Current Homogeneous Freezing Formulations Overestimate Ice Formation

Homogeneous freezing of super-cooled aqueous aerosol particles is an important pathway in cirrus formation and therefore affects the earth's radiation balance. The formation rate of cirrus ice crystals by homogeneous freezing of aqueous particles, mainly sulphuric acid or mixed particles containing sulphuric acid and organics, is believed to be well understood and formulated in terms of e.g. the concept of effective freezing temperatures or the water activity dependent ice nucleation rates. Series of recent cirrus cloud simulation experiments at the cloud chamber facility AIDA at the Karlsruhe Institute of Technology (former Forschungszentrum Karlsruhe) together with process modeling studies demonstrated, that these freezing formulations tend to show a low bias in the humidity onset thresholds for homogeneous ice formation at temperatures below about 210 K, and furthermore overestimate the ice formation rate by at least a factor of 2.

During the previous years, we re-investigated the homogeneous freezing rates of aqueous sulphuric acid aerosol particles at temperatures between -40°C and -80°C, using the cloud chamber facility AIDA (Aerosol Interactions and Dynamics in the Atmosphere) which enables laboratory studies of cloud formation processes, in particular ice formation, in a realistic and dynamic cloud simulation environment. It consists of a large evacuable vessel (volume 84 m3) which is placed inside an insulating box with homogeneous temperature control. Pure sulphuric acid aerosol particles were generated by nucleation from the vapour phase and added to the chamber at number densities between 100 and 10000 cm-3. Expansion cooling and humidity increase are induced by controlled pumping to the cloud chamber, with typical cooling rates of -0.1 to -5 K/min. With increasing relative humidity the particles grow by taking up water and thereby continuously dilute. Both the relative humidity and the bulk aerosol composition are directly measures with a tunable diode water detection technique and an FTIR extinction spectrometer, respectively. The freezing onset and the ice formation rates are measured with an in situ laser scattering and depolarisation setup and an optical particle counter.

The experimental results will be summarized and discussed together with atmospheric implications of a new parameterization for homogeneous freezing rates of sulphuric acid particles which was derived from the experimental data set.