Size-dependent ice nucleation by airborne particles during dust events in the Eastern Mediterranean

Abstract. Predictions of cloud ice formation in climate models remain a challenge, partly due to the complexity of ice-related processes. Mineral dust is a prominent aerosol in the troposphere and is known to be an important contributor to ice nucleation in mixed phase clouds, as dust can initiate ice heterogeneously at relatively low supercooling conditions. We characterized the ice nucleation properties of size-segregated mineral dust sampled during dust events in the Eastern Mediterranean. The sampling site allowed us to compare between the properties of airborne dust from several sources with diverse mineralogy that passed over different atmospheric paths. We focused on particles with six size-classes, determined by the Micro-Orifice Uniform Deposit Impactor (MOUDI) cut-off sizes: 5.6, 3.2, 1.8, 1.0, 0.6 and 0.3 µm. Ice nucleation experiments were conducted in the WeIzmann Supercooled Droplets Observation on Microarray (WISDOM) setup, where the particles are immersed in nanoliter droplets using a microfluidics technique. We observed that the activity of airborne particles depended on their size-class, where supermicron and submicron particles had different activities, possibly due to different composition. The concentrations of ice nucleating particles and the density of active sites (ns) increased with the particle size and particle concentration. The supermicron particles in different dust events showed similar activity, which may indicate that freezing was dominated by common mineralogical components. Combining recent data of airborne mineral dust, we show that current predictions, which are based on natural dust or standard mineral dust, overestimate the activity of airborne dust, especially for the submicron class, and therefore we suggest to include information of particle size in order to increase the accuracy of ice formation and, thus, weather and climate predictions.