Ice nucleation activity of large particles and its dependence on mineralogical composition and size in the immersion freezing mode

Mineral dust is one of the most abundant aerosol types in the atmosphere and an important climate constituent. Once lifted into the atmosphere, mineral dust particles can travel thousands of kilometers and persist for several days, with particle size distributions (PSD) extending up to hundreds of micrometers in diameter. During transport, mineral dust interacts with atmospheric components, among which clouds are particularly relevant due to their effects on the radiative balance, hydrological cycle, and energy budget. Of special importance are mixed-phase clouds (MPCs), in which liquid and solid phases coexist and which exert strong climatic influence through their global cover and long lifetime.
Mineral dust promotes the formation of ice particles in MPCs through ice nucleation, thereby altering cloud properties. Among the different mechanisms, immersion freezing is most relevant for MPCs. The efficiency of mineral dust as ice-nucleating particle in immersion freezing mode depends on particle properties such as surface characteristics, mineralogy, and size. While extensive research has characterized the ice nucleation activity (INA) of dust from low- and mid-latitude sources, high-latitude dust sources remain poorly studied. ... mehrFurthermore, the role of particles larger than 10 µm and the influence of mineralogy on INA are still insufficiently understood.
This thesis addresses these gaps by investigating the INA of mineral dust in immersion freezing mode from both low- and high-latitude sources. Moroccan (low-latitude) and Icelandic (high-latitude) dust samples were systematically collected during field campaigns, characterized for their mineralogy, and size-separated in PSDs that span different size ranges and extend over three orders of magnitude (10−1 to 102 µm). The INA was determined using the ice nucleation active site (INAS, ns ) density framework, in which nucleation occurs at specific surface sites and depends only on temperature. As part of the MICOS campaign and with the participation of multiple researchers, including the author, more than 300 ice-nucleation experiments were conducted in the AIDA cloud chamber, the AIDA mini-chamber (AIDAm), and the Ice Nucleation Spectrometer (INSEKT) at the Karlsruhe Institute of Technology (KIT). Parameterizations developed from these measurements were implemented in the ICON-ART atmospheric model and tested for the summer of 2021 for the northern hemisphere, using a global resolution of 20 km with a nested domain of 10 km over Iceland. A newly implemented source-tagging technique enabled the separation of Icelandic dust from global emissions.
Based on the laboratory data obtained in this study, the INAS density parameterizations developed for both sources showed an exponential dependence on temperature. Moroccan dust showed INAS density values comparable to feldspar. No size dependence was observed for Moroccan samples. In contrast, Icelandic samples displayed a size dependence. When comparing the developed Icelandic parameterization to previous studies, the largest discrepancies occurred at warmer temperatures, particularly in INSEKT measurements. Overall, Icelandic dust was found to be less ice active than Moroccan dust. To better capture mineralogical and size effects, a new metric was introduced—the “effective surface area”—which modifies ns based on the contribution of mineralogical fractions.
The results indicate that the commonly used assumption of size-independent ns (T ) is not universally valid, particularly for polydisperse samples including particles larger than 10 µm. As is the case for smaller particles, larger particles can undergo long-range transport and substantially affect cloud micro-physics. Given their size, they likely exhibit different mineralogical compositions than smaller particles. This highlights the need to explore the dependence of INA with mineralogical composition and size effects in future parameterizations. The ICON-ART simulations with the newly implemented source-tagging technique and parameterizations showed that Icelandic dust was located mainly within the planetary boundary layer and was deposited predominantly over Iceland and the adjacent ocean. Test cases considering the default parameterization in ICON and the newly developed parameterizations indicated that Icelandic dust made only a weak contribution (lower than 5%) to the total INP in immersion freezing mode. Despite small differences in MPC properties among the test cases, significant differences in the cloud radiative effects were found, with the new parameterizations improving the agreement with satellite observations. In conclusion, this work expands our understanding of mineral dust as an ice-nucleating particle by systematically contrasting high- and low-latitude sources and extending particle size distributions beyond the commonly studied micrometer range. The findings emphasize the importance of considering source variability, mineralogy, and particle size when characterizing INA and when representing dust–cloud interactions in climate models. This approach, combining field campaigns, laboratory experiments, and atmospheric modeling, provides new insights into the role of dust in immersion freezing mode and its broader climatic implications.