Clouds are essential for Earth¿s climate system, weather phenomena, and the hydrological cycle. Cloud microphysics determine cloud albedo in the visible and infrared spectral ranges and cloud lifetime, which are essential for the effect of clouds on the radiation balance of the Earth-atmosphere system and, together with the precipitation properties of clouds, global climate. The impact of ice clouds on the Earth¿s radiation balance is still undetermined, as the higher albedo of ice clouds should result in a cooling effect, but as they are more likely to precipitate, they may reduce the total cloud albedo and lead to a warming effect. In all these processes, aerosol particles play a crucial role by acting as cloud condensation nuclei (CCN) for liquid droplets and as ice nuclei for the formation of ice particles. Although the formation of warm clouds containing liquid droplets has been studied extensively, much less is known about ice clouds. Several mechanisms of ice particle formation have been proposed, but even the chemistry and structure of ice nuclei is still under investigation. Ice nuclei can have quite different chemical composition. The ice nucleation ability of e.g. mineral dust particles has been studied fairly well, but very little is known about carbonaceous or biological particles and their ability to act as ice nuclei.

In this project, we focus on the ice nucleation abilities of carbonaceous particles and especially on combustion soot and secondary organic aerosol particles. Current laboratory studies of their ice nucleation abilities often give contradictory results indicating that ice nucleation models may be too simplistic to adequately describe the complexity of heterogeneous ice nucleation. An adequate parameterization of particles¿ surface properties as well as the time dependence of the ice nucleation process is still under discussion. The concept of active sites, i.e. areas on particles where ice can form preferentially, is widely used, and we propose to study the structure and ice activation of particles with respect to their active sites in laboratory experiments under well-defined conditions. We will investigate ice nucleation on a large range of well characterized ice nuclei under conditions found in cirrus cloud formation under controlled temperature and cooling rates.