Partitioning of Aerosol Particles in Mixed-phase Clouds at a High Alpine Site

Abstract

6 German Aerospace Centre, D-82234, Wessling, Germany * Now at Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland Abstract The partitioning of aerosol particles between the cloud and the interstitial phase (i.e., unactivated aerosol) has been investigated during several Cloud and Aerosol Characterization Experiments (CLACE) conducted in winter and summer 2004 and winter 2005 at the high alpine research station Jungfraujoch (3,580 m altitude, Switzerland). Ambient air was sampled using different inlets in order to determine the scavenged fraction of aerosol particles, F Scav , and of black carbon, F Scav,BC . They denote the fraction of the aerosol volume concentration and of the black carbon (BC) mass, respectively, that has been incorporated into cloud droplets and ice crystals. They are both found to increase with increasing liquid water content, and to decrease with increasing particle number concentration. The scavenged fraction also decreases with increasing cloud ice mass fraction and with decreasing temperature from 0 to −25°C. This can be explained by the WegenerBergeron-Findeisen process, which describes the effect of a water vapour flux from liquid droplets to ice crystals, thus releasing the formerly activated particles back into the interstitial phase. The presence of ice could also have prevented additional particles from activating. BC was found to be scavenged into the cloud phase to the same extent as the bulk aerosol, which suggests that BC was covered with soluble material through aging processes, rendering it more hygroscopic. However, BC was found to be enriched in small ice crystals compared to the bulk aerosol, indicating that BC containing particles preferentially act as ice nuclei. If this finding is representative, it would mean that in addition to an indirect effect on liquid cloud