Institut für Meteorologie und Klimaforschung Atmospherische Aerosol Forschung (IMK-AAF)

IMK-AAF studies fundamental processes that govern the formation, transformations and sinks of atmospheric aerosols. These processes are of great relevance for climate, cloud and precipitation formation and for human health (IPCC AR5, Chapter 7). Its research is embedded in the Research Program “Atmosphere and Climate” of the Helmholtz-Association, where it contributes to Topic 1, Topic 3 and Topic 4. The institute operates the large Aerosol- and Cloud Simulation Chamber AIDA (Aerosol Interaction and Dynamics in the Atmosphere). Access to this facility is supported via the EU program EUROCHAMP.

Within KIT, IMK-AAF is contributing to the Center for Climate and Environment which addresses societal challenges resulting from changes of the climate and the environment in the context of demographic, economic and technical developments.

Research Topics at the IMK-AAF

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Aerosol-Cloud Interactions

In clouds of the lower and middle troposphere, the formation of ice often triggers a cascade of secondary processes which are of importance for the formation, distribution and intensity of precipitation events. In the upper troposphere, the different pathways of ice formation influence the radiative properties of cirrus clouds and by that their important role in the climate system. We are using the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) facilities to investigate the role of various atmospheric aerosol types in cloud ice formation under simulated cloud formation conditions. The experiments cover the full range of tropospheric temperature and humidity, and provide a unique data set to develop and improve formulations like the ice nucleation active site (INAS) concept, which can be used to quantify primary ice formation in cloud, weather and climate models.

Ice Nucleating Particles

Ice Nucleating Particles (INPs) are a very minor fraction of atmospheric aerosol particles, which are needed to form ice crystals at high temperatures or low ice supersaturation, and by that have important impact on the formation of precipitation and the net radiative effect of cirrus clouds in the Earth’s climate system. The atmospheric abundance and distribution of INPs strongly depends on the temperature as well as the concentration and type of aerosol particles. We are using the aerosol filter based method INSEKT (Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology) and the newly developed PINE (Portable Ice Nucleation Experiment) instrument for INP measurements at about 10 different locations in Europe and in China. We are in particular interested in long-term observations at field sited influences by aerosol from different natural and anthropogenic sources.

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Aerosol-Cloud Interactions

In clouds of the lower and middle troposphere, the formation of ice often triggers a cascade of secondary processes which are of importance for the formation, distribution and intensity of precipitation events. In the upper troposphere, the different pathways of ice formation influence the radiative properties of cirrus clouds and by that their important role in the climate system. We are using the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) facilities to investigate the role of various atmospheric aerosol types in cloud ice formation under simulated cloud formation conditions. The experiments cover the full range of tropospheric temperature and humidity, and provide a unique data set to develop and improve formulations like the ice nucleation active site (INAS) concept, which can be used to quantify primary ice formation in cloud, weather and climate models.

Secondary Ice

Reliable modeling of cloud processes for weather predictions and climate change projections requires a sound understanding of the ice formation in mixed-phase clouds. However, ice crystal concentrations measured in the clouds often exceed the concentration of ice nucleating aerosol particles by many orders of magnitude. To elucidate this discrepancy, we simulate the atmospheric processes of ice multiplication upon freezing of water droplets levitated in an electrodynamic trap (EDB) coupled with sophisticated optical and infrared video recording system. In the past few years, we have achieved significant progress in understanding the physics of freezing of freely suspended cloud droplets and currently aim at full quantification of the secondary ice production mechanism.

Ice Nucleation and Growth via Deposition from Vapor Phase

The ice crystals in atmospheric clouds governs their life time, optical properties, and are part of the precipitation pathway. In the troposphere, ice crystals form mostly via heterogeneous nucleation, assisted by the presence of aerosol. However, only few aerosol particles would serve as efficient ice nucleators. To understand the role of aerosol particles and to identify the ice nucleating active sites, we conduct laboratory experiments with an Environmental Scanning Electron Microscope (ESEM), allowing us to study morphology and chemical composition of individual aerosol particles and conduct the ice nucleation experiments directly in the specimen chamber of the ESEM.

Biological Ice Nucleators in the Atmosphere

Living organisms have developed ways of controlling ice nucleation and growth to prevent cell damage caused by freezing of intracellular water. As a result, many microorganisms, such as bacteria, fungi, or plant pollen, have become able to trigger freezing of water at just a few degrees of supercooling, as compared to -40°C degrees required to freeze microscopic droplet of supercooled water homogeneously. Some bacterial and animal proteins are also responsible for inhibiting the ice crystal growth by attaching themselves to the crystalline faces of growing ice. In cooperation with microbiologists from KIT and Aarhus university (Denmark), we study how certain bacteria affect freezing of atmospheric water and how this affects their chance of survival and dissemination pathways.

Atmospheric Surfaces

Atmospheric reactions e.g. heterogeneous ice nucleation, gas deposition, particle oxidation and photosensitization or secondary aerosol and biogenic particle formation dependent on the physical and chemical interactions occurring at interfaces. Understanding the factors that influence atmospheric interactions, particularly on the molecular level, is a major unsolved and pressing problem in our understanding of climate. Our aim is to investigate fundamental processes in the atmosphere on the molecular level using nonlinear optical spectroscopy, mainly second-harmonic generation (SHG) and sum-frequency generation (SFG). SHG and SFG are second order nonlinear optical effects that are only active at surfaces and interfaces where the inversion symmetry is broken.

Atmospheric Nanoscience

The smallest long lived nanoparticles in the atmosphere (radius<2 nm) condense from evaporated meteoric material in the mesopause region (h~85 km). There they give rise to an abnormally high radar reflectivity and during summertime at high latitudes act as nuclei for the spectacular noctilucent clouds. We investigate the chemical and physical processes leading to the formation and growth of these ice clouds in the laboratory. To do so, we have developed a unique simulation chamber to expose nanoparticles to the extreme conditions of the mesopause.

In-Situ Aerosol and Cloud Particle Optics

We investigate the interaction of ultraviolet and visible light with atmospheric aerosol and cloud particles in order to (i) measure their shortwave spectral optical properties, (ii) deduce the bioaerosol content, and (iii) study the structural details of ice particles. We develop optical instrumentation that is deployed in aircraft missions, mountain top studies, and cloud chamber experiments.