title: Metastable Nitric Acid Trihydrate in Ice Clouds.
abstract: The composition of high-altitude ice clouds is still a matter of intense discussion. The constituents in question are ice and nitric acid hydrates, but the exact phase composition of clouds and its formation mechanisms are still unknown. In this work, conclusive evidence for a long-predicted phase, alpha-nitric acid trihydrate (alpha-NAT), is presented. This phase was characterized by a combination of X-ray and neutron diffraction experiments, allowing a convincing structure solution. Furthermore, vibrational spectra (infrared and inelastic neutron scattering) were recorded and compared with theoretical calculations. A strong interaction between water ice and alpha-NAT was found, which explains the experimental spectra and the phase-transition kinetics. On the basis of these results, we propose a new three-step mechanism for NAT formation in high-altitude ice clouds.
authors.affiliation: Institut für Materialchemie, Technische Universität Wien, Getreidemarkt 9/BC/01, 1060, Wien, Austria.
authors.forename: Fabian
authors.identifier: http://orcid.org/0000-0002-6976-6595
authors.initials: F
authors.lastname: Weiss
authors.affiliation: Institut für Chemische Technologie und Analytik, TU Wien, Austria.
authors.forename: Frank
authors.identifier: http://orcid.org/0000-0003-2861-957X
authors.initials: F
authors.lastname: Kubel
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Madrid, Spain.
authors.forename: Óscar
authors.identifier: http://orcid.org/0000-0003-2963-9599
authors.initials: Ó
authors.lastname: Gálvez
authors.affiliation: Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische Universität München, Germany.
authors.forename: Markus
authors.identifier: http://orcid.org/0000-0002-4163-2018
authors.initials: M
authors.lastname: Hoelzel
authors.affiliation: ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK.
authors.forename: Stewart F
authors.identifier: http://orcid.org/0000-0002-3228-2570
authors.initials: SF
authors.lastname: Parker
authors.affiliation: Institut für Materialchemie, Technische Universität Wien, Getreidemarkt 9/BC/01, 1060, Wien, Austria.
authors.forename: Philipp
authors.identifier: http://orcid.org/0000-0002-0356-2223
authors.initials: P
authors.lastname: Baloh
authors.affiliation: Paul Scherrer Institute, Laboratory for Atmospheric Chemistry, 5232, Villigen, Switzerland.
authors.forename: Riccardo
authors.identifier: http://orcid.org/0000-0002-6185-6975
authors.initials: R
authors.lastname: Iannarelli
authors.affiliation: Paul Scherrer Institute, Laboratory for Atmospheric Chemistry, 5232, Villigen, Switzerland.
authors.forename: Michel J
authors.identifier: http://orcid.org/0000-0003-3504-695X
authors.initials: MJ
authors.lastname: Rossi
authors.affiliation: Institut für Materialchemie, Technische Universität Wien, Getreidemarkt 9/BC/01, 1060, Wien, Austria. grothe@tuwien.ac.at.
authors.forename: Hinrich
authors.identifier: http://orcid.org/0000-0002-2715-1429
authors.initials: H
authors.lastname: Grothe
keywords: atmospheric chemistry; diffraction; ice clouds; polar stratospheric clouds; vibrational spectroscopy