Counterflow Virtual Impactor (CVI)

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.

  • Setup of the ground-based CVI for HCCT-2010 at Schmücke (Thuringian Forest), source: Stephan Mertes/TROPOS

  • Setup of the ground-based CVI for PRADACS at East Peak (Puerto Rico), source: Stephan Mertes/TROPOS

  • Setup of the ground-based CVI for ACRIDICON-Zugspitze at the Schneefernerhaus (Bavarian Alps), source: Stephan Mertes/TROPOS

  • Setup of the ground-based Ice-CVI for INUIT at the Jungfraujoch (Swiss Alps), source: Stephan Mertes/TROPOS

  • Airborne CVI for warm clouds and small aircrafts, source: enviscope GmbH

  • Airborne CVI mounted on HALO (HALO-CVI), source: Andreas Minikin/DLR

In order to advance the scientific significance in cloud field experiments, a complementary interstitial inlet is operated besides the CVI for the parallel collection and characterization of non-activated particles and the gas phase. The CVI versions available at TROPOS are:

  • Ground-based CVI for warm clouds: operated at Hill Cap Cloud Thuringia (HCCT-2010), Puerto Rico African Dust and Cloud Study (PRADACS) and ACRIDICON-Zugspitze ] (ACRIDICON-Zugspitze) (Mertes et al., 2005a, Mertes et al., 2005b)
  • Ground-based CVI for mixed-phase clouds (Ice-CVI, Mertes et al. (2007)): operated at CLACE2013 (CLACE2013) and INUIT (INUIT)
  • Laboratory-CVI downstream an ice nucleus counter (pumped IN-CVI) : operated at CLACE2013 and INUIT
  • Airborne CVI for warm clouds (in collaboration with enviscope GmbH, enviscope GmbH)
  • Airborne CVI for the research aircraft HALO (HALO-CVI): future missions ML-CIRRUS (Mid-Latitude Cirrus )and ACRIDICON (Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems)


  • Mertes, S., K. Lehmann, A. Nowak, A. Massling and A. Wiedensohler (2005a). "Link between aerosol hygroscopic growth and droplet activation observed for hill-capped clouds at connected flow conditions during FEBUKO." Atmospheric Environment 39(23-24): 4247-4256.
  • Mertes, S., D. Galgon, K. Schwirn, A. Nowak, K. Lehmann, A. Massling, A. Wiedensohler and W. Wieprecht (2005b). "Evolution of particle concentration and size distribution observed upwind, inside and downwind hill cap clouds at connected flow conditions during FEBUKO." Atmospheric Environment 39(23-24): 4233-4245.
  • Mertes, S., B. Verheggen, S. Walter, P. Conolly, M. Ebert, J. Schneider, K. N. Bower, J. Cozic, S. Weinbruch, U. Baltensperger and E. Weingartner (2007). "Counterflow virtual impactor based collection of small ice particles in mixed-phase clouds for the physico-chemical characterization of tropospheric ice nuclei: Sampler description and first case study." Aerosol Science and Technology 41(9): 848- 864, doi:10.1080/02786820701501881.