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 reaction rates. In the present study, ice films are formed in such a reactor, and their surface areas are determined in-situ from BET (Brunauer, Emmett, 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 196 K are consistent with previous data obtained by using thicker nonuniform ice films. In a separate apparatus, environmental scanning electron microscopy (ESEM) is used to obtain particle sizes and shapes and to investigate the morphology of the ices prepared on borosilicate, aluminum, and silicon substrates. Ice films on borosilicate substrates are found to comprise micron-sized granules randomly packed in layers. The uptake of HCl in ice films prepared at 196 K using the same now-tube 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. Similar to the surface areas, the total uptake is strongly proportional to the mass of ice deposited. The combined evidence suggests that even for thin ice films (1) surface areas are greater than the geometric area of the flow-tube reactor and (2) interaction of pore diffusion with surface reaction should be accounted for in the determination of uptake and heterogeneous reaction rates.