[Zurück]

F. Weiss, F. Kubel, O. Galvez, M. Hölzel, S. F. Parker, P. Baloh, R. Iannarelli, J Rossi, H. Grothe:
"Metastable nitric acid trihydrate in ice clouds";
Vortrag: 4th Workshop - Microphysics of ice clouds, Vienna; 16.04.2016 - 17.04.2016.

The composition of high altitude ice clouds is still a matter of intense discussion. The constituents in question
are ice and nitric acid hydrates. The identification and formation mechanisms, however, are still unknown but are
essential to understand atmospheric processing such as the seasonal ozone depletion in the lower polar stratosphere
or the radiation balance of planet Earth. We found conclusive evidence for a long-predicted phase, which has been
named alpha nitric acid trihydrate (alpha-NAT). This phase has been proven by combination of X-ray and neutron
diffraction experiments allowing a convincing structure solution. Additionally, vibrational spectra (infrared and
inelastic neutron scattering) were recorded and compared with theoretical calculations. A strong affinity between
water ice and alpha-NAT has been found, which explains the experimental spectra and the phase transition kinetics
essential for identification in the atmosphere. On the basis of our results, we propose a new three-step mechanism
for NAT-formation in high altitude ice clouds.

nitric acid hydrates, alpha NAT, atmospheric chemistry, ice clouds