Lidar observations of sodium density depletions in the presence of polar mesospheric clouds

Since 1997, sodium resonance lidar and Rayleigh lidar measurements have been conducted simultaneously at the Sondrestrom upper atmosphere research facility near Kangerlussuaq, Greenland (67.0°N, 309.1°E) for studying the arctic mesosphere. The summertime lidar observations during the typical polar mesospheric cloud (PMC) season from June through August are used to investigate changes in the mesospheric sodium layer related to the presence of PMCs. Our observations demonstrate, on average, a 20% reduction in sodium density below the sodium peak altitude when PMCs are detected by the Rayleigh lidar. Individual PMC events can produce appreciable depletions in the lower half of the sodium layer. We have observed nearly 50% reduction in sodium column abundance below 92 km in the presence of a PMC with moderate backscatter strength. The magnitude of the depletion is correlated with PMC backscatter strength. New modeling results that include the photolysis of sodium bicarbonate have suggested that the reduction in sodium density may be associated with adsorption of sodium atoms onto the surface of the ice particles. The observed sodium depletion is not as significant as the depletion in iron density observed from the South Pole during the austral PMC summer months. This difference in depletion may result from the fact that the sodium layer resides a few kilometers higher than the iron layer, and, therefore, is exposed to presumably smaller ice particles (with less surface area) and, thus, has a reduced uptake rate on ice. Our observations extend previously reported reductions in iron and potassium in the presence of PMCs to include sodium, and support the role of heterogeneous chemistry on ice particles as a new and important process for sodium chemistry near the mesopause.