Uptake of chlorine dioxide by model polar stratospheric cloud surfaces: Ultrahigh-vacuum studies

An investigation of the interaction of chlorine dioxide (OClO) with the surface of ice is reported. Experiments were carried out under ultrahigh vacuum (UHV) on films between 10 and 100 water monolayers thick. The initial OClO adsorption probability on ice at 100 K is high, approaching unity on both the crystalline and amorphous surfaces. At low coverages, OClO is quantitatively incorporated into the ice bulk, where it resides until sublimation of the ice film near 185 K. An adsorbed state of OClO is formed at moderate exposures, Chlorine dioxide desorbs from ice at approximately 130 K; desorption is first order, with an activation energy equal to 23 kJ mol(-1) and a prefactor of 2 x 10(9) s(-1). At high exposures, OClO condenses to form a multilayer film, which sublimes near 135 K; the sublimation energy is 38 kJ mol(-1). If adsorbed OClO is covered with an ice film 13 monolayers thick, OClO is trapped within the film, and desorption is completely suppressed until the onset of ice sublimation.