The Meteoric Ni Layer in the Upper Atmosphere

The first global atmospheric model of Ni (WACCM-Ni) has been developed to understand recent observations of the mesospheric Ni layer by ground-based resonance lidars. The three components of the model are: the Whole Atmospheric Community Climate Model (WACCM6); a meteoric input function derived by coupling an astronomical model of dust sources in the solar system with a chemical meteoric ablation model; and a comprehensive set of neutral, ion-molecule, and photochemical reactions pertinent to the chemistry of Ni in the upper atmosphere. In order to achieve closure on the chemistry, the reaction kinetics of three important reactions were first studied using a fast flow tube with pulsed laser ablation of a Ni target, yielding k(NiO + O) = (4.6 +/- 1.4) x 10(-11), k(NiO + CO) = (3.0 +/- 0.5) x 10(-11), and k(NiO2 + O) = (2.5 +/- 1.2) x 10(-11) cm(3) molecule(-1) s(-1) at 294 K. The photodissociation rate of NiOH was computed to be J(NiOH) = 0.02 s(-1). WACCM-Ni simulates satisfactorily the observed neutral Ni layer peak height and width, and Ni+ measurements from rocket-borne mass spectrometry. The Ni layer is predicted to have a similar seasonal and latitudinal variation as the Fe layer, and its unusually broad bottom-side compared with Fe is caused by the relatively fast NiO + CO reaction. The quantum yield for photon emission from the Ni + O-3 reaction, observed in the nightglow, is estimated to be between 6% and 40%.