Mesospheric Na layer at extreme high latitudes in summer

Summertime observations of the mesospheric Na layer at high latitudes are reported from the 1993 Airborne 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 (NaHCO 3 + H → Na + H 2 O + CO 2 ) = 1.1 × 10 −11 exp (−910/ T ) cm 3 molecule −1 s −1 . 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, NaHCO 3 . 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 × 10 8 cm −2 K −1 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 NaHCO 3 , and the remainder by the temperature dependence of the odd‐oxygen/odd‐hydrogen chemistry in the upper mesosphere.