Extending measurements of ice nucleation activity to large-size mineral dust particles

Mineral dust is considered one of the most important seeds for heterogeneous ice nucleation in clouds. In the past decades, several studies have worked on establishing a relationship between mineral dust, number concentration, nucleation temperature, supersaturation, and the number of ice crystals. The explored dust particle-size range was usually limited to a few micrometers for two main reasons: (1) larger and heavier particles are difficult to keep suspended in an experimental setting; and (2) the fraction of coarser aerosol was considered negligible. However, recent studies have shown that dust particles as large as 100 μm or even more can be transported over long distances, leaving a knowledge gap concerning their role as ice-nucleating particles.

In this work, we aim to contribute to closing this gap by investigating the ice nucleation activity for large-size mineral dust particles, extending the studied size range to particles of up to several tens of microns. For this purpose, we used natural dust samples with different mineralogical composition, collected consistently during field campaigns in Morocco and in Iceland, and segregated into five different size classes. In the framework of the MICOS (Dust-induced ice nucleation: effects of Mineralogical COmposition and Size) campaign, we conducted experiments with the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber and with the Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology (INSEKT), in which the size-segregated samples were tested at different temperatures in the range between -16 and -27 °C. The ice nucleation efficiency was quantified in terms of the ice nucleation active surface site (INAS) density approach for the immersion freezing mode. Preliminary results from the AIDA and INSEKT experiments are presented, in which we extended the size range at which cloud chamber experiments are typically conducted.