[Zurück]

H. Grothe, I. Ortega, D Waller, D Stokes:
"Metastable nitric acid hydrates - PSC constituents";
Vortrag: EGU General Assembly 2007, Wien; 15.04.2007 - 20.04.2007; in: "Geophysical Research Abstracts", European Geosciences Union, 9 (2007), Paper-Nr. 07284, 2 S.

Nitric acid hydrates are important constituents of solid PSC and Cirrus cloud particles.
Due to the phase diagram only hexagonal ice and nitric acid trihydrate have a reasonable
thermodynamic stability and are commonly observed in the stratosphere. Besides,
also a number of metastable modifications might exist: -NAD, -NAD, -NAT, NAP
and cubic ice. The existence of these metastable compounds remains uncertain and has
not been proofed yet.
In the laboratory we have developed a model procedure in order to prepare and investigate
all hydrates as pure phases. The investigation methods are X-ray diffraction
[1], [2], FTIR spectroscopy [3], Raman spectroscopy [4] and Environmental SEM [5].
The aim is to verify the phase composition by diffraction and collect the spectroscopic
data of the respective phases, which are needed for interpretation of field measurements
and aerosol chamber experiments. Here, the morphology of the particles has to
be considered since it has an important impact on the respective extinction spectra.
Particular interest has been directed towards the modifications of NAD, which have
been discriminated for the first time by spectroscopic methods. The morphologies of
NAD particles have been recorded by ESEM and the respective extinction spectra
have been determined by T-matrix calculations. Beside the mid-IR region, where normal
vibrations have been assigned, we have also focused on the low-frequency region
below 200 cm−1 where phonon bands are observed by Raman spectroscopy. These
intermolecular vibrations are closely connected to the symmetry and structure of the
unit cell of a hydrate crystal [6]. Thus, a distinction between the phases becomes easily
accessible.
[1] H. Tizek, E. Knözinger, H. Grothe, PCCP 4 (2002), 5128.
[2] H. Tizek, E. Knözinger, H. Grothe, PCCP 6 (2004), 972.
[3] H. Grothe, C.E. Lund Myhre, H. Tizek, Vibr. Spectr. 34 (2004), 55.
[4] H. Grothe, C.E. Lund Myhre, C.J. Nielsen, JPC A 110 (2006), 110, 171.
[5] H. Grothe, H. Tizek, D Waller, D Stokes, PCCP 8 (2006), 2232.
[6] R. Escribano, D. Fernández-Torre, V. Herrero, B. Martín-Llorente, B. Maté, I.
Ortega, H. Grothe, Vibr. Spectr. 43 (2007), 254.

http://www.cosis.net/abstracts/EGU2007/07284/EGU2007-J-07284.pdf