Abstract. This paper describes a laboratory study into the chemical pathways by which Na + is converted to Na in the upper atmosphere. The termolecular clustering reactions of Na + with shown that atomic O will ligand switch with Na. N2 + but not with Na. CO2 +, and that the former reaction proceeds essentially at the Langevin collision frequency. The neutralization of Na + in the upper atmosphere is therefore rather complex. The first step is formation of the Na. N2 + ion from the recombination of Na + with N2.•his cluster ion can then either switch with CO2, which leads to a stable cluster ion that will undergo dissociative electron recombination to form Na; or switch with atomic O, which reforms Na*. The restfit of this is that the lifetime of Na + changes very rapidly from more than a day above 100 km to just a few minutes at 90 km.Furthermore, the rate of neutralization is largely independent of the electron concentration. A simple model describing the conversion of Na + to atomic Na in a descending sporadic E layer demonstrates that this ion-molecule mechanism appears to fulfil many of the major criteria for producing sporadic sodium layers.

Abstract. The reaction between sodium bicarbonate (NaHCO3) and atomic H is the only likely route (apart possibly from daytime photolysis) for recycling this major sodium reservoir back to atomic Na in the upper mesosphere.

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Abstract. Summertime observations of the mesospheric Na layer at high latitudes are reported from the 1993 Airbome Noctilucent Cloud (ANLC-93) campaign in the Canadian Arctic and at the Amundsen-Scott South Pole Station. Measurements at the South Pole reveal a layer that has a smaller column abundance and is significantly higher and thinner than at midlatitudes. Using a model that was essentially optimized to wintertime conditions at high northern latitudes, the South Pole layer can be modeled satisfactorily if the rate coefficient for the reaction between sodium bicarbonate and atomic hydrogen is set to k(NaHCO3 + H • Na + H20 + CO2) = 1.1 x 10 -• exp (-910/T) cm 3 molecule 4 s 4. In particular, the model is able to reproduce the small scale height of about 2 km observed on the underside of the layer. It is then shown that this steep gradient in the atomic Na mixing ratio can be sustained against vertical eddy diffusion because of the sufficiently rapid chemical cycling between Na its major reservoir, NaHCO3. This justifies the assumption in the model that the vertical transport of Na species can be treated in terms of a single continuity equation describing total sodium. The observations from the campaigns in both hemispheres show that the Na abundance has a temperature dependence of about 2 x 108 cm -2 K 4 at temperatures below 170 K, in good accord with the model. About 40% of this dependence appears to be caused by the activation energies of the reactions that partition sodium between atomic Na and NaHCO3, and the remainder by the temperature dependence of the odd-oxygen/odd-hydrogen chemistry in the upper mesosphere.

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