Development of a New Portable Immersion Mode Cooling Chamber (PIMCA) for Ambient In-situ measurements on Immersion Ice Nuclei

Clouds play an important role in the Earth's energy budget. In order to estimate their influence it is crucial to accurately understand cloud formation processes in the atmosphere. The type of ice nucleation mechanism significantly affects the cloud microphysical properties and is a key process for precipitation initiation. In mixed-phase clouds immersion freezing is the dominant mechanism forming ice crystals, whereby ice nuclei (IN) first act as cloud condensation nuclei (CCN) and are activated to cloud droplets. With decreasing temperature the immersed IN can initiate freezing.

There are a number of experimental methods and techniques to investigate the IN ability in the immersion mode, however, most of the IN counters are only suitable for laboratory measurements. A portable instrument to investigate immersion mode IN has yet to be developed and deployed. In-situ atmospheric studies are needed to understand the ice formation processes correctly. Laboratory conditions simulate conditions of atmospheric processes like ageing or coating but are still ideal. We present in this work the new Portable Immersion Mode Cooling chAmber (PIMCA) suitable for field deployment for the investigation of ambient IN. It will provide the opportunity for IN measurements in field sites to study the IN ability of natural atmospheric aerosol. The instrumental setup is based on the laboratory version of the immersion mode chamber (IMCA) that has been used in combination with the Zurich Ice Nucleation Chamber (ZINC). The new portable setup consists of three devices, namely 1) the newly developed immersion mode instrument as a vertical extension to 2) the Portable Ice Nucleation Chamber (PINC) and 3) the Ice Optical Depolarization DEtector (IODE) that can distinguish between liquid droplets and ice crystals. In comparison to the laboratory setup IMCA-ZINC, the new chamber is based on an equivalent functional principle, although with adaptions for flow conditions, chamber dimension and cooling system. The resulting differences concerning residence time for ice nucleation conditions and droplet evolution during the experiment are presented. In addition, empirical validations of the temperature and relative humidity in PIMCA are presented. The new setup is simulated with FLUENT to obtain particle trajectories. Initial calculations including diffusional growth and evaporation processes in the chamber using FLUENT particle trajectories for sample temperatures of T_PIMCA=313K and T_PINC=239K are performed. The result show a residence time at ice nucleation conditions of about 10-12s and the water droplets remain larger than 3-5µm in radius and will be detectable by IODE.

After a successful implementation and validation of the new chamber following in-situ measurements will gain us further insights into the immersion mode IN properties of ambient aerosol particles. This will include knowledge about the IN activity of biological aerosol (e.g. bacteria, fungi and pollen). The goal will be to develop parameterizations that can be used in climate models to better understand the importance of freezing processes in mixed-phase clouds and help to estimate their influence on climate.