Homogeneous and immersion freezing of inorganic and organic aerosol particles at cirrus temperatures

Cirrus clouds influence the global climate system by both a cooling effect due to scattering visible sunlight back to space and a warming effect by absorbing and scattering infrared radiation back towards the earth surface. The net effect of either cooling or warming critically depends on the number concentration, size distribution and shape of cirrus ice crystals. The number concentration of ice crystals in upper tropospheric cirrus clouds is influenced by the primary ice formation processes during cirrus formation, which can either be dominated by heterogeneous ice nucleation, leading to low ice crystal number concentrations, or is also influenced by homogeneous freezing of solution aerosols leading to higher ice crystal number concentrations. Though of crucial importance to predict the climatic effect of cirrus clouds, it is often not clear and sometimes discussed controversially whether heterogeneous or homogeneous ice formation processes dominate cirrus formation. In many cases model predictions of cirrus ice crystal number concentrations disagree to field observations. In the AIDA aerosol and cloud chamber facility at the Karlsruhe Institute of Technology we investigated both heterogeneous and homogeneous ice formation processes of cirrus clouds at temperatures between -35°C and -80°C. In this contribution we focus on homogeneous freezing of inorganic and organic aqueous solution particles and the immersion freezing effect of crystallites in organic solution particles. The results of the homogeneous freezing experiments clearly show that freezing rate formulations currently used in global climate models clearly show a low bias in the humidity onset thresholds for homogeneous ice formation at temperatures below about 210 K, and furthermore overestimate the ice formation rate by at least a factor of 2. The experimental results will be summarized and a new empirical fit to the experimental data will be suggested for use in atmospheric models. In the second part of the contribution we will discuss the effect of partial crystallization of oxalic acid particles on their ice nucleation behavior.