Dependence of ice nucleation activity on mineralogy and particle size of surface dust from Morocco and Iceland in immersion freezing mode.

Mineral dust is one of the most abundant types of aerosol particles in the atmosphere, playing a crucial role in various atmospheric processes. A key interaction within clouds is its ability to produce ice-nucleating particles (INPs), influencing cloud properties such as phase, lifetime, and water content. The efficiency of mineral dust as INPs depends on factors such as mineralogy, composition, and particle size. This study investigates the role of particle size and mineralogy in INP efficiency, contrasting Morocco and Iceland, i.e. a mid- and a high-latitude dust sources by using developed parameterizations in ICON-ART.

For this purpose, we used experimental results obtained with the AIDA chamber and the INSEKT freezing assay. The experiments tested ice production in immersion freezing mode for samples from Morocco and Iceland with different size distributions including large particles (greater than 10 µm in diameter). Our analysis revealed notable differences in INP efficiency for the two source locations. The ice nucleation active surface site (INAS) density indicated no significant size dependence for Moroccan samples. In contrast, Icelandic samples exhibited a subtle size dependence, with larger particles showing slightly reduced activity. This behavior was linked to the dust mineralogical composition, specifically the presence of pyroxene. For Icelandic samples, the pyroxene relative volume fraction decreases with increasing particle size, which correlates with the observed reduction in INP activity.

Based on these insights, we developed a new INAS density parameterization for Icelandic dust and proposed a modification to the equation used to compute INAS density to represent the variation in the efficiency of ice nucleation activity at different diameters. We then used this new parameterization in the ICON-ART model to test the impact of a different ice nucleation efficiency of Icelandic dust on a regional scale. By using ICON-ART global dust distributions were simulated and surface dust was used as input for the new parameterizations at different temperatures, recreating typical field experiments on ice nucleation.

This study underscores the importance of characterizing both mineralogical composition and its size dependence when developing parameterizations for INP activity. Our results challenge the assumption that larger particles are always more efficient INPs in immersion freezing mode, as their efficiency is linked not only to their large surface areas but also to their mineralogical composition, which can vary for different sizes. Additionally, size distribution shapes should be considered as another factor influencing INP concentration as the abundance of particles at different diameters might determine the efficiency of the sample.