Low Temperature Laboratory Models of Atmospheric Processes

Atmospheric aerosol models of different phase, composition, and morphology will be prepared and characterized by IR spectroscopy and X-ray powder diffraction. In agreement with field measurements and thermodynamic data on stratospheric and tropospheric aerosols sulfuric acid, nitric acid, and water were selected as model constituents. The characterized solid and liquid aerosol models will then be exposed to relevant trace gases under atmospheric conditions. Examples are chlorine and bromine containing compounds as well as sulfur and nitrogen oxides. The resulting interaction with the aerosol model surface is then monitored by IR reflectance spectroscopy. In addition, adsorption/desorption studies using mass spectroscopic detection are planned in order to shed light on subsequent chemical reactions which may occur on the surface and in the bulk. It is intended to implement the proposal presented here in the form of an international cooperation. The partner is Prof. Dr. Aharon Loewenschuss (Department of Inorganic and Analytical Chemistry of the Hebrew University of Jerusalem) who will provide fundamental IR matrix spectroscopic information on intermolecular complexes. Such data is badly needed for the interpretation of IR spectra related to surface complexes. In addition, Dr. Loewenschuss is an expert in the preparation and characterization of icy surfaces which often play an important role in atmospheric heterogeneous processes.

This bilateral project (Vienna University of Technology and Hebrew University of Jerusalem) focused on the elucidation of the complex heterogeneous chemistry on aqueous aerosol and icicles in the atmosphere via laboratory investigations. A remarkable consequence of these heterogeneous processes is the annually observed ozone depletion in the lower polar stratosphere which can be related to the conversion of inactive reservoir substances into active halogen compounds on the surface of Polar Stratospheric Clouds (PSC). The reaction pathways and the trace gas abundances in the stratosphere depend on the physical state of the cloud particles, which is strongly influenced by specific nucleation processes. Considerable efforts have been undertaken in this field during the last decade. Nevertheless, important questions concerning the phase composition of PSC still remained unsolved. The complexity of PSC formation is due to the fact that both the supercooled liquid and the solid state have been found in field experiments. Moreover, its main constituents (nitric acid and water) may form different thermodynamically stable and metastable phases. The main interest of this project was directed on the characterization of the non-equilibrium phase diagram of solid nitric acid/ water mixtures. Therefore, a preparation technique was developed which allows the production of metastable nitric acid hydrates that subsequently are subjected to an analysis by X-ray diffractometry (XRD) and infrared spectroscopy (FTIR). XRD - a tool which is seldom used in atmospheric sciences - provides exact information on the long-range order, of the prevailing phases, whereas FTIR analyses the short-range order of the sample constituents. By these means, several stable as well as metastable hydrate phases of nitric acid were produced. Their limits of existence in terms of composition and temperature as well as their characteristics of nucleation and crystal growth were determined and thus helped to assess the atmospheric relevance of the respective phases. In addition, the combined application of XRD and FTIR enabled us to corroborate previous speculative model assumptions which were based on FTIR spectroscopic data only. The project considerably profited from the contribution of the Israeli partner who extended the scientific scope to the interaction of relevant trace gases with the substrate. Matrix isolation studies and surface ice studies helped to resolve the fundamental steps of gas phase/ solid reactions, which are of crucial importance in nucleation processes. These results were supported by theoretical ("ab initio") calculations.