Kinetic Study of the Reactions of AlO and OAlO Relevant to Planetary Mesospheres

Abstract

Aluminum atoms are injected into planetary upper atmospheres by meteoric ablation. Rapid oxidation of Al by O2 to form AlO is then likely to be followed by reactions with O3, O2, and CO2 to form larger oxides and carbonates, which can also be reduced by atomic O and CO. The reactions listed below were investigated experimentally using both pulsed laser photolysis of an Al precursor in a slow flow reactor, and pulsed laser ablation of an Al target in a fast flow tube, with laser-induced fluorescence detection of AlO. The experimental results were interpreted using electronic structure theory calculations and Rice–Ramsperger–Kassel–Markus theory. The low pressure limiting rate coefficients for the two recombination reactions are: log10(krec,0 (AlO + O2 + N2, 192–812 K)) = −35.137 + 6.1052 log10(T) – 1.4089 (log10(T))2 and log10(krec,0(AlO + CO2 + N2, 193–813 K)) = −38.736 + 8.7342 log10(T) – 2.0202 (log10(T))2 cm6 molecule–2 s–1, with a ±20% uncertainty over the experimental temperature range. The following bimolecular reactions were also studied at 295 K: k(AlO + O3 → OAlO + O2) = (1.25 ± 0.19) × 10–10; k(AlO + CO → Al + CO2) = (1.95 ± 0.35) × 10–12; k(OAlO + CO → AlO + CO2) = (2.6 ± 0.7) × 10–11 and k(OAlO + O → AlO + O2) = (1.9 ± 0.8) × 10–10 cm3 molecule–1 s–1. In the terrestrial atmosphere between 65 and 110 km, AlO is mostly removed by recombination with O2 below 85 km, and reaction with O3 above 90 km. On Mars, recombination with CO2 is much more important than with O2, although reduction of AlO by CO should maintain a significant density of Al atoms. Here we show that in both atmospheres, AlOH is likely to be an important reservoir.