During the 1988/89 Airborne Arctic Stratospheric Expedition (AASE), observations of the chemical composition, aerosol characteristics and atmospheric state were obtained from two aircraft, a NASA ER-2 and a DC-8. In this paper we present a diagnosis of observations obtained using the ER-2 on January 24, 1989, using a Lagrangian coupled microphysical-photochemical model. We interpret the high chlorine monoxide mixing ratios observed from the ER-2 on the afternoon of January 24, 1989 as a result of the in situ heterogeneous release of reactive chlorine from the reservoirs HCl and ClONO2 on type 1 polar stratospheric cloud particles observed to be present at that time. This essential element in theories of polar ozone depletion has never before been observed directly in the stratosphere. ¿ American Geophysical Union 1990