Collection of small ice particles by means of a novel counterflow virtual impactor system for the determination of microphysical and chemical properties of tropospheric ice nuclei

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Ice formation in lower and middle tropospheric clouds is of great importance for the water cycle and global climate. The appearance of ice particles initiates precipitation processes and might change cloud radiative properties. In the troposphere ice particles nucleate at temperatures below -5°C due to heterogeneous ice formation mechanisms induced by a subset of atmospheric aerosol particles, named ice nuclei (IN). Up to now the knowledge about atmospheric particles acting as IN and about the atmospheric relevance of different heterogeneous ice nucleation mechanisms is marginal especially for tropospheric clouds. Here the ice phase emerges between preexisting super-cooled droplets complicating the empirical investigation due to icing problems and due to the experimental necessity to distinguish between liquid and ice phase. A dedicated sampling system based on the counterflow virtual impactor (CVI) technique has been proven in a first feasibility evaluation to exclusively collect freshly formed ice particles within a mixed-phase cloud and rejecting interstitial particles, liquid droplets and large ice aggregates that are supposed to contain scavenged aerosol particles as a result of coagulation and riming processes. Upon separation and sampling, the small ice particles are evaporated in the so-called ICE-CVI releasing dry residual particles. These residuals are considered to be the centers of ice nucleation, because in the cloud the freshly formed ice particles solely grow by water vapor diffusion and do not scavenge particles during this period of cloud processing. Thus, a physico-chemical characterization of the ice particle residuals will offer the identification of ice nuclei that have truly formed ice in tropospheric clouds. Particularly, the role of anthropogenic and desert dust particles for ice formation in tropospheric clouds can be elucidated with this approach. Moreover it will be possible to obtain information about the associated heterogeneous nucleation processes. Cloud condensation nuclei (CCN), whose chemical composition (highly soluble matter) is different from ice nuclei (insoluble matter) should be present as residual particles if ice formation appears via droplet freezing. Another process, evaporative freezing, that came into focus just recently, would yield only CCN components to be measured by the ICE-CVI, because the evaporative cooling of super-cooled droplets is assumed to cause freezing without participation of ice nuclei. Obviously, a specific analysis of the ice particle residuals is absolutely necessary for this methodology. Particularly, single particle analysis, like mass spectrometry and electron microscopy is required, which both can be realized by a participation of the novel ICE-CVI in joint ice phase projects.