What are the Implications of Nonequilibrium in the O+OH and O+HO₂ Reactions?

Abstract

Vibrationally excited O2, OH, and HO2 species have been suggested (J. Phys. Chem. A 2004, 108, 758) to provide clues for explaining the “ozone deficit problem” and “HOx dilemma” in the middle atmosphere under conditions of local thermodynamic disequilibrium (LTD), but the question arises of how much LTD will affect the title ozone sink reactions. Besides providing novel kinetic results, it is shown that LTD tends to disfavor ozone depletion relative to traditional atmospheric modeling under Boltzmann equilibration, which is partly due to competition between the various reactive channels. The calculations also suggest that the title LTD processes can be important sources of highly vibrationally excited O2 in the middle atmosphere. Moreover, LTD is shown to offer an explanation for the fact that some down revision of the O+HO2 rate constant, or the ratio of the O+HO2 to O+OH rate constants, is required to improve agreement between the predictions of traditional modeling and observation. This, in turn, provides significant evidence supporting LTD at such altitudes.

Implications of the O + OH reaction in hydroxyl nightglow modeling

“…The calculations employed the QCT method and the popular double many-body potential energy surface (Pastrana et al, 1990) (DMBE IV) for HO 2 ( 2 A ). The DMBE IV form is an ab initio based potential function that has been extensively employed in studies of the O + OH reaction and its reverse, using both quantum (Lin et al, 2006) and classical (Varandas, 2005b) methods. Indeed, special care has been taken to achieve a realistic description of the atomdiatom long-range forces, through the inclusion of dispersion and electrostatic energy components.…”

“…For example, although it has been noted (AdlerGolden, 1997) that the reaction O + HO 2 could be a potential candidate, previous work (Kaye, 1988) has shown that it is unlikely to play a crucial role in studies of the high atmosphere. Although this assumption is possibly valid for HO 2 in the ground vibrational state as the dynamics only yields OH(v ≤ 6), the argument no longer holds if non-LTE is considered, since up to OH(v = 15) products can then be formed (Silveira et al, 2004;Varandas, 2005b). In addition, the reaction of OH + O 3 has been shown to yield HO 2 vibrationally excited above the classical asymptote for dissociation, and thus can be another potential source of OH(v ) Zhang and Varandas, 2001).…”

“…This is justifiable since these state-tostate vibrational-excitation rate constants are expected to be very small at the temperatures of interest (Varandas, 2004a) and to the best of our knowledge reliable estimates for these rate constants are lacking. Of course, collisional processes involving vibrationally excited O 2 and other species Varandas, 2005b) are not considered in this judgement.…”

“…Well known mysteries in the terrestrial atmosphere , many still unsolved, have stimulated the study of reactions and chemical cycles (Slanger et al, 1988;Toumi et al, 1996;Varandas, 2002Varandas, , 2005b in the middle atmosphere (stratosphere and mesosphere). One important contribution arises from the non-local thermodynamic equilibrium or non-LTE hypothesis (also referred to in previous work by local thermodynamic disequilibrium, Varandas, 2003) for the molecular internal degrees of freedom (López-Puertas and Taylor, 2001).…”

“…When non-LTE conditions hold, the chemistry changes dramatically due to the differences in the internal energy content of the species involved: endothermic reactions become exothermic with rate constants substantially larger, at times orders of magnitude larger (Silveira et al, 2004;Varandas, 2005b). Although several studies have been devoted to the role of vibrationally excited species in reactions, particularly for the HO x cycle, atmospheric chemistry models of the middle-upper atmosphere neglect non-LTE (see, e.g., von Clarmann et al, 2010 and references therein).…”