Heterogeneous ice cloud formation at extreme conditions (T<160K)

Heterogeneous ice cloud formation at extreme cold temperatures below 150K can be encountered in planetary atmospheres, e.g. polar mesospheric clouds on Earth or water ice clouds on Mars. In the case of polar mesospheric clouds on Earth, ice nucleation is activated on sub 2nm meteoric smoke particles. The formation process of ice clouds at such cold temperatures on nanometer sized nuclei is not well understood because no ice nucleation experiments for such conditions existed so far. In this contribution, we present results from a laboratory experiment which is designed to study ice nucleation processes below 150K on nanoparticles on a microphysical scale. We determine the binding energy/desorption energy of water molecules on the particle material, which allows us to parameterize the amount of water molecules which are adsorbed on a surface as function of temperature and saturation. In addition, we are able to evaluate for the temperature dependent sublimation flux of water molecules from the phase growing on the particles and conclude that the deposited ice polymorph below 150K is amorphous solid water (ASW). We show that the saturation vapor pressure of ASW is higher than previously assumed which significantly affects the ice growth behavior of the particles. We present a new parameterization for the saturation vapor pressure of ASW. Finally, we determine critical saturations for the onset of ice growth and present a method to predict critical saturations needed for cloud formation below 150K. Moreover, we will discuss the influence of the particle charge on the ice formation process.