A series of balloon flights conducted in March 1993 from Aire-sur-Adour, in southern France, aimed at making in situ measurements of a variety of trace gases in the polar vortex during a dynamically active period. Meteorological analyses indicate that the balloons were launched when the vortex edge was passing aloft. These mid latitude balloon flights revealed coincident layering in long-lived tropospheric source gases such as nitrous oxide, methane and halocarbons as well as in ozone and water vapor. At low altitudes, well-resolved laminations appeared clearly in all these gases. A layer of mid latitude air, enriched in trace gases, was detected at the highest sampled level, near 15 mbar, inducing a pronounced reversal in the profiles of tropospheric source gases. Using global meteorological analyses, trace constituent data derived from the UARS observations and high-resolution advection models, fine-scale distributions of ozone, nitrous oxide, methane, and halocarbons have been constructed. The calculations show qualitatively how such profile reversal arises near 15 mbar, when a filament of air enriched in trace gases is drawn into the outer portion of the vortex. Very high resolution calculations furthermore provide reasonable quantitative agreement between predicted tracer mixing ratios and in situ balloon-borne measurements. ¿ 1998 American Geophysical Union