Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 6 8 2006 10.5194/acp-6-2073-2006 http://www.atmos-chem-phys.net/6/2073/2006/ http://www.atmos-chem-phys.net/6/2073/2006/acp-6-2073-2006.html http://www.atmos-chem-phys.net/6/2073/2006/acp-6-2073-2006.pdf 2073 2090 2006-06-20 Modelling the budget of middle atmospheric water vapour isotopes A. Zahn andreas.zahn@imk.fzk.de P. Franz C. Bechtel J.-U. Grooß T. Röckmann Institute of Meteorology and Climate Research, Forschungszentrum Karlsruhe, Germany Max-Planck-Institute for Nuclear Physics, Heidelberg, Germany National Institute for Water and Atmospheric Research (NIWA), Wellington, New Zealand Robert Bosch GmbH, Bühl, Germany Institute for Marine and Atmospheric Research Utrecht, Utrecht University, The Netherlands Institute of Chemistry and Dynamics of the Geosphere, Forschungszentrum Jülich, Germany A one-dimensional chemistry model is applied to study the stable hydrogen (D) and stable oxygen isotope (¹⁷O, ¹⁸O) composition of water vapour in stratosphere and mesosphere. In the troposphere, this isotope composition is determined by "physical'' fractionation effects, that are phase changes (e.g. during cloud formation), diffusion processes (e.g. during evaporation from the ocean), and mixing of air masses. Due to these processes water vapour entering the stratosphere first shows isotope depletions in D/H relative to ocean water, which are ~5 times of those in ¹⁸O/¹⁶O, and secondly is mass-dependently fractionated (MDF), i.e. changes in the isotope ratio ¹⁷O/¹⁶O are ~0.52 times of those of ¹⁸O/¹⁶O. In contrast, in the stratosphere and mesosphere "chemical'' fractionation mechanisms, that are the production of H_O due to the oxidation of methane, re-cycling of H₂O via the HO_x family, and isotope exchange reactions considerably enhance the isotope ratios in the water vapour imported from the troposphere. The model reasonably predicts overall enhancements of the stable isotope ratios in H₂O by up to ~25% for D/H, ~8.5% for ¹⁷O/¹⁶O, and ~14% for ¹⁸O/¹⁶O in the mesosphere relative to the tropopause values. The ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios in H₂O are shown to be a measure of the relative fractions of HO_x that receive the O atom either from the reservoirs O₂ or O₃. Throughout the middle atmosphere, MDF O₂ is the major donator of oxygen atoms incorporated in OH and HO₂ and thus in H₂O. In the stratosphere the known mass-independent fractionation (MIF) signal in O₃ is in a first step transferred to the NO_x family and only in a second step to HO_x and H₂O. In contrast to CO₂, O(¹D) only plays a minor role in this MIF transfer. The major uncertainty in our calculation arises from poorly quantified isotope exchange reaction rate coefficients and kinetic isotope fractionation factors. Anderson, S. M., Morton, J., and Mauersberger, K.: Laboratory measurements of ozone isotopomeres by tunable diode laser absorption spectroscopy, \CPL, 156, 175-180, 1989. Baertschi, P.: Absolute $^18$O content of standard mean ocean water, \EPS, 31, 341-344, 1976. Barth, V. and Zahn, A.: Oxygen isotope composition of carbon dioxide in the middle atmosphere, \JGR, 102, 12 995-10 007, 1997. Brasseur, G. P. and Solomon, S.: Aeronomy of the middle atmosphere, D. Reidel Publishing Company, Dordrecht/Boston/Lancaster/Tokyo, 1986. Cliff, S. S. and Thiemens, M. H.: The $^18$O/$^16$O and $^17$O/$^16$O ratios in atmospheric nitrous oxide: A mass-independent anomaly, §ci, 278, 1774-1775, 1997. Cliff, S. S., Brenninkmeijer, C. A. M., and Thiemens, M. H.: First measurement of the $^18$O/$^16$O and $^17$O/$^16$O ratios in stratospheric nitrous oxide: A mass-independent anomaly, \JGR, 104, 16 171-16 175, 1999. Coplen, T. B., Bohlke, J. K., De Bievre, P., Ding, T., Holden, N. E., Hopple, J. A., Krouse, H. R., Lamberty, A., Peiser, H. S., Revesz, K., Rieder, S. E., Rosman, K. J. R., Roth, E., Taylor, P. D. P., Vocke, R. D., and Xiao, Y. K.: Isotope-abundance variations of selected elements† (IUPAC Technical Report), \PAC, 74, 10, 1987-2017, 2002. Craig, H.: Standard for reporting concentrations of deuterium and oxygen-18 in natural waters, §ci, 133, 1833-1834, 1961. Crutzen, P. J.: Methane's sinks and sources, \Nat, 350, 380-381, 1991. Crutzen, P. J.: On the role of CH$_4$ in atmospheric chemistry: sources, sinks and possible reductions in anthropogenic sources, \Amb, 24, 52-55, 1995. DeMore, W. B., Howard, C. J., Sander, S. P., Ravishankara, A. R., Golden, D. M., Kolb, C. E., Hampson, R. F., Molina, M. J., and Kurylo, M. J.: Chemical kinetics and photochemical data for use in stratospheric modeling, Pasadena, CA: JPL Publication 97-4, 1997. Dessler, A. E., Weinstock, E. M., Anderson, J. G., and Chan, K. R.: Mechanisms controlling water vapor in the lower stratosphere: A tale of two stratospheres, \JGR, 100, 23 167-23 172, 1995. Dessler, A. E. and Sherwood, S. C.: A model of HDO in the tropical tropopause layer, Atmos. Chem. Phys., 3, 2173-2181, 2003. DeWit J. C., Van der Straaten, C. M., and Mook, W. G.: Determination of the absolute D/H ratio of V-SMOW and SLAP, \GeN, 4, 33-36, 1980. Dinelli, B. M., Lepri, G., Carlotti, M., Carli, B., Mencaraglia, F., Ridolfi, M., Nolt, I. G., and Ade, P. A. R.: Measurements of the isotope ratio distribution of HD$^16$O and H$_2^18$O in the 20-38 km altitude range from far-infrared spectra, \GRL, 24, 2003-2006, 1997. Dinelli, B. M., Carli, B., and Carlotti, M.: Measurement of stratospheric distribution of H$_2^16$O, H$_2^18$O, H$_2^17$O and HD$^16$O from far infrared spectra, \JGR, 96, 7509-7514, 1991. Dubey, K., Mohrschladt, R., Donahue, N. M., and Anderson, J. G.: Isotope specific kinetics of hydroxyl radical (OH) with water (H$_2$O): Testing models of reactivity and atmospheric fractionation, \JPC , 101, 1494-1500, 1997. Engel, A., Schiller, C., Schmidt, U., Borchers, R., Ovarlez, H., and Ovarlez, J.: The total hydrogen budget in the Arctic winter stratosphere during EASOE, \JGR, 101, 14 495-14 503, 1996. Feilberg, K. L., Johnson, M. S., and Nielsen, C. J.: Relative reaction rates of HCHO, HCDO, DCDO, H$^13$CHO, and HCH$^18$O with OH, Cl, Br, and NO$_3$ radicals, \JPC, 108, 7393-7398, 2004. Forster, P. M. de F. and Shine, K. P.: Stratospheric water vapour changes as a possible contributor to observed stratospheric cooling, \GRL, 26, 3309-3312, 1999. Forster, P. M. de F. and Shine, K. P.: Assessing the climate impacts of trends in stratospheric water vapor, \GRL, 29, doi:10.1029/2001GL013909, 2002. Franz, P. and Röckmann, T.: High-precision isotope measurements of H$_2^16$O, H$_2^17$O, H$_2^18$O, and the $^17$O-anomaly of water vapor in the southern lowermost stratosphere, Atmos. Chem. Phys., 5, 2949-2959, 2005. Friedmann, I. and Scholz, T. G.: Isotopic composition of atmospheric hydrogen, 1967-1969, \JGR, 79, 785-788, 1974. Froidevaux, L. and Yung, Y. L.: Radiation and chemistry in the stratosphere: Sensitivity to O$_2$-absorption cross sections in the Herzberg continuum, \GRL, 9, 854-857, 1982. Gettelman, A., Holton, J. R., and Rosenlof, K. H.: Mass fluxes of O$_3$, CH$_4$, N$_2$O and CF$_2$Cl$_2$ in the lower stratosphere calculated from observational data, \JGR, 102, 19 149-19 159, 1997. Gierczak, T., Talukdar, R. K., Herdon, S., Vaghjiani, G. L., and Ravishankara, A. R.: Rate coefficients for the reactions of hydroxyl radicals with methane and deuterated methanes, \JPC, 101, 3125-3134, 1997. Greenblatt, G. D. and Howard, C. J.: Oxygen Atom Exchange in the Interaction of $^18$O with Several Small Molecules, \JPC , 93, 1035-1042, 1989. Grooß, J.-U.: Modelling of stratospheric chemistry based on HALOE/UARS satellite data, PhD thesis, University of Mainz, Verlag Shaker, Aachen, 1996. Günther, J., Erbacher, B., Krankowsky, D., and Mauersberger, K.: Pressure dependence of two relative ozone formation rate coefficients, \CPL, 306, 209-213, 1999. Guo, J., Abbas, M. M., and Nolt, I. G.: Stratospheric H$_2$$^18$O distribution from infrared observations, \GRL, 16, 1277-1280, 1989. Hagemann, R., Nier, G., and Roth, E.: Absolute isotopic scale for deuterium analysis of natural waters. Absolute D/H ratio for SMOW, \Tel, 22, 712-715, 1970. Holdsworth, G., Fogarasi, S., and Krouse, H. R.: Variation of stable isotopes of water with altitude in the Saint Elias Mountains of Canada, \JGR, 96, 7483-7494, 1991. Holton, J. R., Haynes, P. H., McIntyre, M. E., Douglass, A. R., Rodd, R. B., and Pfister, L.: Stratosphere-troposphere exchange, \ReG, 33, 403-439, 1995. Hurst, D. F., Dutton, G. S., Romashkin, P. A., Wamsley, P. R., Moore, F. L., Elkins, J. W., Hintsa, E. J., Weinstock, E. M., Herman, R. L., Moyer, E. J., Scott, D. C., May, R. D., and Webster, C. R.: Closure of the total hydrogen budget of the northern extratropical lower stratosphere, \JGR, 104, 8191-8200, 1999. IPCC, Climate Change 2001: The Scientific Basis, Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), edited by: Houghton, J. T., Ding, Y., Griggs, D. J., Noguer, M., van der Linden, P. J., and Xiaosu, D., Cambridge University Press, UK, pp. 944, 2001. Janssen, C., Guenther, J., Krankowsky, D., and Mauersberger, K.: Relative formation rates of $^50$O$_3$, and $^52$O$_3$ in $^16$O-$^18$O mixtures, \JCP, 111, 7179-7182, 1999. Johnson, D. G., Jucks, K. W., Traub, W. A., and Chance, K. V.: Isotopic composition of stratospheric ozone, \JGR, 105, 9025-9031, 2000. Johnson, D. G., Jucks, K. W., Traub, W. A., and Chance, K. V.: Isotopic composition of stratospheric water vapor: Measurements and photochemistry, \JGR, 106, 12 211-12 218, 2001a. Johnson, D. G., Jucks, K. W., Traub, W. A., and Chance, K. V.: Isotopic composition of stratospheric water vapor: Implications for transport \JGR, 106, 12 219-12 226, 2001b. JPL (Ed.): Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, JPL, 2003. Kaiser, J. and Röckmann, T.: Absence of isotope exchange in the reaction of N$_2$O+O($^1$D) and the global $^17$O budget of nitrous oxide, \GRL,, 32, L15808, doi:10.1029/2005GL023199, 2005. Kaye, J. A. and Strobel, D. F.: Enhancement of heavy ozone in the Earth's atmosphere?, \JGR, 88, 8447-8452, 1983 Kaye, J. A.: Analysis of the origins and implications of the $^18$O content of stratospheric water vapour, \JAC, 10, 39-51, 1990. Keith, D. W.: Stratospheric-tropospheric exchange: Inferences from the isotopic composition of water vapour, \JGR, 105, 15 167-15 173, 2000. Khalil, M. A. K., Khalil, M. A. K., Shearer, M. J., and Rasmussen, R. A.: Methane sinks and distribution, in: Atmospheric Methane: Sources, Sinks, and Role in Global Change, edited by: Khalil, M. A. K., NATO ASI Ser. I, 13, Springer-Verlag, New York, 1993. Kirk-Davidoff, D. B., Hintsa, E. J., Anderson, J. G., and Keith, D. W.: The effect of climate change on ozone depletion through changes in stratospheric water vapour, \Nat, 402, 399-401, 1999. Kuang, Z., Toon, G. C., Wennberg, P. O., and Yung, Y. L.: Measured HDO/H$_2$O ratios across the tropical tropopause, \GRL, 30(7), 1372, doi:10.1029/2003GL017023, 2003. Lary, D. J. and Toumi, T.: Halogen-catalyzed methane oxidation, \JGR, 102, 23 421-23 428, 1997. Lelieveld, J. and Crutzen, P. J.: Influences of cloud photochemical processes on tropospheric ozone, \Nat, 343, 227-233, 1990. Lelieveld, J. and Crutzen, P. J.: Role of deep convection in the ozone budget of the troposphere, §ci, 264, 1759-1761, 1994. Lelieveld, J., Crutzen, P. J., and Dentener, F. J.: Changing concentration, lifetime and climate forcing of atmospheric methane, \Tel, 50B, 128-150, 1998 Le Texier, H., Solomon, S., and Garcia, R. R.: The role of molecular hydrogen and methane oxidation in the water vapour budget of the stratosphere, \QJR , 114, 281-295, 1988. Li, W., Baoling, N., Deqiu, J., and Qingliang, Z.: Measurement of the absolute abundance of oxygen-17 in VSMOW, Kexue Tongbao (Chinese Science Bulletin), 33, 1610-1613, 1988. López-Valverde, M. A., López-Puertas, M., Remedios, J. J., Rodgers, C. D., Taylor, F. W., Zipf, E. C., and Erdman, P. W.: Validation of measurements of carbon monoxide from the improved stratospheric and mesospheric sounder, \JGR, 101, 9929-9955, 1996. Luz, B., Barkan, E., Bender, M. L., Thiemens, M. H., and Boering, K. A.: Triple-isotope composition of atmospheric oxygen as a tracer of biosphere productivity, \Nat, 400, 547-550, 1999. Luz, B. and Barkan, E.: Assessment of oceanic productivity with the triple-isotope composition of dissolved oxygen, §ci, 288(5473), 2028-2031, 2000. Lyons, J. R.: Transfer of mass-independent fractionation in ozone to other oxygen-containing radicals in the atmosphere, \GRL, 28, 3231-3234, 2001. Masgrau, L., González-Lafont, A., and Lluch, J. M.: Mechanism of the gas-phase $\rm HO + H2_O \rightharpoonup H_2O + OH$ reaction and several associated isotope exchange reactions: A canonical variational transition state theory plus multidimensional tunneling calculation, \JPC A, 103, 1044-1053, 1999. Massie S. T. and Hunten, D. M.: Stratospheric eddy diffusion coefficients from tracer data, \JGR, 86, 9859-9868, 1981. Mauersberger, K., Erbacher, B., Krankowsky, D., Günther, J., and Nickel, R.: Ozone isotope enrichment: Isotopomer-Specific rate coefficients, §ci, 283, 370-372, 1999. Mauersberger, K., Lämmerzahl, P., and Krankowsky, D.: Stratospheric ozone isotope enrichments - revisited, \GRL, \it 28, 3155-3158, 2001. McCarty, M. C., Boering, K. A., Rahn, T., Eiler, J. M., Rice, A. L., Tyler, S. C., Schauffler, S., Atlas, E., and Johnson, D. G.: The hydrogen isotopic composition of water vapor entering the stratosphere inferred from high-precision measurements of δD-CH$_4$ and δD-H$_2$, \JGR, 109, doi:10.1029/2003JD004003, 2004. Michelsen, H. A., Irion, F. W., Manney, G. L., Toon, G. C., and Gunson, M. R.: Features and trends in Atmospheric Trace Molecules Spectroscopy (ATMOS) version 3 stratospheric water vapor and methane measurements, \JGR, 105, 11 713-22 724, 2000. Morton, J., Barnes, J., Schueler, B., and Mauersberger, K.: Laboratory studies of heavy ozone, \JGR, 95, 901-907, 1990. Moyer, E. J., Irion, F. W., Yung, Y. L., and Gunson, M. R.: ATMOS stratospheric deuterated water and implications for troposphere-stratosphere transport, \GRL, 23, 2385-2388, 1996. Oltmans, S. J. and Hofmann, D. J.: Increase in lower-stratospheric water vapour at a mid-latitude Northern Hemisphere site from 1981 to 1994, \Nat, 374, 146-149, 1995. Pollock, W., Heidt, L. E., Lueb, R., and Ehhalt, D. H.: Measurement of stratospheric water vapour by cryogenic collection, \JGR, 85, 5555-5568, 1980. Quay, P., Stutsman, J., Wilbur, D., Snover, A., Dlugokencky, E., and Brown, T.: The isotopic composition of atmospheric methane, \GBC, 13, 445-461, 1999. Rahn, T., Eiler, J. M., Boering, K. A., Wennberg, P. O., McCarthy, M. C., Tyler, S., Schauffler, S., Donnelly, S., and Atlas, E.: Extreme deuterium enrichment in stratospheric hydrogen and the global atmospheric budget of H$_2$, \Nat, 424, 918-921, 2003. Randel, W. J., Wu, F., Gettelmann, A., Russell III, J. M., Zawodny, J. M., and Oltmans, S. J.: Seasonal variation of water vapor in the lower stratosphere observed in Halogen Occultation Experiment data, \JGR, 106, 14 313-14 325, 2001. Rhee, T. S., Brenninkmeijer, C. A. M., and Röckmann, T.: The overwhelming role of soils in the global atmospheric hydrogen cycle, Atmos. Chem. Phys., 6, 1611-1625, 2006. Ridal, M., Jonsson, A., Werner, M., and Murtagh, D. P.: A one-dimensional simulation of the water vapor isotope HDO in the tropical stratosphere, \JGR, 106, 32 283-32 294, 2001. Ridal, M.: Isotopic ratios of water vapor and methane in the stratosphere: comparison between ATMOS measurements and a one-dimensional model, \JGR, 107, doi:10.1029/2001JD000708, 2002. Rinsland, C. P., Gunson, M. R., Foster, J. C., Toth, R. A., Farmer, C. B., and Zander, R.: Stratospheric profiles of heavy water vapour isotopes and CH$_3$D from analysis of the ATMOS Spacelab 3 infrared solar spectra, \JGR, 96, 1057-1068, 1991. Rinsland, C. P., Goldman, A., Malathy Devi, V., Fridovich, B., Snyder, D. G. S., Jones, G .D., Murcray, F .J., Murcray, D. G., Smith, M. A. H., Seals Jr., R. K., Coffey, M. T., and Mankin, W. G.: Simultaneous stratospheric measurements of H$_2$O, HDO, CH$_4$ from balloon-borne and aircraft infrared solar absorption spectra and tunable diode laser spectroscopy of HDO, \JGR, 89, 7259-7266, 1984. Röckmann, T., Kaiser, J., Brenninkmeijer, C. A. M., Crowley, J. N., Borchers, R., Brand, W. A., and Crutzen, P. J.: Isotopic enrichment of nitrous oxide ($^15$N$^14$NO, $^14$N$^15$NO, $^14$N$^14$N$^18$O) in the stratosphere and in the laboratory, \JGR, 106, 10 403-10 410, 2001. Röckmann, T., Kaiser, J., Crowley, J. N., Brenninkmeijer, C. A. M., and Crutzen, P. J.: The origin of the anomalous or "mass-independent" oxygen isotope fractionation in atmospheric N$_2$O, \GRL, 28, 503-506, 2001. Rozanski, K., Araguas-Araguas, L., and Gonfiantini, R.: Isotopic patterns in modern global precipitation, Climate change in continental isotopic records, \GeM, 78, 1-36, 1993. Rosenlof, K. H., Tuck, A. F., Kelly, K. K., Russell III, J. M., and McCormick, M. P.: Hemispheric asymmetries in water vapour and inferences about transport in the lower stratosphere, \JGR, 102, 13 213-13 234, 1997. Rosenlof, K. H., Oltmans, S. J., Kley, D., Russell, J. M., Chiou, E. W., Chu, W. P., Johnson, D. G., Kelly, K. K., Michelsen, H. A., Nedoluha, G. E., Remsberg, E. E., Toon, G. C., and McCormick, M. P.: Stratospheric water vapor increases over the past half-century, \GRL, 1195-1198, 2001. Rosenlof, K. H.: Transport changes inferred from HALOE water and methane measurements, \JMSJ, 80, 4B, 831-848, 2002. Saueressig, G., Bergamaschi, P., Crowley, J. N., and Fischer, H.: D/H kinetic isotope effect in the reaction CH$_4$+Cl, \GRL, 23, 3619-3622, 1996. Saueressig, G., Crowley, J. N., Bergamaschi P., Brühl, C., Brenninkmeijer, C. A. M., and Fischer, H.: Carbon 13 and D kinetic isotope effects in the reactions of CH$_4$ with O($^1$D) and OH: New laboratory measurements and their implications for the isotopic composition of stratospheric methane, \JGR, 106, 23 127-23 138, 2001. Sharma, H. D., Jervis, R. E., and Wong, K. Y.: Isotopic exchange reactions in nitrogen oxides, \JPC, 74, 923-933, 1970. Smith, R. B.: Deuterium in North Atlantic storm tops, \JAS, 49, 2041-2057, 1992. SPARC: SPARC assessment of upper tropospheric and stratospheric water vapour, edited by: Kley, D., Russell III, J. M., and Phillips, C., SPARC report No. 2, 2000. Stowasser, M., Oelhaf, H., Wetzel, G., Friedl-Vallon, F., Maucher, G., Seefeldner, M., Trieschmann, O., and Clarmann, T. V.: Simultaneous measurements of HDO, H$_2$O, and CH$_4$ with MIPAS-B: Hydrogen budget and indication of dehydration inside the polar vortex, \JGR, 104, 19 213-19 225, 1999. Talukdar, R. K. and Ravishankara, A. R.: Rate coefficients for $\rm O(^1D)+H_2, D_2,$ HD reactions and H atom yield in $\rm O(^1D) + HD$ reaction, \CPL , 253, 177-183, 1996. Taylor, C. B.: Vertical distribution of deuterium in atmospheric water vapour: problems in application to assess atmospheric condensation models, \Tel, 36B, 67-72, 1984. Thiemens, M. H. and Heidenreich III, J. E.: The mass-independent fractionation of oxygen: A novel isotope effect and its possible cosmochemical implications, §ci, 219, 1073-1075, 1983. Thiemens, M. H. and Jackson, T.: New experimental evidence for the mechanism for production of isotopically heavy ozone, \GRL, 15, 639-642, 1988. Thiemens, M. H. and Jackson, T.: Pressure dependency for heavy isotope enhancement in ozone formation, \GRL, 17, 717-719, 1990. Tse, R. S., Wong, S. C., and Yuen, C. P.: Determination of deuterium/hydrogen ratios in natural waters by Fourier transform nuclear magnetic resonance spectrometry, \AnC, 52, 2445-2447, 1980. Tyler, S. C., Ajie, H. O., Rice, A. L., Cicerone, R. J., and Tuazon, E. C.: Experimentally determined kinetic isotope effects in the reaction of CH$_4$ with Cl: Implications for atmospheric CH$_4$, \GRL, 27, 1715-1718, 2000. Webster, C. R. and Heymsfield, A. J.: Water Isotope Ratios D/H, $^18$O/$^16$O, $^17$O/$^16$ in and out of Clouds Map Dehydration Pathways, §ci, 302, 1742-1745, 2003. Wen, J. and Thiemens, M. H.: Multi-isotope study of the O($^1D$)+CO$_2$ exchange and stratospheric consequences, \JGR, 98, 12 801-12 808, 1993. Yang, H. and Tung, K. K.: Cross-isentropic stratosphere-troposphere exchange of mass and water vapour, \JGR, 101, 9413-9423, 1996. Yung, Y. L., DeMore, W. B., and Pinto, J. P.: Isotope exchange between carbon dioxide and ozone via O($^1$D) in the stratosphere, \GRL, 18, 13-16, 1991. Yung, Y. L., Lee, A. Y. T., Irion, F. W., DeMore, W. B., and Wen, J.: Carbon dioxide in the atmosphere: Isotopic exchange with ozone and its use as tracer in the middle atmosphere, \JGR, 102, 10 857-10 866, 1997. Zahn, A.: Constraints on 2-way transport across the Arctic tropopause based on O$_3$, stratospheric tracer (SF$_6$) ages, and water vapor isotope (D, T) tracers, \JAC, 39, 303-325, 2001. Zahn, A., Barth, V., Pfeilsticker, K., and Platt, U.: Deuterium, Oxygen-18 and tritium as tracers for water vapour transport in the lower stratosphere and tropopause region, \JAC, 30, 25-47, 1998. Zöger, M., Engel, A., McKenna, D. S., Schiller, C., Schmidt, U., and Woyke, T.: Balloon-borne in situ measurements of stratospheric H$_2$O, CH$_4$ and H$_2$ in midlatitudes, \JGR, 104, 1817-1825, 1999.