Lidar observations, conducted at the South Pole by University of Illinois researchers,
are used to characterize the seasonal variations of mesospheric Na and Fe above the site.
The annual mean layer abundances are virtually identical to midlatitude values, and the
mean centroid height is just 100 m higher for Na and 450 m higher for Fe compared with
40 N. The most striking feature of the metal profiles is the almost complete absence of Na
and Fe below 90 km during midsummer. This leads to summertime layers with
significantly higher peaks, narrower widths, and smaller abundances than are observed at
lower latitudes. The measurements are compared with detailed chemical models of these
species that were developed at the University of East Anglia. The models accurately
reproduce most features of these observations and demonstrate the importance of rapid
uptake of the metallic species on the surfaces of polar mesospheric clouds and meteoric
smoke particles. The models show that vertical downwelling in winter, associated with the
meridional circulation system, must be less than about 1 cm s 1 in the upper
mesosphere in order to avoid displacing the minor constituents O, H, and the metal
layers too far below 85 km. They also show that an additional source of gas-phase
metallic species, that is comparable to the meteoric input, is required during winter to
correctly model the Na and Fe abundances. This source appears to arise from the
wintertime convergence of the meridional flow over the South Pole.Ope