Part of the atmospheric aerosol particles influences the radiation budget of the earth due to their cloud serving as cloud drop nuclei (CCN, indirect aerosol forcing). Anthropogenic CCN might change the microphysical cloud properties (concentration and size of drops) and that way exert influence on the climatic impact of clouds.
Investigating indirect aerosol forcing becomes even more complex because of the ice formation in super-cooled clouds. At temperatures above -38°C this is caused by atmospheric particles (heterogeneous ice nucleation), the so-called ice nuclei (IN). Beside different radiative properties of ice particles, ice formation is of great importance for the initiation of precipitation processes at mid-latitudes.
One way to improve the knowledge about drop and ice particle formation and to discover an anthropogenic influence is the microphysical and chemical characterization of aerosol particles, which already formed liquid and iced hydrometeors in the atmosphere. For this purpose, drops and ice particles are sampled inside clouds by means of a Counterflow Virtual Impactor (CVI). The non-activated interstitial particles are segregated during the collection process, while the hydrometeors are evaporated in a dry and particle free carrier air. Thus, non-volatile CCN and IN remain airborne as dry residual particles.
At the Leibniz Institute for Tropospheric Research a ground-based and an airborne CVI system exists for liquid clouds. A third, ground-based system is especially designed for ice particle collection in mixed-phase clouds (ICE-CVI). .
