Scavenging of black carbon in mixed phase clouds at the high alpine site Jungfraujoch

[摘要] The scavenging of black carbon (BC) in liquid and mixed phase clouds wasinvestigated during intensive experiments in winter 2004, summer 2004 andwinter 2005 at the high alpine research station Jungfraujoch (3580 m a.s.l.,Switzerland). Aerosol residuals were sampled behind two well characterizedinlets; a total inlet which collected cloud particles (droplets and iceparticles) as well as interstitial (unactivated) aerosol particles; aninterstitial inlet which collected only interstitial aerosol particles. BCconcentrations were measured behind each of these inlets along with thesubmicrometer aerosol number size distribution, from which a volumeconcentration was derived. These measurements were complemented by in-situmeasurements of cloud microphysical parameters. BC was found to be scavengedinto the condensed phase to the same extent as the bulk aerosol, whichsuggests that BC was covered with soluble material through aging processes,rendering it more hygroscopic. The scavenged fraction of BC(F_Scav,BC), defined as the fraction of BC that is incorporated intocloud droplets and ice crystals, decreases with increasing cloud ice massfraction (IMF) from F_Scav,BC=60% in liquid phase clouds toF_Scav,BC~5–10% in mixed-phase clouds with IMF>0.2. Thiscan be explained by the evaporation of liquid droplets in the presence ofice crystals (Wegener-Bergeron-Findeisen process), releasing BC containingcloud condensation nuclei back into the interstitial phase. In liquidclouds, the scavenged BC fraction is found to decrease with decreasing cloudliquid water content. The scavenged BC fraction is also found to decreasewith increasing BC mass concentration since there is an increasedcompetition for the available water vapour.

[学科分类] 大气科学