Do vibrationally excited OH molecules affect middle and upper atmospheric chemistry?

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)+O 3 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)+O 3 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 O x and O 3 compared to other reactions involving, e.g., atomic oxygen. Thus the impact of vibrationally excited OH on the ozone or O x 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 HO 2 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 Correspondence to: T. von Clarmann (thomas.clarmann@kit.edu) 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)+O 3 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.

Implications of the O + OH reaction in hydroxyl nightglow modeling

“…Such a value seems to be corroborated (Pickett et al, 2006) by upper atmospheric measurements, although it has also been questioned (von Clarmann et al, 2010) since its use results in predictions of a smaller population of hydroxyl radicals than observed. Quoting the authors (von Clarmann et al, 2010), "it may provide an upper limit for the reaction". Indeed, the topic has long been a matter of uncertainty.…”

“…Since the relaxation rate constant for v > 1 was unknown, a vibrationally independent effective quenching was fixed at twice the value of the experimental total removal rate (Spencer and Glass, 1977). Such a value seems to be corroborated (Pickett et al, 2006) by upper atmospheric measurements, although it has also been questioned (von Clarmann et al, 2010) since its use results in predictions of a smaller population of hydroxyl radicals than observed. Quoting the authors (von Clarmann et al, 2010), "it may provide an upper limit for the reaction".…”

“…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). Indeed, 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), but consideration of these effects in chemistry modelling has been neglected (von Clarmann et al, 2010). One of the reasons for this, as has been pointed out Adler-Golden, 1997), is the lack of accurate state-to-state deactivation and state-specific rate constants for key reactions involved in the atmospheric cycles.…”

HO2•$_{2}^{\bullet }$+O3 → OH•+2O2 reaction: A potential source of vibrationally hot OH radicals in the atmosphere

“…Vibrationally excited species are key to various important processes in the atmosphere at higher altitudes as well as at lower altitudes 1–9 . At higher altitudes, where lifetime of most of the vibrationally excited species is high, they contribute to night airglow, a phenomenon which acts as a window to understand the chemistry of upper atmosphere 1–4 .…”

“…In addition, vibrationally excited species can affect the reaction rate significantly 9–17 . For example, in a recent study, 18 it was found that excitation of the HO$$_{2}^{\bullet }$$ in its OH vibrational mode can increase the rate of H‐abstraction path for HO$$_{2}^{\bullet }$$+O→ OH • +O 2 reaction by ∼3 orders of magnitude.…”