Kristina Höhler

A New Ice Nucleation Active Site Parameterization for Desert Dust and Soot

AbstractBased on results of 11 yr of heterogeneous ice nucleation experiments at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber in Karlsruhe, Germany, a new empirical parameterization framework for heterogeneous ice nucleation was developed. The framework currently includes desert dust and soot aerosol and quantifies the ice nucleation efficiency in terms of the ice nucleation active surface site (INAS) approach.The immersion freezing INAS densities nS of all desert dust experiments follow an exponential fit as a function of temperature, well in agreement with an earlier analysis of AIDA experiments. The deposition nucleation nS isolines for desert dust follow u-shaped curves in the ice saturation ratio–temperature (Si–T) diagram at temperatures below about 240 K. The negative slope of these isolines toward lower temperatures may be explained by classical nucleation theory (CNT), whereas the behavior toward higher temperatures may be caused by a pore condensation and freezing mechan...

Immersion freezing of clay minerals and bacterial ice nuclei

The immersion mode ice nucleation efficiency of clay minerals and biological aerosols has been investigated using the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber. Both monodisperse and polydisperse populations of (1) various clay dust samples as well as (2) Snomax® (a proxy for bacterial ice nucleators) and (3) hematite are examined in the temperature range between −4°C and −35°C. The temperature dependence of ice formation inferred by the INAS (Ice Nucleation Active Surface-Site) density is investigated and discussed as a function of cooling rate and by comparing to predicted nucleation rates (i.e., classical nucleation theory with θ-probability density function nucleation scheme). To date, we observe that maintaining constant AIDA temperature does not trigger any new ice formation during the immersion freezing experiments with clay dust samples and Snomax®, implying strong temperature dependency (and weak time dependency) within our time scales and conditions of experiments. ...

Crystallization and immersion freezing ability of oxalic and succinic acid in multicomponent aqueous organic aerosol particles: Immersion freezing by oxalic acid

This study reports on heterogeneous ice nucleation efficiency of immersed oxalic and succinic acid crystals in the temperature range from 245 to 215 K, as investigated with expansion cooling experiments using suspended particles. In contrast to previous laboratory work with emulsified solution droplets where the precipitation of solid inclusions required a preceding freezing/evaporation cycle, we show that immersed solids readily form by homogeneous crystallization within aqueous solution droplets of multicomponent organic mixtures, which have noneutonic compositions with an excess of oxalic or succinic acid. Whereas succinic acid crystals did not act as heterogeneous ice nuclei, immersion freezing by oxalic acid dihydrate crystals led to a reduction of the ice saturation ratio at freezing onset by 0.066–0.072 compared to homogeneous freezing, which is by a factor of 2 higher than previously reported laboratory data. These observations emphasize the importance of oxalic acid in heterogeneous ice nucleation.

Parameterizations of ice formation derived from AIDA cloud simulation experiments

Since 2003, the AIDA cloud chamber has been used for comprehensive series of ice nucleation experiments with a variety of different aerosols and in wide ranges of temperature, relative humidity and cooling rate. Ice nucleation onset and ice formation rates have been obtained as a function of aerosol parameters, ice supersaturation, temperature and cooling rate for homogeneous freezing of water droplets and solution particles, immersion freezing at and below water saturation, and deposition ice nucleation between ice and water saturation. The AIDA team has started a consistent and comprehensive re-analysis of the 10 year data set to provide a new set of parameters for formulating the ice formation in atmospheric models as function of aerosol properties, temperature and humidity. Here we present basic concepts and some selected results.