MORPHOLOGY AND SURFACE-AREAS OF THIN ICE FILMS

Thin ice films formed by deposition from the vapor phase in flow-tube reactors have been used to simulate polar stratospheric cloud surfaces in order to obtain laboratory data on uptake and heterogeneous reacti on rates. In the present study, ice films are formed in such a reactor , and their surface areas are determined in-situ from BET (Brunauer, E mmett, and Teller) analysis of gas adsorption isotherms. The measured surface areas are found to be strongly dependent on the total mass of ice deposited. The specific surface areas of ices prepared at 77 or 19 6 K are consistent with previous data obtained by using thicker nonuni form ice films. In a separate apparatus, environmental scanning electr on microscopy (ESEM) is used to obtain particle sizes and shapes and t o investigate the morphology of the ices prepared on borosilicate, alu minum, and silicon substrates. Ice films on borosilicate substrates ar e found to comprise micron-sized granules randomly packed in layers. T he uptake of HCl in ice films prepared at 196 K using the same now-tub e reactor is also measured to be approximately 1 x 10(14) molecules/cm (2) when a partial pressure of HCl of about 5 x 10(-7) Torr is used. S imilar to the surface areas, the total uptake is strongly proportional to the mass of ice deposited. The combined evidence suggests that eve n for thin ice films (1) surface areas are greater than the geometric area of the flow-tube reactor and (2) interaction of pore diffusion wi th surface reaction should be accounted for in the determination of up take and heterogeneous reaction rates.