Evidence from repeated Experiments in the Dynamic Cloud Simulation Chamber AIDAd for Immersion Freezing of Mineral Dust by the Surface Site Density Process

Improving quantitative and aerosol-based formulations for microscale processes is still among the major challenges in aerosol, cloud, and climate research. The AIDA (Aerosol Interaction and Dynamics in the Atmosphere) atmospheric simulation chamber at the Karlsruhe Institute of Technology (KIT) is in operation since more than 25 years for a wide range of trace gas, aerosol, cloud and climate research related experiment series. It is also used as an expansion-type cloud simulation chamber to investigate ice nucleation processes at simulated mixed-phase cloud and cirrus cloud conditions.

In 2022, the newly developed dynamic cloud simulation chamber AIDAd (Aerosol Interaction and Dynamics in the Atmosphere – dynamic version) came into operation at the Karlsruhe Institute of Technology. The walls of the cloud chamber located inside a vacuum chamber can actively be cooled at constant rates up to 10 K min-1 (equivalent to an updraft velocity of about 15 m s-1 in the atmosphere) within the temperature range of +30°C to -55°C. This unique technical design enables the simulation and investigation of tropospheric cloud processes in a wide range of cooling rates and liquid condensation levels.

The performance of AIDAd was demonstrated in series of test runs with cooling rates between 1 and 10 K min-1, and with cloud droplet freezing experiments, which showed the homogeneous freezing of pure water droplets to occur, as expected, at temperatures between -35°C and -37°C. Results from these test runs and proof-of-concept experiments will be discussed in the first part of the presentation. The second part will focus on the ice-nucleation activity of dust aerosols in the immersion freezing mode. Repeated experiments with different mineral dust aerosols were performed to investigate the nature of immersion freezing and its formulation or parameterization as a more stochastic or active site related process. Series of experiments have been conducted at different temperatures and cooling rates. The results from these experiments provide clear evidence for the ice-nucleation active site (INAS) concept to be appropriate for quantifying the ice-nucleation activity of mineral dust aerosol in the immersion freezing mode.