Nonlocal thermodynamic equilibrium effects in stratospheric HF by collisional energy transfer from electronically excited O_2 and implications for infrared remote sensing

Abstract

A possible nonlocal thermodynamic equilibrium (non-LTE) effect involving stratospheric HF arising from the direct photochemical excitation of vibrationally excited HF by collisional energy transfer from electronically excited O(2) [O(2)((1)Delta), O(2)((1)Sigma)] is presented. Although this non-LTE effect is smaller than one associated with the direct solar excitation of both HF(nu= 1) and HF(nu= 2), calculations show that inclusion of the mechanism proposed here into retrieval algorithms is necessary if correct daytime upper stratosphere HF profiles are to be inferred in future infrared thermal emission measurements. Accurate determinations of the state-to-state rate constants of the reactions involved in these excitation processes and of the quenching rates of vibrationally excited HF are needed if IR thermal emission techniques are to be accurately employed in the measurement of stratospheric HF.