Mechanisms for varying non-LTE contributions to OH rotational temperatures from measurements and modelling. II. Kinetic model

作者：Noll Stefan^*; Proxauf Bastian; Kausch Wolfgang; Kimeswenger Stefan

来源：Journal of Atmospheric and Solar-Terrestrial Physics, 2018, 175: 100-119.

DOI：10.1016/j.jastp.2018.05.005

• OH emission
• Temperature
• Non-LTE
• Kinetic model

OH rotational temperatures T-rot deviate from kinetic temperatures T-kin if the rotational level populations derived from the measured OH lines are not in local thermodynamic equilibrium (LTE). In particular,. from OH bands wi T-rot th high upper vibrational level appear to be affected by incomplete rotational relaxation and corresponding non-LTE temperature excesses Delta T-NLTE. The processes that lead to these non-LTE effects and their variations have not been investigated in detail so far since this requires thorough modelling of the rotational relaxation based on suitable observing data. We performed such an effort for the vibrational level nu = 9. As it is the highest nu that can be populated by the OH-producing hydrogen-ozone reaction, the relaxation processes are the least complex ones. Nevertheless, there are many systematic uncertainties. Therefore, we studied the Delta T-NLTE depending on the Einstein A-coefficients used, the very uncertain rate coefficient for the nu = 9 deactivating collisions of OH with atomic oxygen, and the input profiles for T-kin, air density, and the volume mixing ratios of N-2, 0(2), O, and 0(3). For the profiles, we used SABER products, where we adapted the O retrieval for a better agreement with the model, and the empirical NRLMSISE-00 model. The profiles were selected or calculated for the Cerro Paranal region in Chile since this allowed us to combine OH T-rot measurements based on high-resolution spectra of the UVES spectrograph at the Very Large Telescope with the SABER T-kin and OH emission profiles for an alternative, fully observation-based Delta T-NLTE derivation. Moreover, we measured OH(nu = 9) rotational level populations up to the rotational state N = 10 based on UVES OH(9-3) and OH(9-4) line measurements. With these data in comparison with a grid of models, we derived the best-fitting crucial rate coefficients for the rotational relaxation at nu = 9, which have not been studied before. With the final models, we calculated Delta T-NLTE climatologies and compared them with those from the UVES-based data. Significant Delta T-NLTE and similar variability structures could be identified. Moreover, we analysed the model in order to find the mechanisms that cause the variations. Here, the steep increase of Delta T-NLTE with altitude combined with the height variations of the OH layer appears to be the most important process.

• 出版日期2018-10