Metallic layers in the mesopause and lower thermosphere region

Abstract The layers of neutral metal atoms such as Na and Fe, which occur in the mesosphere and lower thermosphere as a result of meteoric ablation, can be observed with excellent spatial and temporal resolution by lidar. This has proved to be a powerful technique for studying dynamical processes between 80 and 105 km. However, an important assumption has been that these metal atoms behave as chemically inert tracers. The validity of this depends on the kinetic response of the metal atoms and their compounds to dynamical perturbations in both the temperature and the concentrations of minor species such as O 3 , H 2 O, O and H. This paper describes laboratory studies of several important reactions involving Na and Fe species. The reaction NaHCO 3 + H → Na + H 2 O + CO 2 was investigated using a fast flow tube, where sodium bicarbonate was produced in situ from a flow of Na vapour mixed sequentially with N 2 O, H 2 and CO 2 , and then reacted with an excess of atomic H. The results appear to provide complete closure for the upper mesospheric chemistry of sodium. The reactions of FeO with O 3 , O 2 , H 2 O and CO 2 were studied by the pulsed laser photolysis of a ferrocene/NO 2 mixture, followed by time-resolved laser induced fluorescence of FeO. The resulting rate coefficients, together with ab initio quantum calculations on the Fe-containing species formed by these reactions, indicate that FeO 3 is probably the most important reservoir for iron immediately below the atomic Fe layer. A wave-driven model was then employed to compare Na and Fe as dynamical tracers. This shows that, whereas Na is essentially conserved as a tracer between 80 and 105 km, Fe could undergo significant chemical amplification of dynamical perturbations below 87 km, especially during summer. Differences in the ion-molecule chemistries of these metals indicate that the Fe Na ratio should exhibit large wave-driven perturbations above 100 km.