Atmospheric aerosol particles are solid or liquid particles suspended in air. Processes that control formation, transformation and the removal of atmospheric aerosols are of great interest in atmospheric science. The reason is that these particles, which are often smaller than 1 micrometer in diameter, play an important part in Earth's radiation budget through the scattering of sunlight and through the interaction with clouds. Human activities, such as burning of fossil fuels and land use, change the properties of the aerosol and may therefore influence climate. This can be either directly through an increase in aerosols or indirectly through the way the anthropogenic aerosols change the way clouds form. To assess the role of aerosols in our environment and the influence by anthropogenic emissions requires an understanding of the life cycle and transport patterns for aerosol particles as well as a detailed knowledge of how cloud formation mechanisms depend on the properties of the pre-existing aerosols. We study a particular type of clouds: cirrus clouds. These exist in the upper troposphere, at altitudes between about 7 to 17 km. They can appear as wide sheets, wispy filaments, and also as sub-visible cloud layers. Cirrus clouds are composed of non-spherical ice particles with particle sizes and number densities varying considerably with meteorological conditions and origin of the air in which they form. Because cirrus clouds cover up to 30% of the Earth they play an important role in atmospheric chemistry and climate. Observations indicate that the mean global cirrus cloud occurrence frequency over the oceans has increased by several percent per decade. This increase in cirrus may be related to human activities such as air traffic. However, since the exact mechanisms of formation for cirrus clouds are still unknown it is difficult to predict how anthropogenic activities will change cloud abundances in the future. Therefore, we study how different types of aerosol particles influence the formation of cirrus clouds.
The viscosity of a liquid normally increases upon cooling. When the viscosity reaches a value of about 1012 Pa s, molecular motion becomes so slow, that the liquid vitrifies at the glass transition temperature Tg. Such glasses are amorphous substances that behave mechanically like solids, but lack the long-range order of crystals. The ability of a liquid to form a glass depends strongly on its chemical and physical properties and its mixing state. Glasses are ubiquitous in nature and are widely applied in bioengineering, food technology, pharmaceutical industries or cryobiology. Atmospheric aerosol particles may also form glasses. In particular, the organic fraction of aerosol particles tends to remain liquid instead of crystallizing as the temperature is decreased and, thus, organic aerosol particles may form highly viscous liquids and glasses. While the glassy state of a bulk sample is usually obtained by cooling, aerosol particles can transform into glasses also upon drying. We study how temperature, composition, water content, and humidity affect the glass transition in aqueous solutions and organic aerosol particles.
The nucleation of ice from water and aqueous solutions is important for a wide range of topics such as the freezing of cells in biological tissues or the formation of ice clouds in the atmosphere. We study ice nucleation processes experimentally using various methods in the lab, and we have also developed novel thermodynamic models for describing ice nucleation from aqueous solutions. This water-activity-based approach is widely used in atmospheric models for predicting ice nucleation from aerosol particles.
In the biosphere, many organisms have to cope with subzero temperatures, either temporary or permanently. Therefore, fishes, insects and plants have developed strategies to protect their living cell membranes from the destructive growth of ice crystals by producing antifreeze proteins (AFP). These biopolymer molecules adsorb preferentially to specific facets of ice crystals, thereby retarding ice growth perpendicular to this facet. In addition, the recrystallization process of many small ice crystals into few larger ice crystals is impeded through the adsorption of antifreeze polymers. We study the effects of such ice-binding molecules of biological as well as synthetic origin in cryo-microscopy experiments.
Water is a remarkable liquid as it possesses a number of properties that are highly unusual when compared to other liquids. One well known peculiarity is water's density anomaly, that is liquid water expands at temperatures below 277 Kelvin. The "mysterious" properties of liquid water become even more pronounced in the supercooled state below 273.15 K. With few precautions liquid water can be cooled to about 235 K before ice nucleation occurs. This is in contrast to the common conception that below 273.15 K only ice can exist. Adding solutes to water decreases the lowest attainable temperatures even further and some concentrated aqueous solutions can be held in the metastable liquid state for very long timescales. The peculiar properties of water are a result of the hydrogen bonding network formed by the oxygen and hydrogen atoms of the water molecules. Supercooled metastable water and aqueous solutions are more than just an interesting curiosity since they do exists in many places in the real world. In the cold and temperate regions of our planet much of biology takes place when water is in the supercooled regime. Also, many high altitude clouds consist of supercooled water and aqueous solutions. In order to understand the processes occurring in such environments, we study the properties of supercooled water and aqueous solutions both experimentally and theoretically.