Black carbon enrichment in atmospheric ice particle residuals observed in lower tropospheric mixed phase clouds

[1] The enrichment of black carbon (BC) mass in residuals of small ice crystals was investigated during intensive experiments in winter 2004 and 2005 at the high alpine research station Jungfraujoch (3580 m asl, Switzerland). Two inlets were used to sample the bulk aerosol (residuals of cloud droplets and ice crystals and nonactivated aerosol particles) and the residual particles of small ice crystals (diameter 5-20 mu m). An enrichment of the BC mass fraction in the ice particle residuals was observed by investigating the measured BC mass concentration as a fraction of the bulk (submicrometer) aerosol mass concentration sampled by the two inlets. On the average, the BC mass fraction was 5% for the bulk aerosol but 27% for the ice particle residuals. The observed enrichment of BC in ice particle residuals suggests that some BC-containing particles may preferentially act as ice nuclei, with important implications for the indirect aerosol effect via glaciation of clouds if these particles represent a significant fraction of the number of ice crystals nucleated.