The photolysis of FeOH and its effect on the bottomside of the mesospheric Fe layer

Metal layers in the upper mesosphere and lower thermosphere are created through meteoric ablation. They are important for understanding the temperature structure, dynamics and chemistry of this atmospheric region. Recent lidar observations have shown a regular downward extension of the Fe layer bottomside which correlates with solar radiation. In this study we combine lidar observations, quantum chemical calculations and model simulations to show that this bottomside extension is primarily caused by photolysis of FeOH. We determine the photolysis rate to be J(FeOH)=(6±3)×10−3s−1. We also show that the reaction FeOH+H→FeO+H2 is slower at mesospheric temperatures than previous estimates. With these updated rate coefficients, we are able to significantly improve the modelling of the Fe layer bottomside. The calculations further show the nearly complete depletion of FeOH during sunlit periods. This may have implications for cloud nuclei in the middle atmosphere.