Lidar Soundings of the Mesospheric Nickel Layer Using Ni(3F) and Ni(3D) Transitions

During six nights between January and March 2018 we observed the mesospheric Ni layer by lidar from Kuhlungsborn, Germany (54 degrees N, 12 degrees E). For most of the soundings we utilized for the first time a transition from the low-lying excited Ni(D-3) state at 341nm. For additional soundings we used the ground-state Ni(F-3) transition at 337nm, giving similar results but a worse signal-to-noise ratio. We observed nightly mean Ni peak densities between approximate to 280 and 450cm(-3) and column abundances between 3.110(8) and 4.910(8)cm(-2). Comparing with iron densities we get a Fe/Ni ratio of 38, which is a factor of 2 larger than the ratio in CI chondrites and factor of 32 larger than the Fe/Ni ratio observed by the only previous measurement of mesospheric Ni (Collins et al., 2015, ). The underabundance of Ni compared to CI chondrites suggests that Ni is more efficiently sequestered as Ni+ or neutral reservoir species than Fe. Plain Language Summary In the upper mesosphere between 80- and 100-km altitude, layers of gaseous metals occur whose source is the evaporation of cosmic dust particles, which undergo severe heating when entering the atmosphere. Metals like sodium or iron have been observed for many years by ground-based laser radar instruments (lidars). Here we report the second-ever set of observations of nickel (Ni) in the mesosphere. For the first time we used an absorption line in the UV from a slightly excited state of Ni, which provides a much stronger signal-to-noise ratio compared to earlier soundings. We observed Ni peak densities between 280 and 450/ccm, which is a factor of 2 lower than expected from Fe measurements and the relative abundances of these elements in cosmic dust. This suggests small differences in chemical reaction rates, converting Ni into Ni-containing molecules, which are invisible to the lidar, compared to Fe. Observed densities are a factor of 32 lower with respect to Fe than reported in the earlier study in Alaska, and this needs to be further examined.