Abstract
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 1470 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 1.5% for polar winter and 2.5% for midlatitude night conditions). For OH(v=0...9)+O the effective rate coefficients are lower by up to 11% than those involving OH in its vibrational ground state. The effects on the odd oxygen sink are negative and can reach −3% (midlatitudinal nighttime lowermost thermosphere), i.e. neglecting vibrational excitation overestimates the odd oxygen sink. The OH sink is overestimated by up to 10%. After a solar proton event, when upper atmospheric OH can be enhanced by an order of magnitude, the excess relative odd oxygen sink by consideration of vibrational excitation in the reaction of OH(v=0...9)+O3 is estimated at up to 0.2%, and the OH sink by OH(v=0...9)+O can be reduced by 12% in the thermosphere by vibrational excitation.
Highlights
In the 1970s and early 1980s, a large number of studies were performed to assess reactions involving vibrationally excited molecules (e.g. Wieder and Marcus, 1962; Coltharp et al, 1971; Worley et al, 1972; Gordon and Lin, 1976; Hui and Cool, 1978; Kneba and Wolfrum, 1980)
While it has become a standard procedure to consider non-LTE radiative processes associated with vibrational and rotational excitation in radiative transfer modeling and remote sensing data analysis whenever appropriate (e.g. Funke et al, 2001a,b; Kaufmann et al, 2003; Yankovsky and Manuilova, 2006), consideration of these effects in chemistry modelling is by far no standard today, it is quite plausible that excess energy in the form of vibrational excitation will make it easier for the reactants to reach the activation energy
Since larger populations of excited molecules are found for species whose excess populations are driven by their status nascendi rather than the radiance field, we focus this study on the reactions OH(v = 0...9)+O3 (Sect. 2) and OH(v = 0...9)+O
Summary
In the 1970s and early 1980s, a large number of studies were performed to assess reactions involving vibrationally excited molecules (e.g. Wieder and Marcus, 1962; Coltharp et al, 1971; Worley et al, 1972; Gordon and Lin, 1976; Hui and Cool, 1978; Kneba and Wolfrum, 1980). Due to low collision rates, quenching happens not frequently enough to redistribute the vibrational energy that molecules may have via the status nascendi or via absorption of a photon, leading to non-LTE (LopezPuertas and Taylor, 2001). Funke et al, 2001a,b; Kaufmann et al, 2003; Yankovsky and Manuilova, 2006), consideration of these effects in chemistry modelling is by far no standard today, it is quite plausible that excess energy in the form of vibrational excitation will make it easier for the reactants to reach the activation energy. Since larger populations of excited molecules are found for species whose excess populations are driven by their status nascendi rather than the radiance field, we focus this study on the reactions OH(v = 0...9)+O3 We discuss the limitations of this study and identify necessary future work (Sect. 4)
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