The Intergovernmental Panel on Climate Change 2007 report underlines the importance of aerosol and cloud particles for the water cycle and radiation balance, and thus for the global climate. Nitric acid hydrates are important constituents of solid cloud particles in the lower polar stratosphere (polar stratospheric clouds) and the upper troposphere (cirrus clouds). In the nitric acid – water phase diagram, apart from hexagonal ice and solid nitric acid, there are two stable hydrate phases: nitric acid monohydrate (NAM) and nitric acid trihydrate (b-NAT). In addition, there are metastable phases: a low-temperature modification of NAT (a-NAT), two metastable modifications of nitric acid dihydrate (a-NAD and b-NAD), and a nitric acid pentahydrate (NAP), as well as cubic ice. These metastable modifications have particular relevance for atmospheric chemistry, but have been ignored up to now. We have developed a quenching technique to prepare amorphous ice and hydrate samples, from which we grow the respective crystalline phases, both metastable and stable. Inelastic neutron scattering on Tosca has helped us to close gaps in the vibrational spectra of these samples.
H Grothe, P Baloh (Technische Universität Wien, Austria), SF Parker (ISIS)
Research date: December 2008
Grothe et al., Phys. Chem. Chem. Phys., submitted for publication.