We present an example of observations within a single air mass of ClO, ClONO2, and HCl from the instruments on the UARS spacecraft. A three-dimensional chemistry-transport calculation for HCl is used to show that this air mass has been transported through regions cold enough for polar stratospheric cloud formation and chlorine activation through heterogeneous chemical reactions. These data, together with modeling of HCl with and without heterogeneous loss as a qualitative measure of polar processing, indicate directly the transformation of the reservoirs HCl and ClONO2 to reactive chlorine species. ¿ American Geophysical Union 1995