Effect of updated relaxation rate constants on the H2O vibrational level populations and ro-vibrational spectra in the mesosphere and lower thermosphere

This paper discusses the formation mechanisms of infrared radiation in H2O ro-vibrational bands in the mesosphere and lower thermosphere (MLT). At these heights and above, the vibrational levels of the molecules involved in radiative transitions are not in local thermodynamic equilibrium (LTE) with the surrounding medium, and the biggest uncertainty source in modeling the IR radiation in molecular bands is associated with the corresponding vibrational kinetics model parameters. In this study, we re-analyze available experimental data of Barnes et al. (2004) and Zittel and Masturzo (1991) and update the rate constant of V–V exchange (k) corresponding to the second vibrational number v2 increase by two and the first (or the third) quantum number decrease by one. The estimated values of k for quenching by N2 and O2 are 1.7 × 10−12 and 1.3 × 10−12 cm3 s−1, respectively. These values are about four times larger than the values used in all earlier models of non-LTE populations of vibrational levels and IR radiation in ro-vibrational bands of H2O. We state that using the same k for all processes of vibrational–vibrational energy exchange does not correspond to laboratory experiments and can lead to offsets both in calculating the vibrational level populations and in H2O concentration retrievals from IR spectral radiance. We provide the estimates of these offsets as well as a list of spectral microwindows, in which H2O radiance is sensitive to uncertainty of k.