Physico-chemical Characterisation of Ice Particle Residuals in Tropospheric Mixed-phase Clouds

Ice nucleation in tropospheric, super-cooled clouds is the main initiation mechanism for precipitation in middle latitudes and change the cloud optical properties. In the lower and middle troposphere ice particles are formed by heterogeneous nucleation that is induced by a special but almost unknown subset of atmospheric aerosol particles named ice nuclei (IN).

In order to collect aerosol particles that truly have formed ice particles in tropospheric mixed phase clouds a novel sampling system based on a counterflow virtual impactor (CVI) has been designed. The so-called ICE-CVI pre-segregates supercooled drops and restricts the collection of ice particles to sizes between 5 and 20 µm. The collected ice particles remain airborne and are completely sublimated in a dry and particle free carrier air stream. In this way dry residual particles that can be interpreted as IN are released and analysed.

For atmospheric field measurements the ICE-CVI was operated on the high alpine site Jungfraujoch (Suisse Alps, 3580 m asl) within the framework of the international Cloud and Aerosol Characterisation Experiment (CLACE-3 and CLACE-4) conducted in February/March 2004 and 2005. Downstream of the ICE-CVI inlet, number concentration, size distribution, black carbon and non-refractory matter content of the residuals were measured. Furthermore, an impactor was connected for the off-line single particle analysis using environmental scanning electron microscopy (ESEM). The sampled ice water content (IWC) was determined by two Lyman-alpha hygrometers. The obtained results are related to measurements of the total aerosol particles collected by a corresponding inlet and to in-situ cloud microphysical measurements carried out with a cloud particle imager (CPI).

Beside a verification of the ICE-CVI sampling properties under realistic conditions first case studies for IN characterisation were carried out. A good agreement was found between IWC, residual particle concentration measured with the ICE-CVI on one hand and IWC, ice particle concentration obtained from the CPI on the other hand for a similar ice particle size range. This implies an efficient sampling of ice particles and a complete pre-segregation of liquid drops by the ICE-CVI. The scavenging ratio of particles contained in the sampled ice particles indicates the preference of large particles to serve as IN, although the absolute majority of IN were found below 1 µm. From the different chemical analysis methods it can be concluded that the supermicron IN are mostly mineral dust particles, whereas black carbon and non-volatile organic matter is enriched in submicron residual particles with regard to the abundant total aerosol particles. Thus, these substances are additionally regarded to serve as IN, which implies an anthropogenic influence on ice formation in midlatitude, tropospheric clouds.