Reflections on Aluminium: Some Thoughts on the Mesospheric Processing of Ablated Meteoric Al+

Abstract

Environmental Context.As the second most abundant main-group metal, Al is a prominent constituent of meteors. Other metal-containing products of meteoric infall have been implicated in noctilucent cloud nucleation and polar stratospheric cloud formation. Aluminium is also the principal metallic component of the space debris in low Earth orbit. Re-entry of this debris is an anthropogenic route to neutral and ionized Al atoms in the upper atmosphere, the consequences of which are as yet unknown. The calculations reported here suggest that natural mechanisms exist for the processing of Al+ to yield neutral Al-containing structures that are likely to be highly reactive with trace upper-atmosphere constituents. Abstract.Quantum-chemical calculations, at the CP-dG2thaw level of theory, are used to characterize several novel Al-containing molecular and cluster ions that are likely to participate in the chemistry of Al+ resulting from meteoric ablation in the upper atmosphere. The calculations suggest that two overall processes may be dominant in aluminium’s ion chemistry in the mesosphere/lower thermosphere (MLT) region. The first of these processes is an efficient cycle (Al+ → AlN2+ → AlO+ → Al+), with addition of N2 to Al+ the apparent rate-determining step on the overall cycle, and with weak competition from neutralization processes. The second possible process involves conversion of Al+ to the species HOAlOH+, by (we propose) reaction with H2O and O (3P) respectively. Although water vapour is only a trace species within the mesosphere, production of HOAlOH+ appears pivotal: This molecular ion is intrinsically stable against substitution or abstraction involving other mesospheric species, but is highly activated towards further addition of N2, CO2 and H2O. We propose that, once formed, HOAlOH+ is rapidly consumed through condensation reactions, producing larger cluster ions whose eventual neutralization through dissociative recombination will most probably yield hydrated AlOn structures. These Al-containing neutrals may act as nucleation sites for noctilucent and polar stratospheric clouds.