Heterogeneous Freezing of Droplets with immersed Mineral Dust Particles – the Roles of Droplet Volume and Nucleation Time

Heterogeneous ice nucleation in mixed phase clouds is one of the main processes controlling the formation of precipitation in the mean latitudes and therefore a crucial process in understanding and predicting the behaviour of such clouds. However our knowledge of the physical and chemical processes underlying heterogeneous ice formation is limited (Kärcher and Lohmann, 2003). Even concerning the physical nature of the heterogeneous ice nucleation process, there is a still ongoing debate whether it should be treated as a stochastical (stochastical hypothesis) or a deterministic (singular hypothesis) process (Marcolli et al., 2007; Vali, 2008).

To contribute to this debate and help solving this important cloud microphysical issue, at the Leibniz Institute for Tropospheric Research's LACIS chamber (Leipzig Aerosol Cloud Interaction Simulator), both experimental and theoretical investigations have been performed concerning the roles of droplet volume and nucleation time in the immersion freezing process.

LACIS is a laminar flow tube of 7m length with a diameter of 15 mm. The temperatures inside LACIS can be varied from 298 down to 223K under operational pressures from 700 hPa to ambient values. Residence times are in the order of seconds up to 1 minute. Inside LACIS super-saturation with respect to water and/or ice are achieved by a combined heat and vapor diffusion process.

For the investigations described here, size segregated, monodispers Arizona Test Dust (ATD) particles were used as ice nuclei (IN). Due to the operation principle of LACIS (Stratmann et al., 2004), each droplet contained only one IN. In the course of the experiments ice fractions (number of frozen droplets divided by the number of all droplets) were determined as function of temperature, droplet size, and nucleation time. For the layout of experiments and interpretation of the data gained, a numerical model FLUENT/FPM (particle dynamics, 2005), was used. FLUENT/FPM features the coupled, 2-dimensional solution of the fluid- (flow, heat/mass transfer) and cloud microphysical processes (particle/cloud droplet hygroscopic growth, activation, dynamic growth, and heterogeneous freezing) taking place inside LACIS.

Summarizing the experimental results, it can be stated that for the investigated immersion freezing of droplets containing a single size selected Arizona Test Dust particle, in the parameter range (temperatures below 243 K, droplet sizes in the order of micrometers, nucleation time in the order of seconds) considered, no volume or time dependence could be found. The latter fact should be not be viewed as prove, that the immersion freezing process is not of statistical nature. Instead it should be taken as a hint that current views and theories might just be missing some important features of the heterogeneous nucleation process.

References: Kärcher, B., and Lohmann, U. (2003). J. Geophys. Res., 108, 4402, doi:10.1029/2002JD003220. Marcolli, C. et al. (2004). Atmos. Chem. Phys., 7, 5081-5091. Particle dynamics (2005), Fine Particle Model (FPM) for FLUENT, particle dynamics GmbH, Leipzig, Germany. (www.particle-dynamics.de) Stratmann, F. et al. (2004). J. Atmos. Oceanic Technol., 21, 876-887. Vali, G. (2008). Atmos. Chem. Phys., 8, 5017-5031.