. The smaller r-value for Min 1 is understandable because of the stronger influence of local tropospheric weather conditions. In a discussion of any Be7 concentration trends over durations of decades, the effect of the sunspot activity should therefore be considered. This can be done either by using concentration ratios within a year where the sunspot activity influence is cancelled out or by taking the sunspot effect directly into account. ¹he trend of Be7 concentration In the following, I will discuss Be7 ground level air concentrations. To discern any influence of the increasing amount of AIAG and subsequent changes of atmospheric dynamics on Be7-concentrations complete and uninterrupted data sequences over decades are required, which are very sparse. Certainly there
Changing production rate With regard to the possibility of a changing Be7 production rate, the yield of spallation itself as the cause can be excluded. If we discard the possibility of a changing number of target nuclei per volume (i.e. N , O ) at a given altitude the only possibility is 2 2 a change in the spectrum of the incoming cosmogenic particles. And indeed such a change is possible due to the solar sunspot cycle (Lal, 1967). Selection of time interval To study the variations in atmospheric tracer concentrations caused by an exchange during a limited
Fig. 1. Yearly average Be7 ground-level concentration for Braunschweig (Germany) and the average of five French stations compared with the inverse function of the sunspot activity. Added is the month of the year with the lowest activity for the average of the five French stations (Min 1 France)
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Fig. 2. Linear regressions for different stations and time spans of the month with the lowest concentration per year. All regressions show uniformly a positive regression gradient. The values for the gradient for the regression lines in the succession of citation are: Skibotn 10.3; Ris+ 49.8; Braunschweig 13.2; France 11.7; Braunschweig 66.6; France 76.9.
exist quite a few Be7 ground level air concentration measurements (e.g. Feely, 1989; Larsen, 1995) but most of the long-term data have interruptions. With these interruptions it is difficult to judge their influence on the evaluation without detailed knowledge of the reasons for the data’s abscence. In Fig. 2, linear regressions of Min. 1 are shown for Braunschweig/Germany (1965—94, 1982—94), for 5 stations in France (1965—95, 1982—95), for Ris+/Denmark (1982—95) (Aarkrog, 1996) and for Skibotn/Sweden (1975—90) (Kolb, 1995). The unambigious trend is increasing concentrations for all six data sets. ¹he influence of sun spot activity Several causes are possible for that effect as already mentioned as items (a)—(c). Because an influence of the sunspot activity (Lal, 1967) principally cannot be excluded (item (a)), in Fig. 3, the linear regression for the inverse sunspot activity (1965—93) is shown together with Min. 1 concentrations of the French stations for the same period of time (1965—93) and the corresponding Be7 linear regression. The gradient of the sunspot regression is strongly negative (!15) whereas that of the concentration values is positive (#5). (The scales of the ordinates for both calculations had been converted to make a correct comparison possible.) That means that the increasing Min 1-concentration values are not caused by the sunspot activity. On the contrary, for that time span the sunspot activity would mitigate this effect. Removal processes To discuss a possible influence of wet removal processes (item (b)), the correlation between the amount of precipitation and Be7 ground-level air concentration has been calculated for the Braunschweig station over the identical time series (i.e. 1965—1994). Under wet removal processes, wash out as well as rain out is understood. The correlation coefficient amounts to
Fig. 3. Minimal monthly Be7 ground-level concentration per year for the five French stations and the inverse sunspot activity between 1965 and 1993. The according linear regressions show different signs of the gradient. For a better comparison for the calculations the two scales of the ordinates are normalized to the amplitudes for the two curves.
r"!0.202 with a standard deviation of s"0.278. The r-value represents the average of the annual means. That is to say that the correlation coefficient indicates no dependency between precipitation and ground-level concentration of Be7 for the data set under consideration. In recent publications the annual concentration ratios of Be7, Max i , i"1,2, 6 r" i Min i have been discussed for Braunschweig (Hartwig, 1996) and Skibotn (Hartwig, 1995) data sets. This ratio is an indicator for the elongation of the annual concentration course and because of that indicates the influence of regions of high production rate or high radiation age on the ground-level concentration. r is the con1 centration ratio of the month with the highest to that of the lowest concentration within one year and r is 2 the month with the second highest to that of the second lowest concentration, etc. Discussing these concentration ratios has the advantage that for longtime data sets over decades the influence of the sunspot cycle is cancelled out. This influence is especially important for concentrations influenced by air of higher altitudes. In addition, peculiarities of meteorological conditions extending over a year cancel out too, which would otherwise deteriorate regressions over decades. The gradient of the regression lines of the r (i"1, 2, 6) from 1964 to 1995 have nearly i a continous run from r with the highest value to 1 r with the lowest which is understandable on phys6 ical grounds and will be discussed later. The set of gradients with rising i is: !0.0105, !0.0124, !0.0106, !0.0036, !0.0037, !0.0003. For the precipitation of corresponding ratios and months, the data set of gradients is: !0.0356, !0.0266, #0.0005, #0.015, !0.0199, #0.0301. Obviously, the precipitation data show a stochastic distribution
Dynamics and the stability of the atmosphere
Fig. 4. Linear regressions of Be7 concentration ratios of the month with the highest to the lowest concentration r per 1 year, the second highest to the second lowest r per year, etc. 2 for five stations in France. The regression gradient steadily becomes smaller. The bars indicate the standard deviation.
which again makes no influence of precipitation on the Be7 data trend plausible. Besides precipitation theoretically dry removal processes could play a role. Usually in European midlatitudes dry deposition on the average accounts for 10—20% of the entire deposited amount. Therefore, it is not very likely that dry deposition can explain the Be7 data trend. That leaves item (c) as the dynamic processes. Dynamic processes Indeed a change of atmospherical dynamics can be responsible for the effects under consideration. Feely (1989) has discussed the effects which can cause seasonal variations in Be7 concentrations in surface air. With regard to horizontal transfer, he believes that his data support the view that a lateral transport from middle latitudes and higher altitude (Mauna Loa, 3400 m) to the pole region exists. He discussed that situation by means of data sets from the South Pole and Mauna Loa. If this poleward transfer exists, it could only explain the Min1 data trend at our latitudes if we have been experiencing a continuous decrease of this kind of transport over the last 30 years connected with an according influence of higher tropospherical regions to the surface area. That leads to a possible influence of the tropospheric vertical transport. ¹ropospheric stability During a rather early stage of the discussion about the structure of the atmosphere it was pointed out (Hartwig, 1973) that the troposphere is not a wellmixed box as was sometimes assumed during those days but that the vertical eddy diffusivity can be very different at different altitudinal atmospheric layers. This assertion was based on rather strong altitudenal concentration gradients of spallation product concentrations which were measured and on the discussion of the concentration ratio of different spallation prod-
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ucts with different radioactive half-lives. Such ratios give evidence about the duration of atmospheric transport processes. In the light of these findings, the Be7 data trend could be caused by a gradual and long-term change of tropospheric eddy diffusivity or by enhanced STE or by both of these effects. It is not possible to decide on the basis of the Min 1 regression data alone which effect is responsible. Nevertheless, a supplementary consideration of the r -values disi cussed elsewhere (Hartwig, 1995, 1996) suggests that at least part of the effect is caused by STE. A decline of the r values over three decades can be caused by a rise i of the Min values, by a decline of the Max itself or by i i both. The rise of the Min itself is stronger than the i decline of the r , which indicates the possibility of i a slight increase of the Max . This leads to the coni clusion that at least part of the effect is due to the enhanced STE. Unfortunately, because of the interference of other effects (e.g. sunspot activity), it is not advisable to calculate Max regressions alone. i CONCLUSION
Data sets of Be7 ground-level air concentration of 8 stations in 4 countries (Denmark, France, Germany, Sweden) with sampling time periods between 15 and 32 years show a unique and steady trend of change for all stations. It can be excluded that this change of concentration is caused by the production rate, i.e. by a change of cosmic ray intensity or modulations due to the 11 year sunspot cycle. Because of the results of investigations with precipitation data, it seems also very unlikely that this trend is caused by atmospheric removal processes as discussed in some publications for other latitudes and stations (Feely, 1989). This change must therefore be caused by a change in the dynamics and the stability patterns in the atmosphere, i.e. changes of the (dynamical) structure of the only two layers of the atmosphere which contribute to the ground-level concentration of Be7, the troposphere and stratosphere. On the basis of the presented Be7 data sets it is not possible to decide whether a change in STE alone or a combination of changing STE plus a change of tropospheric eddy diffusion is responsible for that effect. Considerations show that it is to be expected that the stability properties at the altitude of the tropopause are changing steadily due to the growing amount of AIAG which alters the spatial structure of the heat source distribution in the atmosphere independent of additional dynamic heat flows within the troposphere and stratosphere. The time dependency of natural heat sources (ozone layer and Earth’s surface) due to the yearly cycle of the input of solar radiation and the relative strength is different to that caused by the AIAG. Therefore, the apparent changes in the atmospheric dynamics also have a yearly pattern and a trend which can be related to the growing concentration of the AIAG. Alltogether it is suggested that the discussed trend of the Be7 ground-level
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concentration is commensurate and in line with a expected change of the atmospheric dynamics without being able to decide whether STE or STE plus tropospheric vertical eddy diffusion is responsible for the trend. The findings described here add a new step to a better understanding of the risks of climatic gases. Acknowledgements—The processing of part of the data by E. Puls is gratefully acknowledged. In addition I am indebted to the unknown reviewers for very helpful comments.
REFERENCES
Aarkrog, A. et al. (1996) Radioaktivita¨ten i Ris+-I-571 (EN) Roskilde, Denmark Brost, R. A., Feichter, I. and Heimann (1991) Three-dimensional simulation of Beryllium-7 in a global climate model. Journal of Geophysical Research 96 (D12), 22,423—22,445. Cubasch, U., Hasselmann, K., Ho¨ck, H., Maier-Reimer, E., Mikolajewicz, U., Sauer, B. and Sausen, R. (1992). Timedependent greenhouse warming computations with a coupled ocean—atmosphere model. Climate Dynamics 8, 55—69. Cubasch, U., Santer, B. and Hegerl, G. (1995) Klimamodelle — wo stehen wir? Physikalische Bla¨ tter 51(4), 269—276. Dibb, J. (1989) Atmospheric deposition of beryllium 7 in the Chesapeake Bay Region. Journal of Geophysical Research 94(D2), 2261—2265. Dibb, J., Talbot, R. and Gregory, G. (1992) Beryllium 7 and lead 210 in the Western Hemisphere Arctic Atmosphere: observations from three recent aircraft-based Sampling programs. Journal of Geophysical Research 97(D15), 16709—16715. Dutkiewicz, V. and Husan, C. (1985) Stratospheric and tropospheric components in surface air. Journal of Geophysical Research 90(D3), 5783—5788. Feely, H. W., Larsen, R. J. and Sanderon, C. G. (1989) Factors that cause seasonal variations in beryllium-7 concentrations in surface air. Journal of Environmental Radioactivity 9, 223—249. Georgii, H. W., Heudorfer, W. and Jung, H. J. (1981), Struktur und Analyse von Klimamodellen und der Einflu{ der Spurengase auf den Temperaturhaushalt der Atmospha¨re, In: Die Auswirkungen von CO -Emissionen auf das 2 Klima, eds S. Hartwig et al. pp. 264—380. Hartwig, S. and Sittkus, A. (1969) Radioaktive Isotope als Luftmassenindikatoren II., Die Produktion von P33, Zeitschrift fur Naturforschung 24a, 908—912. Hartwig, S. and Sittkus, A. (1973), Be7 measurement in ground level air and the Austausch within the lower part of the troposphere. Nature, Physica scripta 241/106, 36—37.
Hartwig, S., Berner, W., Bienewitz, H. K., Georgii, H. W., Heiman, M., Heudorfer, W., Jung, H. J., Kamber, D., Lieth, H., Neftel, A., Oberbacher, B., Oeschger, H., Siegenthaler, U. and Zumbrunn, R. (1981) Die Auswirkungen von CO -Emissionen auf das Klima Forschungsbericht 2 No. 104 606 des Umweltbundesamtes Berlin, erstellt durch das Battelle-Institut. Hartwig, S. (1995) Further evidence of changing stability of the atmosphere. Radiocarbon 37(3), 961—962. Hartwig, S. (1996) Langja¨hrige Be7-Bodenluftmessungen lassen A®nderung des atmospha¨rischen Austauschverhaltens wa¨hrend der letzten Jahrzehnte vermuten. Zeitschrift fur Naturforschung 51a, 1139—1143. Holton, J. R., Hayness, P. H., McIntyre, M. E., Douglass, A. R., Road, R. B. and Pfister, L. (1995) Stratosphere—troposphere exchange. Reviews of Geophysics, 33(4), 403—439. IPSN/DPE (anonymous) (1996) Le cesium 137 et le be´ryllium 7 dans les ae´rosol atmosphe´riques. CE/Cadarache, France. Koch, D. H., Jacob, D. J. and Graustein, W. C. (1996) Vertical transport of tropospheric aerosols as indicated by Be7 and Pb210 in a chemical tracer model. Journal of Geophysical Research 101(D13), 18,651—18,666. Kolb, W. (1992/1995) Aktivita¨tskonzentration von Radionukliden in der bodennahen Luft Norddeutschlands und Nordnorwegens im Zeitraum von 1963 bis 1990. PTB-Ra-29-Report, Physikalisch-Technische Bundesanstalt, Braunschweig. Lal, D. and Peters, B. (1962) Cosmic ray produced isotopes and their applications to problems in geophysics In Progress in Cosmic Ray and Elementary Particle Physics, Vol. 6, pp. 3—74. North Holland, New York. Lal, D. and Peters, B. (1967) Cosmic ray produced radioactivity on the earth. In Handbuch der Physik, ed. S. Flu¨gge, Band XLVI/2: Kosmische Strahlung II, Ed. K. Sitte, pp. S. 551-612, 46/2, Springer, Berlin. Larsen, R. J., Sanderson, C. G. and Kada, J. (1995) EM¸ Surface Air Sampling Program, 1990—1993 Data. USDOE Report EML-572. Mu¨h, H., Sittkus, A., Albrecht, A. and Hartwig, S. (1966), Radioaktive Isotope als Luftmassenindikatoren I. Zeitschrift fur Naturforschung 21a, 1123—1128. Mu¨h, H. and Sittkus, A. (1969) Naturwissenschaften 56, 211—212. Raymond, S., Bradley, F., Kleining, T. and Diaz, H. (1993) Changes in Arctic temperature inversions in CDIAC communications carbon dioxide informations Analysis Center. Oak Ridge, TN 37831-8335, Fall 1993, 19, 1—4. Reiter, E. R. (1975) Reviews in Geophysics and Space Physics 13, 459—474. World Met. Org. (1988) WMO-Report 18, Genf. World Met. Org. (1990) WMO-Report 20, Genf. World Met. Org. (1988) WMO-Bulletin, Advisory Group on Greenhouse gases, Vol. 37/2, p. 111.
Atmospheric Environment Vol. 32, No. 16, pp. 2731—2736, 1998 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1352–2310(98)00055–7 1352—2310/98 $19.00#0.00
INFRARED ACTIVE GASES ARE LIKELY TO CHANGE THE DYNAMICS AND THE STABILITY OF THE ATMOSPHERE S. HARTWIG Universita¨t Wuppertal, 42097 Wuppertal, Germany (First received 1 May 1997 and in final form 25 January 1998. Published June 1998) Abstract—Examination of the monthly ground-level air concentration of the spallation product Be7 from up to three decades at 8 stations in 4 European countries shows a twofold trend for some specific months of the year. This trend is related to the month with the lowest concentration of the year and to the ratio of the month with the highest to that with the lowest concentration. Discussion of the possible causes for that trend leads to the conclusion that a steady change of the dynamic of the atmosphere can explain these findings. The positive trend of some seasonal Be7 concentrations indicate a positive trend of the stratospheric/tropospheric exchange possibly connected with higher tropospheric vertical diffusion. The general finding is commensurate with the trend of exponentially growing concentration of climatic gases within the atmosphere during the last decades. ( 1998 Elsevier Science Ltd. All rights reserved Key word index: Climatic gases, change of atmospheric stability, spallation products, Be-7, fossil fuel risk, long-term evaluation
INTRODUCTION
For over 30 years a growing concern has existed among scientists that increases in CO concentrations 2 in the atmosphere may be changing our climate to a considerable extent (Hartwig, 1981). Following sophisticated discussions and increased knowledge, it has become obvious that not only CO but also quite 2 a number of anthropogenic infrared active gases (AIAG) (i.e. CH , N O, FC11/12, etc.) have the 4 2 potential to alter our climate (Georgii 1981; see also Hartwig, 1981). As a result of these deliberations, there is an increasing amount of activity to model climatic changes and to predict their possible consequences. Scientific study groups have been founded all over the world which devote their efforts to help improve our understanding of the impending changes. The international organisations, e.g. the WMO, have become involved in channelling the efforts of these research groups. Numerous papers have been published on this subject by scientists and organisations (e.g. Cubasch, 1992/1995, WMO, 1988/90). The attention to climatic changes was focused by an interest in the matter itself on the one hand and the necessity of dealing with atmospheric parameters and with atmospheric phenomena which had to be easily recognisable against any type of background noise and stochastic variations on the other. Climatic parameters are determined through observation over periods of 30 years or more (WMO) and are therefore less susceptible to stochastic deteriorations than are short time parameters. Nevertheless,
the AIAG will change not only these climatic parameters but also short time conditions, especially short time dynamics of the atmosphere because inter alia they will change the heat source distribution of and the heat flow within the atmosphere (Raymond, 1993). In the following some aspects of a possible change of short-time parameters will be discussed. The atmosphere has a highly structured mode of existence. As with any physical phenomena in nature there exist many ways to describe and model its structure with varying degrees of complexity. In previous years, the atmosphere has been conceived of as a composition of an altitude dependent number of more or less static departments. They were thought of as well-mixed black boxes. The two lowest are the troposphere and the stratosphere separated by the tropopause, which is a dynamic non-static separation layer between the former two. A first step of a more complex description includes the consideration of the non-uniform distribution of the natural heat sources, i.e. the ground and the ozone layer. The heat source for the troposphere being the Earth’s surface, the temperature above the ground decreases and cool air layers are situated above warm areas. The troposphere is therefore usually unstable especially at low altitudes. In the stratosphere, the heat source, i.e. the ozone layer, is situated in the upper part leading to high stability. For many decades it has been observed that in the midlatitudes an exchange of air between stratosphere and troposphere exists (Dutkiewicz, 1985). This causes a characteristic seasonal pattern in the ground-level concentration of
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chemical and radioactive tracers which have their major source in the stratosphere. It was believed that this exchange occurs mainly in the midlatitudes by the tropopause folding related to the jet streams. Experimental evidence with radioactive tracers with different half-lives has shown that this exchange is not a continuous process but occurs in batches mainly during late spring and early summer. The nature and the details of the exchange processes described above have been supported through experimental measurements with spallation and fission products (e.g. Mu¨h, 1966; Hartwig, 1969; Mu¨h, 1969; Dibb, 1989, 1992). A still more complex description of the system would include the internal spatial structure of the troposphere and stratosphere mainly near the region of the tropopause, the temporal variation of the governing parameters and the coupling to the timedependent heat flow including scales of global dimensions. And indeed, in a recent publication (Holton, 1995) it has been shown that the stratosphere—troposphere exchange (STE) can be explained better if it is placed in the frame work of the general atmospheric circulation. It was pointed out that for STE—depending on the timescales—large-scale and small-scale waves mainly in the troposphere play an important role for seasonal and longer durations. In contrast to earlier views at least partially the opinion is now favoured that it is not tropopause-folding events but the concept of non-local control with wave dissipation that leads to the springtime or early summer maximum of radioactive or chemical tracers (Holton, 1995). Nevertheless, despite our growing knowledge about the interdependencies within the troposphere and stratosphere and STE, simple model concepts as sketched above can still play an important role in gaining an insight into the modes of the atmosphere’s behaviour. This is especially true if experimentally established changes of a single parameter are discussed as done in these publications, because complex models often have the freedom of several adaptable parameters which are not matched by reliable experimental data sets. In this publication a change in the seasonal pattern of ground-level air concentration of Be7 over several decades is discussed and put into perspective with possible dynamical changes and the growing amount of climatic gases in the atmosphere. Due to the growing concentration of the AIAG during the last decades, the relative importance of the distribution of the above described heat sources will gradually change because the AIAG will be uniformly distributed throughout the atmosphere in contrast to ground and ozone layer thus diminishing the relative importance of the latter as temperature gradient generating heat sources. Consequently, one can expect that the stability, eddy diffusivity and exchange processes will change too with the consequence that the seasonal pattern of tracer concentrations could change likewise.
MODE OF EVALUATION
Source configuration Spallation products such as Be7, P32, P33, Na35, Na24, etc., are produced by cosmic rays. The target nuclei for spallation in the atmosphere are mainly those of nitrogen, oxygen and argon. Because the number per volume of those nuclei is a function of altitude (barometric gradient) the production rate of the spallation product is also altitude-dependent. Spallation is a different nuclear process than fission. If a cosmic ray particle hits a target nucleus an intermediate ‘‘fire ball’’ is formed from which nucleons evaporate. The energy for that evaporation must be provided by the incoming cosmic ray particle. Therefore, an energy threshold exists above which spallation is possible. The particles should have an energy of 200 MeV or more. Cascading primary cosmic ray particles therefore generate an altitude-dependent particle flux which grows initially with the penetration of the atmosphere but will be less effective at lower altitudes because of insufficient energy per particle due to the growing number of particles and the absorption of energy. Both effects, air density and cascading particle flux, give rise to a strong altitude dependency of spallation products per mass unit of target material with an increase of up to 3 orders of magnitude from the ground to up to 40 km near the pole and a subsequent decrease at higher altitudes and at all latitudes. As spallation implies evaporation of nucleons from the target nucleus—spallation products must have a lower mass number than the original target nucleus thus lower than the number of nitrogen, oxygen or argon. In addition to the altitude dependency, a latitude dependency of spallation product concentration exists because of the terrestrial magnetic field. The magnetic stiffness for protons of cosmogenic origin is highest at the equator and lowest at the poles. Consequently, the spallation product concentration at the pole is higher than at the equator. Alltogether the polar stratosphere experiences the highest production rate and the tropical lower troposphere the lowest (Lal, 1962/67). If the production rate per unit volume is considered, the maximum is at 12 km near the equator and at 16 km near the pole (Mu¨h, 1966). ¹he annual course of Be7 Because of the altitude and latitude dependency of spallation in the atmosphere air concentration measurements of isotopes produced by spallation have been used to investigate atmospheric transport patterns (e.g. Mu¨h, 1966/69; Hartwig, 1973; Dutkiewicz, 1985; Feely, 1989; Brost, 1991) such as STE. It has been shown that the annual pattern of groundlevel-air Be7-concentration in the midlatitude, with its maximum during late spring, for most of the stations can be explained by the flux of air from the stratosphere to the ground which accounts for up to 40% of
Dynamics and the stability of the atmosphere
the amount (Koch, 1996; Dutkiewicz, 1985) of the month with the highest concentration. The rest is of tropospheric origin. Because STE is a consequence inter alia of the dynamics and stability patterns of the troposphere and the stratosphere—and these in essence depend on the heat source distribution and heat flow—it is to be expected that with a gradual change of the heat source pattern over the years, a gradual change of STE and spallation product concentration will likewise occur.
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time span it is advisable to consider as short a period within the year as possible because the signal/noise ratio is getting increasingly worse with longer averaging periods such as annual means. In addition if various changes of dynamics appear at different times of the year their effect can cancel out completely in annual means. Therefore, the month with the lowest concentration of each year (Min1) has been used for the discussion. Sun spot activity
Radiation age Experimental investigations have shown (Lal, 1964; Mu¨h, 1966; Dutkiewicz, 1985) that the radiation age of stratospheric air parcels is much higher due to the low eddy diffusivity compared to the tropospheric situation. Thus, stratospheric air parcels will usually have a very high specific spallation-isotope activity because of the longer radiation age and higher cosmic ray flux (e.g. Lal, 1962/67; Mu¨h, 1966). If the stratosphere becomes less stable due to the increasing AIAG concentration, which mitigate the negative temperature gradient (Raymond, 1993) lower stability and a lower radiation age will result. Consequently, the specific air parcel radioactivity connected with increasing STE will decrease. Causes for deviations In order to obtain information about the dynamics of the atmosphere from ground-level air concentration measurements of spallation products one has to discuss and understand the annual course of the concentration. The concentration of spallation products, such as Be7, in the troposphere is caused primarily by the intensity of the cosmic radiation and by the irradiation period of the air parcel under consideration. Be7 concentration in ground-level air can vary for three groups of reasons. Despite (a) the varying production rates, (b) atmospheric removal processes such as dry and wet deposition can change. In addition, (c) dynamic processes such as stratosphere-troposphere exchange (STE), downward transfer (DT) or horizontal transfer (HT) can change too. The latter on the condition that the dynamic processes connect atmospheric areas with principally strongly different Be7 concentrations.
Figure 1 shows the annual-mean, ground-level Be7 concentrations at five stations in France over three decades (IPSN, 1996) and the average of the Min. 1 concentrations of the same stations. The average concentration at a German station (Braunschweig) is also shown (Kolb, 1995). This can be compared with the inverse sun spot activity over the same period of time which is shown in arbitrary units (Koch, 1996). A correlation can be seen and leads to correlation coefficients of r"0.59 (France), r"0.60 (Braunschweig) and r"0.48 (France Min. 1). The correlation is obviously obscured by atmospheric dynamics and removal processes. The smaller r-value for Min 1 is understandable because of the stronger influence of local tropospheric weather conditions. In a discussion of any Be7 concentration trends over durations of decades, the effect of the sunspot activity should therefore be considered. This can be done either by using concentration ratios within a year where the sunspot activity influence is cancelled out or by taking the sunspot effect directly into account. ¹he trend of Be7 concentration In the following, I will discuss Be7 ground level air concentrations. To discern any influence of the increasing amount of AIAG and subsequent changes of atmospheric dynamics on Be7-concentrations complete and uninterrupted data sequences over decades are required, which are very sparse. Certainly there
Changing production rate With regard to the possibility of a changing Be7 production rate, the yield of spallation itself as the cause can be excluded. If we discard the possibility of a changing number of target nuclei per volume (i.e. N , O ) at a given altitude the only possibility is 2 2 a change in the spectrum of the incoming cosmogenic particles. And indeed such a change is possible due to the solar sunspot cycle (Lal, 1967). Selection of time interval To study the variations in atmospheric tracer concentrations caused by an exchange during a limited
Fig. 1. Yearly average Be7 ground-level concentration for Braunschweig (Germany) and the average of five French stations compared with the inverse function of the sunspot activity. Added is the month of the year with the lowest activity for the average of the five French stations (Min 1 France)
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Fig. 2. Linear regressions for different stations and time spans of the month with the lowest concentration per year. All regressions show uniformly a positive regression gradient. The values for the gradient for the regression lines in the succession of citation are: Skibotn 10.3; Ris+ 49.8; Braunschweig 13.2; France 11.7; Braunschweig 66.6; France 76.9.
exist quite a few Be7 ground level air concentration measurements (e.g. Feely, 1989; Larsen, 1995) but most of the long-term data have interruptions. With these interruptions it is difficult to judge their influence on the evaluation without detailed knowledge of the reasons for the data’s abscence. In Fig. 2, linear regressions of Min. 1 are shown for Braunschweig/Germany (1965—94, 1982—94), for 5 stations in France (1965—95, 1982—95), for Ris+/Denmark (1982—95) (Aarkrog, 1996) and for Skibotn/Sweden (1975—90) (Kolb, 1995). The unambigious trend is increasing concentrations for all six data sets. ¹he influence of sun spot activity Several causes are possible for that effect as already mentioned as items (a)—(c). Because an influence of the sunspot activity (Lal, 1967) principally cannot be excluded (item (a)), in Fig. 3, the linear regression for the inverse sunspot activity (1965—93) is shown together with Min. 1 concentrations of the French stations for the same period of time (1965—93) and the corresponding Be7 linear regression. The gradient of the sunspot regression is strongly negative (!15) whereas that of the concentration values is positive (#5). (The scales of the ordinates for both calculations had been converted to make a correct comparison possible.) That means that the increasing Min 1-concentration values are not caused by the sunspot activity. On the contrary, for that time span the sunspot activity would mitigate this effect. Removal processes To discuss a possible influence of wet removal processes (item (b)), the correlation between the amount of precipitation and Be7 ground-level air concentration has been calculated for the Braunschweig station over the identical time series (i.e. 1965—1994). Under wet removal processes, wash out as well as rain out is understood. The correlation coefficient amounts to
Fig. 3. Minimal monthly Be7 ground-level concentration per year for the five French stations and the inverse sunspot activity between 1965 and 1993. The according linear regressions show different signs of the gradient. For a better comparison for the calculations the two scales of the ordinates are normalized to the amplitudes for the two curves.
r"!0.202 with a standard deviation of s"0.278. The r-value represents the average of the annual means. That is to say that the correlation coefficient indicates no dependency between precipitation and ground-level concentration of Be7 for the data set under consideration. In recent publications the annual concentration ratios of Be7, Max i , i"1,2, 6 r" i Min i have been discussed for Braunschweig (Hartwig, 1996) and Skibotn (Hartwig, 1995) data sets. This ratio is an indicator for the elongation of the annual concentration course and because of that indicates the influence of regions of high production rate or high radiation age on the ground-level concentration. r is the con1 centration ratio of the month with the highest to that of the lowest concentration within one year and r is 2 the month with the second highest to that of the second lowest concentration, etc. Discussing these concentration ratios has the advantage that for longtime data sets over decades the influence of the sunspot cycle is cancelled out. This influence is especially important for concentrations influenced by air of higher altitudes. In addition, peculiarities of meteorological conditions extending over a year cancel out too, which would otherwise deteriorate regressions over decades. The gradient of the regression lines of the r (i"1, 2, 6) from 1964 to 1995 have nearly i a continous run from r with the highest value to 1 r with the lowest which is understandable on phys6 ical grounds and will be discussed later. The set of gradients with rising i is: !0.0105, !0.0124, !0.0106, !0.0036, !0.0037, !0.0003. For the precipitation of corresponding ratios and months, the data set of gradients is: !0.0356, !0.0266, #0.0005, #0.015, !0.0199, #0.0301. Obviously, the precipitation data show a stochastic distribution
Dynamics and the stability of the atmosphere
Fig. 4. Linear regressions of Be7 concentration ratios of the month with the highest to the lowest concentration r per 1 year, the second highest to the second lowest r per year, etc. 2 for five stations in France. The regression gradient steadily becomes smaller. The bars indicate the standard deviation.
which again makes no influence of precipitation on the Be7 data trend plausible. Besides precipitation theoretically dry removal processes could play a role. Usually in European midlatitudes dry deposition on the average accounts for 10—20% of the entire deposited amount. Therefore, it is not very likely that dry deposition can explain the Be7 data trend. That leaves item (c) as the dynamic processes. Dynamic processes Indeed a change of atmospherical dynamics can be responsible for the effects under consideration. Feely (1989) has discussed the effects which can cause seasonal variations in Be7 concentrations in surface air. With regard to horizontal transfer, he believes that his data support the view that a lateral transport from middle latitudes and higher altitude (Mauna Loa, 3400 m) to the pole region exists. He discussed that situation by means of data sets from the South Pole and Mauna Loa. If this poleward transfer exists, it could only explain the Min1 data trend at our latitudes if we have been experiencing a continuous decrease of this kind of transport over the last 30 years connected with an according influence of higher tropospherical regions to the surface area. That leads to a possible influence of the tropospheric vertical transport. ¹ropospheric stability During a rather early stage of the discussion about the structure of the atmosphere it was pointed out (Hartwig, 1973) that the troposphere is not a wellmixed box as was sometimes assumed during those days but that the vertical eddy diffusivity can be very different at different altitudinal atmospheric layers. This assertion was based on rather strong altitudenal concentration gradients of spallation product concentrations which were measured and on the discussion of the concentration ratio of different spallation prod-
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ucts with different radioactive half-lives. Such ratios give evidence about the duration of atmospheric transport processes. In the light of these findings, the Be7 data trend could be caused by a gradual and long-term change of tropospheric eddy diffusivity or by enhanced STE or by both of these effects. It is not possible to decide on the basis of the Min 1 regression data alone which effect is responsible. Nevertheless, a supplementary consideration of the r -values disi cussed elsewhere (Hartwig, 1995, 1996) suggests that at least part of the effect is caused by STE. A decline of the r values over three decades can be caused by a rise i of the Min values, by a decline of the Max itself or by i i both. The rise of the Min itself is stronger than the i decline of the r , which indicates the possibility of i a slight increase of the Max . This leads to the coni clusion that at least part of the effect is due to the enhanced STE. Unfortunately, because of the interference of other effects (e.g. sunspot activity), it is not advisable to calculate Max regressions alone. i CONCLUSION
Data sets of Be7 ground-level air concentration of 8 stations in 4 countries (Denmark, France, Germany, Sweden) with sampling time periods between 15 and 32 years show a unique and steady trend of change for all stations. It can be excluded that this change of concentration is caused by the production rate, i.e. by a change of cosmic ray intensity or modulations due to the 11 year sunspot cycle. Because of the results of investigations with precipitation data, it seems also very unlikely that this trend is caused by atmospheric removal processes as discussed in some publications for other latitudes and stations (Feely, 1989). This change must therefore be caused by a change in the dynamics and the stability patterns in the atmosphere, i.e. changes of the (dynamical) structure of the only two layers of the atmosphere which contribute to the ground-level concentration of Be7, the troposphere and stratosphere. On the basis of the presented Be7 data sets it is not possible to decide whether a change in STE alone or a combination of changing STE plus a change of tropospheric eddy diffusion is responsible for that effect. Considerations show that it is to be expected that the stability properties at the altitude of the tropopause are changing steadily due to the growing amount of AIAG which alters the spatial structure of the heat source distribution in the atmosphere independent of additional dynamic heat flows within the troposphere and stratosphere. The time dependency of natural heat sources (ozone layer and Earth’s surface) due to the yearly cycle of the input of solar radiation and the relative strength is different to that caused by the AIAG. Therefore, the apparent changes in the atmospheric dynamics also have a yearly pattern and a trend which can be related to the growing concentration of the AIAG. Alltogether it is suggested that the discussed trend of the Be7 ground-level
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concentration is commensurate and in line with a expected change of the atmospheric dynamics without being able to decide whether STE or STE plus tropospheric vertical eddy diffusion is responsible for the trend. The findings described here add a new step to a better understanding of the risks of climatic gases. Acknowledgements—The processing of part of the data by E. Puls is gratefully acknowledged. In addition I am indebted to the unknown reviewers for very helpful comments.
REFERENCES
Aarkrog, A. et al. (1996) Radioaktivita¨ten i Ris+-I-571 (EN) Roskilde, Denmark Brost, R. A., Feichter, I. and Heimann (1991) Three-dimensional simulation of Beryllium-7 in a global climate model. Journal of Geophysical Research 96 (D12), 22,423—22,445. Cubasch, U., Hasselmann, K., Ho¨ck, H., Maier-Reimer, E., Mikolajewicz, U., Sauer, B. and Sausen, R. (1992). Timedependent greenhouse warming computations with a coupled ocean—atmosphere model. Climate Dynamics 8, 55—69. Cubasch, U., Santer, B. and Hegerl, G. (1995) Klimamodelle — wo stehen wir? Physikalische Bla¨ tter 51(4), 269—276. Dibb, J. (1989) Atmospheric deposition of beryllium 7 in the Chesapeake Bay Region. Journal of Geophysical Research 94(D2), 2261—2265. Dibb, J., Talbot, R. and Gregory, G. (1992) Beryllium 7 and lead 210 in the Western Hemisphere Arctic Atmosphere: observations from three recent aircraft-based Sampling programs. Journal of Geophysical Research 97(D15), 16709—16715. Dutkiewicz, V. and Husan, C. (1985) Stratospheric and tropospheric components in surface air. Journal of Geophysical Research 90(D3), 5783—5788. Feely, H. W., Larsen, R. J. and Sanderon, C. G. (1989) Factors that cause seasonal variations in beryllium-7 concentrations in surface air. Journal of Environmental Radioactivity 9, 223—249. Georgii, H. W., Heudorfer, W. and Jung, H. J. (1981), Struktur und Analyse von Klimamodellen und der Einflu{ der Spurengase auf den Temperaturhaushalt der Atmospha¨re, In: Die Auswirkungen von CO -Emissionen auf das 2 Klima, eds S. Hartwig et al. pp. 264—380. Hartwig, S. and Sittkus, A. (1969) Radioaktive Isotope als Luftmassenindikatoren II., Die Produktion von P33, Zeitschrift fur Naturforschung 24a, 908—912. Hartwig, S. and Sittkus, A. (1973), Be7 measurement in ground level air and the Austausch within the lower part of the troposphere. Nature, Physica scripta 241/106, 36—37.
Hartwig, S., Berner, W., Bienewitz, H. K., Georgii, H. W., Heiman, M., Heudorfer, W., Jung, H. J., Kamber, D., Lieth, H., Neftel, A., Oberbacher, B., Oeschger, H., Siegenthaler, U. and Zumbrunn, R. (1981) Die Auswirkungen von CO -Emissionen auf das Klima Forschungsbericht 2 No. 104 606 des Umweltbundesamtes Berlin, erstellt durch das Battelle-Institut. Hartwig, S. (1995) Further evidence of changing stability of the atmosphere. Radiocarbon 37(3), 961—962. Hartwig, S. (1996) Langja¨hrige Be7-Bodenluftmessungen lassen A®nderung des atmospha¨rischen Austauschverhaltens wa¨hrend der letzten Jahrzehnte vermuten. Zeitschrift fur Naturforschung 51a, 1139—1143. Holton, J. R., Hayness, P. H., McIntyre, M. E., Douglass, A. R., Road, R. B. and Pfister, L. (1995) Stratosphere—troposphere exchange. Reviews of Geophysics, 33(4), 403—439. IPSN/DPE (anonymous) (1996) Le cesium 137 et le be´ryllium 7 dans les ae´rosol atmosphe´riques. CE/Cadarache, France. Koch, D. H., Jacob, D. J. and Graustein, W. C. (1996) Vertical transport of tropospheric aerosols as indicated by Be7 and Pb210 in a chemical tracer model. Journal of Geophysical Research 101(D13), 18,651—18,666. Kolb, W. (1992/1995) Aktivita¨tskonzentration von Radionukliden in der bodennahen Luft Norddeutschlands und Nordnorwegens im Zeitraum von 1963 bis 1990. PTB-Ra-29-Report, Physikalisch-Technische Bundesanstalt, Braunschweig. Lal, D. and Peters, B. (1962) Cosmic ray produced isotopes and their applications to problems in geophysics In Progress in Cosmic Ray and Elementary Particle Physics, Vol. 6, pp. 3—74. North Holland, New York. Lal, D. and Peters, B. (1967) Cosmic ray produced radioactivity on the earth. In Handbuch der Physik, ed. S. Flu¨gge, Band XLVI/2: Kosmische Strahlung II, Ed. K. Sitte, pp. S. 551-612, 46/2, Springer, Berlin. Larsen, R. J., Sanderson, C. G. and Kada, J. (1995) EM¸ Surface Air Sampling Program, 1990—1993 Data. USDOE Report EML-572. Mu¨h, H., Sittkus, A., Albrecht, A. and Hartwig, S. (1966), Radioaktive Isotope als Luftmassenindikatoren I. Zeitschrift fur Naturforschung 21a, 1123—1128. Mu¨h, H. and Sittkus, A. (1969) Naturwissenschaften 56, 211—212. Raymond, S., Bradley, F., Kleining, T. and Diaz, H. (1993) Changes in Arctic temperature inversions in CDIAC communications carbon dioxide informations Analysis Center. Oak Ridge, TN 37831-8335, Fall 1993, 19, 1—4. Reiter, E. R. (1975) Reviews in Geophysics and Space Physics 13, 459—474. World Met. Org. (1988) WMO-Report 18, Genf. World Met. Org. (1990) WMO-Report 20, Genf. World Met. Org. (1988) WMO-Bulletin, Advisory Group on Greenhouse gases, Vol. 37/2, p. 111.