Mesospheric Na layer at 40 degrees N: Modeling and observations

A complete monthly record of the annual variation of Na and temperature in the upper mesosphere has been obtained from 3 years of nighttime lidar obse rvations at two midlatitude sites, Urbana-Champaign, Illinois (40 degrees N ), and Fort Collins, Colorado (41 degrees N). The Na density exhibits a str ong annual variation at all heights between 81 and 107 km, with the column abundance of the layer peaking in early winter and then decreasing by nearl y a factor of 4 to a midsummer minimum. There are also significant semiannu al components to the variations in the centroid height and thickness of the layer. The nighttime temperature profile between 81 and 105 km exhibits a high winter mesopause at about 101 km and a summer mesopause at about 85 km . During spring and autumn, the mesopause oscillates apparently randomly be tween these states. A seasonal model of the Na layer was then constructed i ncorporating recent laboratory studies of the pertinent neutral and ionic r eactions of the metal. The background atmospheric composition was provided from three off-line models, as well as from UARS/Microwave Limb Sounder sat ellite measurements of H2O. With a small number of permitted adjustable par ameters, the model is able to reproduce many observed features of the Na la yer remarkably well, including the monthly variation in column abundance an d layer shape. The biggest discrepancy is during midsummer, when the modele d layer is displaced 2-3 km above that observed, although a factor contribu ting to this is that the lidar observing period during summer was relativel y short and the effect of the diurnal tide could have been incompletely sam pled. Both the observations and the model show that Na density and temperat ure are highly correlated below 96 km (correlation coefficient equal to 0.8 -0.95), mostly as a result of the influence of odd oxygen/hydrogen chemistr y on the partitioning of sodium between atomic Na and its principal reservo ir species, NaHCO3. Above 96 km, a weak negative correlation (-0.2) is expl ained by the dominance of ion-molecule chemistry. Finally, it was shown tha t if the eddy diffusion coefficient in the middle mesosphere is significant ly smaller or if the global meteoric influx is much larger than the values used in the present model, then processes for permanently removing gas-phas e Na species in the mesosphere, such as polymerisation and deposition onto dust particles, will need to be included.