The NASA Goddard three-dimensional chemistry and transport model (3D CTM), utilizing winds and temperatures from a stratospheric data assimilation, is used to investigate the influence of a large cutofflow (COL) system on the transport of chemically processed stratospheric air. The conversion of hydrogen chloride to reactive chlorine is simulated for the early winter of 1991/1992. This heterogeneously converted air is defined as being processed. Throughout December and early January, simulated processed air remained nearly isolated within the Arctic vortex. In late January there were intrusions of midlatitude air across the boundary of the vortex over the North Atlantic and northwestern Europe. These intrusions, which are associated with horizontal transport associated with the COL system, dilute regions of highly processed air within several days. The simulation demonstrates the importance of synoptic scale events on processed air distribution within the vortex as well as transport out of the vortex base into the upper troposphere and midlatitudes. ¿ American Geophysical Union 1995