Recent determinations of O^ absorption cross sections in the Herz berg continuum, obtained from in situ solar irradiance measurements, have shown that the corresponding laboratory values might be overestimated by 30 to 50 percent. The change in the stratospheric composition due to such a reduction in these cross sections has been calculated by means of a steady state one dimensional chemical model. An accurate determination of the O^ absorption spectrum in the 200-220 nm range is required since

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... the distribution of several trace species is very sensitive to the atmospheric transmissivity in this wavelength region. Résumé La section efficace d'absorption de l'oxygène moléculaire, dans le domaine spectral du continuum de Herzberg, a été déterminée à partir de la mesure de l'éclairement solaire, à deux altitudes connues. Ces Zusammenfassung Die neue Bestimmungen des O^ absorption Durchschnittmesser in das Herzberg Continuum, haben gezeigt dass die Laboratorium Messungen wahrscheinlich von 30% bis 50% zu gross sind. Die Änderungen in der stratosphärischen Zusammensetzung einer solchen Reduktion sind abgeschäftzt worden für ein Eindimensioneies chemisches Modell. Ein genaue Bestimmung des C^ absorption Durchschittmesser zwischen 200 und 220 nm is notwendig, denn die minoritäre Bestandteile der Stratosphäre hängen merklich von die Transmittivität der, Sonnenstrählen in diesem Wellenlänge Bereich, ab. -4-Ogawa (1971) or the tabulation by Hudson and Reed (1979) or by Nicolet (1978) are intermediate. Recently, Frederick and Mentall (1982) as well as Herman and Mental! (1982) have suggested changes in the O 2 absorption cross section in order to explain the observed attenuation of the solar irradiance between two different heights located in the range of 30 to 40 km. The observations have been performed by a balloon borne Fastie-Ebert double monochromator flying over Palestine, Texas. Frederick and Mentall (1982), have deduced from their observations on September 15, 1980, that, in the 200-210 nm range, the Hasson and Nicholls cross sections should be multiplied by factors in -5-Fig. 5.-Same as fig. 4 but for ozone. 4. CONCLUSIONS AND SUMMARY A reduction of the absorption cross sections in of the Herzberg continuum which has been suggested from direct observation in the atmosphere, leads to modifications in the calculated distributions. of stratospheric minor constituents. The results show the necessity of an accurate determination of the absorption spectrum of O^. However, thé discrepancies between observed and computed concentrations for several species which are photolysed around 200 nm are only partly reduced. Additional measurements of stratospheric solar irradiance between 30 and 40 km are required to obtain unambiguous values of the absorption cross section in the 200-220 nm range and of the transmission factor in the Schumann Runge bands. -16-ACKERMAN, M., Ultraviolet solar radiation related to mesospheric processes, in Mesospheric Models and Related Experiment (G. Fiocco, editor), Reidel Publish. Co, Dordrecht, Holland, 1971. ALLEN, M. and J.E. FREDERICK, Effective photodissociation cross sections for molecular oxygen and nitric oxide in the Schumann-Runge bands, J. Atmos. Sei., 1982. BRASSEUR, G., A. DE RUDDER and A. ROUCOUR, The natural and perturbed ozonosphere, Proceedings of the International Conference on Environmental Pollution, Thessaloniki, Greece, 1982. BRASSEUR, G. and P.C. SIMON, Stratospheric Chemical and Thermal Response to long-term Variability in Solar UV irradiance, J. Geophys. Res., 86, 7343-7362, 1981. CIESLIK, Si, Assessment of the atmospheric penetration of solar UV radiation based upon new spectroscopical data. Paper presented at the EGS meeting, Leeds, 1982. DITCHBURN, R.W. and P.A. YOUNG, The absorption of molecular oxygen between 1850 and 2500 Ä, J. Atmos. Terrest. Phys., 24, 127-139, 1962. observation of the attenuated solar irradiance in the Schumann-Runge band absorption region of molecular oxygen, J. Geophys. Res., 86, 9885-9890, 1981. FREDERICK, J.E. and J.E. MENTALL, Solar irradiance in the stratosphere : implications for the Herzberg continuum absorption of O 2 , Geophys. Res. Lett., 9, 461-464, 1982. FROIDEVAUX, L. and YUNG, Y.L., Radiation and Chemistry in the stratosphere : Sensitivity to O 2 absorption cross sections in the Herzberg continuum, Geophys. Res. Lett., 9, 854-857, 1982. -17-HASSON, V. and R.W. NICHOLLS, Absolute spectral absorption measurements on molecular oxygen from 2460-1920 Â : II continuum measurements 2430-1920 Â, J. Phys. B.: Atom. Molec. Phys., 4, 1789-1797, 1971. HERMAN, J.R. and J.E. MENTALL, O 2 absorption cross sections (187-225 nm) from stratospheric solar flux measurements, J. Geophys. Res., 1982. HUDSON, R.D. and E.I. REED, editors, The Stratosphere : Present and Future, NASA Reference Publication 1049, 432 pp. 1979. GIES, Experimentally determined oscillator strengths and linewidths for the Schumann-Runge band system of molecular oxygen -Bruxelles, Belgique, 1978. OGAWA, M., Absorption cross sections of Og and COg continua in the Schumann and far UV regions, J. Chem. Phys.,