[Zurück]

H. Grothe:
"Physicochemical Laboratory Models of Metastable Atmospheric Aerosols";
Technische Universität Wien, Fakultät für Technische Chemie, 2006.

The atmosphere exhibits strong fluctuations in temperature and chemical composition.
Therefore, many aerosol particles are not in thermodynamic equilibrium with their environment.
These non-equilibrium conditions influence the particle´s chemical composition and phase as
well as the morphology, the surface structure and finally the surface chemistry. These
physicochemical properties cannot be determined in field measurements but are accessible by
laboratory model experiments. In the framework of this habilitation different models for aerosol
investigations are presented and their assets and drawbacks are discussed.
Aerosol constituents of different origin have been investigated by physicochemical laboratory
models. However, this thesis focuses particularly on the aerosols of the winter polar
stratosphere. At this region sulphuric acid, nitric acid and water nucleate into supercooled
solutions or crystalline acid hydrates, some of which are metastable phases, namely the low
temperature modification of nitric acid trihydrate, both modifications of nitric acid dihydrate and
the recently discovered nitric acid pentahydrate. It was the aim of this work to crystallize these
metastable structures as essentially pure samples and determine their spectroscopic properties.
This was achieved by a special preparation technique called quenching, where an aerosol of
well-known molar ratio hits a pre-cooled sample support providing a high cooling rate and
consequently the formation of an amorphous sample film. The metastable phases were
crystallized from this disordered sample by careful annealing. For sample characterization it was
indispensable to determine the exact phase composition which was carried out by X-ray
diffraction and interpreted on the basis of the classical nucleation theory. After knowing the
phase ratio, the respective infrared spectra and Raman spectra were recorded. The first midinfrared
spectra of nitric acid dihydrate were collected, but also the low-frequency region was
found to exhibit valuable spectral information concerning the connection of the respective
constituents by hydrogen bonds. The nature of hydrogen bonds was monitored also by matrix
isolation infrared spectroscopy, which is a very abstract model concentrating only on the
interaction of two or three molecular species situated in the cages of a noble gas solid.
For the interpretation of extinction spectra of real solid aerosols, in an aerosol chamber, flow
tube or in the field, the morphology of the crystals and crystalline particles is of crucial
importance. Therefore, the quenching technique was used to prepare samples of nitric acid
trihydrate in an environmental scanning electron microscope. Different morphologies of nitric
acid trihydrate were found depending on the presence of ice in the sample. The ice has also
strong impact on the crystallite sizes, phase transition temperatures and crystallization kinetics
which were measured by X-ray diffraction and evaluated by the Avrami model.
Infrared optical indices were determined for supercooled binary and ternary solutions of the
constituents H2SO4/HNO3/H2O. It was found that far infrared data is essential for a successful
Kramers-Kronig Transformation supplying the indices in question. Differences between series of
optical indices gathered by different laboratories were explained by non-considering the
respective density and dissociation equilibria of the sample, which show strong temperature
dependence.
All results have been compared with measurements from other laboratories using similar
models, but also with models on different scale such as the aerosol chamber. It was found that
particularly for metastable aerosols the application of laboratory models on a bench scale is a
crucial point, since the intrinsic parameters of the aerosol are only accessible when applying
advanced physicochemical measurement techniques. These do, however, necessarily imply
sample geometries which are not available in real aerosols.

aerosols, polar stratospheric clouds, laboratory models, nitric acid hydrates