A global atmospheric model of meteoric iron

Abstract

[1] The first global model of meteoric iron in the atmosphere (WACCM-Fe) has been developed by combining three components: the Whole Atmosphere Community Climate Model (WACCM), a description of the neutral and ion-molecule chemistry of iron in the mesosphere and lower thermosphere (MLT), and a treatment of the injection of meteoric constituents into the atmosphere. The iron chemistry treats seven neutral and four ionized iron containing species with 30 neutral and ion-molecule reactions. The meteoric input function (MIF), which describes the injection of Fe as a function of height, latitude, and day, is precalculated from an astronomical model coupled to a chemical meteoric ablation model (CABMOD). This newly developed WACCM-Fe model has been evaluated against a number of available ground-based lidar observations and performs well in simulating the mesospheric atomic Fe layer. The model reproduces the strong positive correlation of temperature and Fe density around the Fe layer peak and the large anticorrelation around 100 km. The diurnal tide has a significant effect in the middle of the layer, and the model also captures well the observed seasonal variations. However, the model overestimates the peak Fe + concentration compared with the limited rocket-borne mass spectrometer data available, although good agreement on the ion layer underside can be obtained by adjusting the rate coefficients for dissociative recombination of Fe-molecular ions with electrons. Sensitivity experiments with the same chemistry in a 1-D model are used to highlight significant remaining uncertainties in reaction rate coefficients, and to explore the dependence of the total Fe abundance on the MIF and rate of vertical transport.

A new model of meteoric calcium in the mesosphere and lower thermosphere

“…This picture is quite similar to that of Fe + and its oxide ions (Feng et al, 2013 (Broadley et al, 2007), because it is a bimolecular reaction in contrast to the third-order association reactions where Ca + clusters with O2, N2 etc. (Table 1).…”

“…Although there is reasonable agreement in the general shape of the Ca + layer, and the model captures the Ca + ion density at certain heights measured both by the lidar and the two rocket flights, overall the model appears to over-15 predict the Ca + density. This is a problem that we have encountered previously with WACCM modeling of both Na + (Marsh et al, 2013a) and Fe + (Feng et al, 2013), which we have attributed to the absence of electro-dynamical transport of these metallic ions in WACCM. This point is discussed further below.…”

“…S2 in the SI). This MIF-dependence is not really observed in the Na and Fe layers, which have a 10 strong annual variation with a minimum in summer and maximum in winter (Feng et al, 2013;Marsh et al, 2013a). The reason for the Ca layer is strongly influenced by the MIF is because the lifetime of Ca in the layer is short: this can be estimated by dividing the Ca layer column abundance by the integrated Ca MIF, i.e.…”

“…The seasonal variation of these MIFs with latitude ( Fig. S2) was derived by scaling to the variation of the Na and Fe MIFs determined previously using an astronomical dust model (Marsh et al, 2013a;Feng et al, 2013). 25…”

“…Below the Ca layer peak (~90 km), CaCO3 is the dominant gas-phase Ca species; although note that the line labelled CaCO3 refers to the sum CaCO3 + O2CaCO3 + OCaCO3, which is mostly O2CaCO3 below 90 km, as discussed in Section 2.1. Importantly, because of the CaOH and CaCO3 holding cycles, the dimerization of molecular species is much more important than for other meteoric metals such as Fe (Feng et al, 2013) and Na (Marsh et al, 2013a). This is why the profile 25 of the Ca sink (representing polymerized Ca molecules) is the most abundant form of the metal below 97 km.…”