Lab experiments on the complex microphysics controlling PMC formation

Until recently it was believed that surface properties which determine the contact parameter of ice on the ice nuclei surface are the major uncertainty in the description of mesospheric ice particle formation processes. However, the combination of laboratory experiments and numerical simulation studies on Polar Mesospheric Cloud (PMC) formation showed that other parameters such as the absorption coefficient for solar and infrared radiation, atmospheric dynamics and the kinetics of ice particle growth may play a comparably important role in the PMC formation process. Key properties influencing cloud formation such as desorption energy, contact parameter and light absorption coefficient of the materials that are presumed as Meteoric Smoke Particle (MSP) constituents as well as type and saturation vapor pressure of the possible ice phases of PMC particles are insufficiently known at mesospheric conditions. To tackle these issues we performed laboratory experiments using the mesospheric ice cloud chamber MICE-TRAPS on artificially produced MSP analogues of variable iron silicate composition. We present measurements of the desorption energy and contact parameter of the investigated materials. ... mehrWe use these parameters in Classical Nucleation Theory (CNT) and show that despite unmet assumptions the formalism of CNT can be used to predict nucleation rates of water ice on small nanoparticles and therefore forecast critical saturations needed for cloud formation under mesospheric conditions. We also show that the absorption of light can play a significant role in the ice activation of MSPs. In particular, the absorption of light can lead to the startling effect that (depending on seed particle composition) ice nucleation on smaller seed particles may be favored with respect to larger particles. We present measurements on the absorption coefficient of iron silicates and show that great care has to be exercised employing refractive index literature values for the interpretation of absorption data. Furthermore, our experiments show that the actual saturation vapor pressure over mesospheric ice may differ by more than 100% from what is usually assumed for ice at mesospheric conditions. The experiments also provide indications on the shape and morphology of ice particles in PMCs.