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)
Results from field experiments
Sampling principle of the Counterflow Virtual Impactor (CVI)
The principle of the CVI is based on the virtual inertial impaction of accelerated cloud elements. The acceleration results from the air speed of the aircraft or from a wind tunnel in front of the CVI which reaches about 150 m s-1 at the CVI inlet tip. The inlet tubing is equipped with a porous tube through which the so-called supply flow is fed in. The main part of this flow is sucked as so-called sample flow by the instruments. A small flow of excess air streams as so-called counter flow out of the inlet tip against the accelerated cloud air. That way a stagnation plane is created in the tube. Hydrometeors with sufficient inertia to reach this plane are sampled via the sample flow, while interstitial particles are stopped before and removed from the inlet by the counterflow. The lower cut-off diameter of the ground-based CVI systems has been reduced to 5 µm through instrumental developments at the Leibniz-Institute for Tropospheric Research.
Standard instrumentation of the CVI systems
Real-time sensors or sampling devices for gas- or aerosol measurements of IfT or other collaborating institutions can be coupled to the CVI depending on the specified scientific objective. The table below summarizes the standard measurements conducted by the Leibniz-Institute for Tropospheric Research.
| instrument, sampling | measured parameter | remarks |
| Condensation Particle Counter (CPC) | particle concentration | Dp > 15 nm |
| Differential Mobility Particle Sizer (DMPS) | particle size distribution | 25 nm < Dp < 850 nm |
| Particle Soot Absorption Photometer (PSAP) | volume absorption coefficient black carbon |
λ = 565 nm |
| Lyman-alpha hygrometer | liquid water content | λ = 121 nm |
| dew point mirror | dewpoint in the CVI system | - 40 to 60 °C |
| filter | graphitic carbon | Raman-Spectroscopy |
Ground-based CVI for liquid drop collection
Within the first phase of the CVI development, a system was implemented to sample liquid drops in warm clouds. The CVI inlet tip is installed in a wind tunnel which is visible from outside. The total liquid water of the collected drops is transferred into the gas phase inside the vertical evaporation tube, so that the former CCN become available for analysis. The wind tunnel can be oriented horizontally or vertically depending on the outer conditions. This ground-based CVI system was deployed in field campaigns on top of the Puy-de-Dôme (France, EU-project CIME), Mt. Brocken (Germany, BMBF-program AFS), Mt. Schmücke (Germany, FEBUKO, project within the BMBF-program AFO2000) and Pico del Este (Puerto-Rico, PRACS-RICO project).
Figure 2: Horizontal (left) and vertical (right) setup of the ground-based CVI system on top of the Pico del Este on Puerto Rico
CVI-system for the characterization of ice nuclei in tropospheric mixed-phase clouds (ICE-CVI)
Within a project funded by the DFG (German science foundation) a CVI system has been developed to sample small, freshly formed ice particles in a mixed-phase cloud which was operated in March 2004 on the Jungfraujoch during the international field campaigns CLACE. Beside the interstitial particles, which are excluded by the CVI, a contamination due to large ice crystals and super-cooled droplets needs to be prevented. For this purpose a virtual impactor (VI) and a droplet pre-impactor (PI) are used. The VI segregates large cloud elements and in the PI super-cooled droplets freeze on the impaction plates, while the small ice particles bounce and remain in the sample flow.
Figure 3: Schematic und actual setup (on Jungfraujoch, Switzerland) of the ICE-CVI system collecting freshly formed ice particles for the analysis of incorporated ice nuclei.
A Scanning Mobility Particle Sizer (SMPS) from the Paul Scherrer Institute in Villigen, an Aerosol Mass Spectrometer (AMS) from the Max Planck Institute Mainz and an impactor for the Environmental Scanning Electron Microscope of the TU Darmstadt have been employed to analyze the residues, i.e. the IN. Further field campaigns for the characterization of IN in the framework of the DFG-collaborative research center TROPEIS are planned where the ICE-CVI will be coupled to a single particle mass spectrometer.
Airborne CVI equipped with a passive wind tunnel
Cloud measurements can be conducted with improved spatial resolution using a low flight velocity. But velocities below 100 m s-1 are not sufficient to realize the CVI sampling principle. Thus, a passive wind tunnel is constructed, which doubles the air speed at the CVI inlet tip. This system was deployed in March 2003 for the first time onboard a Partenavia aircraft in cooperation with the company enviscope in cumulus clouds.
Figure 4: Airborne CVI aboard the Partenavia. Display window: CVI inlet tip inside the passive wind tunnel
Contact CVI technique and CVI projects: Dr. Stephan Mertes
