Reflections on aluminium: Some thoughts on the mesospheric processing of ablated meteoric Al⁺

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⁺ → A1O⁺ → 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 AlO_n structures. These Al-containing neutrals may act as nucleation sites for noctilucent and polar stratospheric clouds.