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 Kühlungsborn, Germany (54°N, 12°E). For most of the soundings we utilized for the first time a transition from the low‐lying excited Ni(3D) state at 341 nm. For additional soundings we used the ground‐state Ni(3F) transition at 337 nm, giving similar results but a worse signal‐to‐noise ratio. We observed nightly mean Ni peak densities between ∼280 and 450 cm−3 and column abundances between 3.1·108 and 4.9·108 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, https://doi.org/10.1002/2014GL062716). 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. Key Points: First observation of the mesospheric Ni layer from metastable Ni(3D) state revealed peak densities of 280–450 cm−3Compared to Fe and their respective abundance in CI chondrites, Ni is depleted by a factor of about 2Observations hint at faster‐than‐expected conversion of Ni into ions and neutral reservoir molecules [ABSTRACT FROM AUTHOR]