Atmos. Chem. Phys. Discuss., 10, 11449-11481, 2010
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Do vibrationally excited OH molecules affect middle and upper atmospheric chemistry?
T. von Clarmann1, F. Hase1, B. Funke2, M. López-Puertas2, J. Orphal1, M. Sinnhuber3, G. P. Stiller1, and H. Winkler3
1Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Karlsruhe, Germany
2Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain
3Bremen University, Institute of Environmental Physics, Bremen, Germany

Abstract. Except for a few reactions involving electronically excited molecular or atomic oxygen or nitrogen, atmospheric chemistry modelling usually assumes that the temperature dependence of reaction rates is characterized by Arrhenius law involving kinetic temperatures. It is known, however, that in the upper atmosphere the vibrational temperatures may exceed the kinetic temperatures by several hundreds of Kelvins. This excess energy has an impact on the reaction rates. We have used upper atmospheric OH populations and reaction rate coefficients for OH(v=0...9)+O3 and OH(v=0...9)+O to estimate the effective (i.e. population weighted) reaction rates for various atmospheric conditions. We have found that the effective rate coefficient for OH(v=0...9)+O3 can be larger by a factor of up to 1020 than that involving OH in its vibrational ground state only. At altitudes where vibrationally excited states of OH are highly populated, the OH reaction is a minor sink of Ox and O3 compared to other reactions involving, e.g., atomic oxygen. Thus the impact of vibrationally excited OH on the ozone or Ox sink remains small. Among quiescent atmospheres under investigation, the largest while still small (less than 0.1%) effect was found for the polar winter upper stratosphere and mesosphere. The contribution of the reaction of vibrationally excited OH with ozone to the OH sink is largest in the upper polar winter stratosphere (up to 4%), while its effect on the HO2 source is larger in the lower thermosphere (up to 1% for polar winter and 1.7% for midlatitude night conditions). For OH(v=0...9)+O the rate coefficients differ by plus/minus a few percent only from those involving OH in its vibrational ground state. The effects on the odd oxygen sink are negative and can reach −0.7% (polar summer lowermost thermosphere), i.e. neglect of vibrational excitation overestimates the odd oxygen sink. The OH sink is overestimated by up to 2%. After a solar proton event, when upper atmospheric OH can be enhanced by an order of magnitude, the excess relative odd oxygen sink by OH(v=0...9)+O3 is estimated at up to 0.2%, and the excess relative OH sink by OH(v=0...9)+O3 can exceed 4% in the thermosphere.

Citation: von Clarmann, T., Hase, F., Funke, B., López-Puertas, M., Orphal, J., Sinnhuber, M., Stiller, G. P., and Winkler, H.: Do vibrationally excited OH molecules affect middle and upper atmospheric chemistry?, Atmos. Chem. Phys. Discuss., 10, 11449-11481, doi:10.5194/acpd-10-11449-2010, 2010.
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