Impact of the Bergeron-Findeisen process on the release of aerosol particles during the evolution of cloud ice

The paper focuses on the redistribution of aerosol particles (APs) during t he artificial nucleation and subsequent growth of ice crystals in a superco oled cloud. A significant number of the supercooled cloud droplets during i cing periods (seeding agents: C3H8, CO2) did not freeze as was presumed pri or to the experiment but instead evaporated. The net mass flux of water vap our from the evaporating droplets to the nucleating ice crystals (Bergeron- Findeisen mechanism) led to the release of residual particles that simultan eously appeared in the interstitial phase. The strong decrease of the dropl et residuals confirms the nucleation of ice particles on seeding germs with out natural aerosol particles serving as ice nuclei. As the number of resid ual particles during the seedings did not drop to zero, other processes suc h as heterogeneous ice nucleation, spontaneous freezing, entrainment of sup ercooled droplets and diffusion to the created particle-free ice germs must have contributed to the experimental findings. During the icing periods, r esidual mass concentrations in the condensed phase dropped by a factor of 1 .1-6.7, as compared to the unperturbed supercooled cloud. As the Bergeron-F indeisen process also occurs without artificial seeding in the atmosphere, this study demonstrated that the hydrometeors in mixed-phase clouds might b e much cleaner than anticipated for the simple freezing process of supercoo led droplets in tropospheric mid latitude clouds. (C) 2001 Elsevier Science BN. All rights reserved.