CVI working principle
TROPOS developed and built up several counterflow virtual impactors for the in-situ sampling of cloud drops and ice particles and simultaneous pre-segregation of non-activated particles. These different CVI systems mad substantial contributions in research projects concerning drop activation (Drop Activation) and ice nuclei (Ice Nucleation) in real clouds. The CVI working principle is based on the virtual impaction of cloud particles on an artificial generated counterflow, which is blown out of the inlet tip against the sampling direction. This prevents that the complete gas phase and smaller, interstitial particles reach the CVI, while the larger cloud particles (drops, ice particles) overcome the counterflow due to their higher inertia and are collected. The cloud particles are then imbedded in a dry and particle-free carrier air, which leads to the complete evaporation of the water or ice phase, respectively. In this way, dry drop or ice particles residues are released, which can be interpreted as cloud condensation or ice nuclei, for their microphysical and chemical analysis.