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

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