Scavenging of black carbon in mixed phase clouds at the high alpine site JungfraujochReport as inadecuate

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1 Laboratory of Atmospheric Chemistry 2 TROPOS - Leibniz Institute for Tropospheric Research 3 School of Earth 4 IPA - Institute for Atmospheric Physics

Abstract : The scavenging of black carbon BC in liquid and mixed phase clouds was investigated during intensive experiments in winter 2004, summer 2004 and winter 2005 at the high alpine research station Jungfraujoch 3580 m a.s.l., Switzerland. Aerosol residuals were sampled behind two well characterized inlets; a total inlet which collected cloud particles droplets and ice particles as well as interstitial unactivated aerosol particles; an interstitial inlet which collected only interstitial aerosol particles. BC concentrations were measured behind each of these inlets along with the submicrometer aerosol number size distribution, from which a volume concentration was derived. These measurements were complemented by in-situ measurements of cloud microphysical parameters. BC was found to be scavenged into the condensed 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. The scavenged fraction of BC FScav,BC, defined as the fraction of BC that is incorporated into cloud droplets and ice crystals, decreases with increasing cloud ice mass fraction IMF from FScav,BC=60% in liquid phase clouds to FScav,BC~5?10% in mixed-phase clouds with IMF>0.2. This can be explained by the evaporation of liquid droplets in the presence of ice crystals Wegener-Bergeron-Findeisen process, releasing BC containing cloud condensation nuclei back into the interstitial phase. In liquid clouds, the scavenged BC fraction is found to decrease with decreasing cloud liquid water content. The scavenged BC fraction is also found to decrease with increasing BC mass concentration since there is an increased competition for the available water vapour.

Author: J. Cozic - B. Verheggen - S. Mertes - P. Connolly - K. Bower - A. Petzold - Urs Baltensperger - E. Weingartner -



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