Morphology of nitric acid and water ice films.

Ice films have been used to simulate stratospheric cloud surfaces in order to obtain laboratory data on solubilities and heterogeneous reaction rates. To obtain intrinsic uptake and surface reaction probabilities which can be applied to atmospheric models, it is necessary to carefully characterize these films. In the present study, environmental scanning electron microscopy (ESEM) is used to study thin films of both water ice and nitric acid ice near the composition of the trihydrate. The ices are formed by vapor deposition onto aluminum or borosilicate-glass substrates cooled to about 200 K. Micrographs are recorded during the deposition process and during subsequent annealing at higher temperatures. The results show that the ice films are composed of loosely consolidated granules, which range from about 1 to 20 microns in size at temperatures between 197 and 235 K. Cubic water ice is sometimes observed at 200 K and converts to the hexagonal form at slightly higher temperatures. The loose packing of the granules confirms the high porosities of these films obtained from separate bulk porosity measurements. Average surface areas calculated from the observed granule sizes range from about 0.2 to 1 m2 g-1 and agree with surface areas obtained by gas-adsorption (BET) analysis of annealed ice films. For unannealed films, the BET areas are about an order of magnitude higher than the ESEM results and imply that the unannealed ices contain microporosity which is lost during the annealing process. The present results have important implications for the extraction of intrinsic reaction probabilities from laboratory rate data.