Ice might play an important roIe in the composition of the atmospheric gaseous phase : (l) during its formation, it can incorporate some trace gases by simultaneous condensation of gas and water vapor (co-condensation) or by rapid solidification of supercooled droplets in which some gases are dissolved (riming) ; and (2) after the precipitation, some trace gases exchanges can happen between the snow cover and the troposphere. Furthermore, the ice surface can catalyze heterogeneous chemical reactions between trace gases that are inert in the gaseous phase. For example during winter, such reactions between chlorine compounds (HCI,.. . ) are occuring at the surface of ice particles that constitute the polar stratospheric clouds (PSCs) and are responsible for the polar stratospheric ozone depletion. In the present work, we have studied the incorporation of the trace gases HCl and HBr during both of the atmospheric ice formation mechanisms (co-condensation and riming). These species have been chosen because of their atrnospheric interest and for some reasons of experimental possibilities at the ESRF of Grenoble. Therefore, we have developped an infrared spectroscopy set-up which allowed us to produce some cristalline ice films at 190 K (which is the winter polar stratosphere temperature) and to study the interactions between HCI and ice by condensation of an HCl/H20 gaseous mixture at this temperature. Our results show that HCI is incorporated homogeneously into ice by an ionic solvatation process which leads to the deformation of the cristalline structure of the solid. This suggests that the ice of the PSCs would contain many volume and surface defects. Among others, these defects could play an important roIe in the reactivity of the ice that catalyzes the heterogenous reactions leading to the polar stratospheric ozone depletion. In addition, we have studied the environment of HBr in ice formed by riming of aqueous solutions of HBr by EXAFS at the ESRF. Our results are prelirninary and suggest that at about -20°C, an important part of HBr is degasing. The remaining part seems to be incorporated homogeneously into ice, in the form of a sursaturated solid solution. Thus the evolution of the trace gases trapped into the snow cover would be controlled by the melting/refreezing cycle that occurs during the snow metamorphism more than by solid phase diffusion mechanisrns, which are too slow to allow the degasing of these species.
