Localized morphological defects, such as cracks and edges, can enhance ice nucleation of airborne particles through droplet-freezing and water vapor deposition. Herein, we have conducted a comprehensive investigation examining the role of surface milling upon ice nucleation efficiency for chemically homogeneous and uniformly sized hematite particles as a model proxy for atmospheric dust particles. The ice nucleation efficiencies of pristine cubic and milled hematite particles were measured using the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber in the temperature (T) range between -28 ¢ªC and -82 ¢ªC, which is central to both mixed-phase and cirrus cloud formation. We observed that the droplet-freezing efficiency of milled hematite particles is almost an order of magnitude higher at -35.2 ¢ªC < T < -33.5 ¢ªC than that of the cubic hematite particles, indicating a substantial effect of simple morphological modification on the ice activation. Our measurements under water subsaturated conditions (RHwater < 100% < RHice) elucidated several different ice nucleation pathways and how they could vary depending on T-RHice conditions simulated in the AIDA chamber. Specifically, temperature-independent freezing was observed at -50 ¢ªC > T > -60 ¢ªC, where RHice explicitly controls ice nucleation efficiency. Both temperature and RHice played important roles in other regions (-50 ¢ªC < T < -60 ¢ªC). Observation at T colder than -60 ¢ªC revealed that higher ice supersaturation is necessary to maintain constant nucleation efficiency, suggesting that water vapor deposition on particle surface presumably triggers ice nucleation. The predominant T influence on ice nucleation near the water saturation (-50 ¢ªC < T) may be due to water uptake at particle surface and/or within bulk phase in subsaturated conditions. The results of microphysical characterization of hematite surface, including N2-adsorption specific surface area, particle size distribution and surface charge properties, are presented in a context of surface modification. Applications of the fitted parameterization derived from AIDA measurements to modeling simulations with the single column version of the CAM5 (Community Atmospheric Model version 5) are also presented. Modeling exercise particularly merits examining the effect of complex ice microphysical processes as well as temporal evolution of hydrometeor classes.

Diese Veranstaltung ist Teil der Reihe Aerosolphysikalisches Seminar

Referent/in
Dr. Naruki Hiranuma

KIT
IMK-AAF

Veranstalter
Institut für Meteorologie und Klimaforschung
Karlsruher Institut für Technologie (KIT)
Hermann von Helmholtz Platz 1
76344 Eggenstein-Leopoldshafen
Tel: 0721-608-0
E-Mail: sekretariat ∂does-not-exist.imk-asf kit edu

Zielgruppe
Wissenschaftliche Mitarbeitende