Surface Areas and Porosities of Ices Used to Simulate Stratospheric Clouds

Low temperature ices formed by deposition from the vapor phase are used as laboratory simulations of stratospheric ice clouds. To obtain intrinsic reactivities of these ices, detailed information on their physical structure is required. Surface areas are obtained from BET analysis of gas adsorption isotherms. Bulk densities and porosities are determined photogrammetrically. Scanning electron microscopy is used to obtain particle sizes and to study the morphology of the ices at several temperatures. Water ice and HNO 3-H 2O ice films deposited from the vapor at temperatures below 200 K exhibit large BET surface areas and are highly porous. When these ices are annealed by warming above 200 K, a large decrease in BET surface area occurs without any large change in bulk density or particle size. For the annealed ices, external surface areas calculated from the observed particle sizes agree reasonably well with the BET areas. This is evidence that the annealed ices are composed of nonporous particles and that the porosity of these ices is due to interstices among the particles. For unannealed ices at 200 K, the BET surface areas are almost an order of magnitude higher than the external areas; this indicates that these ices are comprised of particles which have internal porosity. Pore size analysis of the unannealed ices yields internal or pore surface areas. At 200 K, the sum of the external and internal surface areas accounts for only about 30-40% of the BET area and suggests that part of the BET value may be due to pore filling and, thus, cannot be considered a true surface area in this case.