A temporal comparison of potential vorticity, ozone mixing ratio, and carbon monoxide mixing ratio along the flight path of the CV-990 on April 20, 1984, is presented and discussed. The flight path was designed to obtain in situ measurements of O3 and CO in the lower stratosphere of a rapidly developing cyclone and above, within, and below an associated folded tropopause structure extruded into the troposphere. The potential vorticity is computed from vertical cross-sectional analyses of potential temperature and wind speed along the flight path. These analyses, in turn, were derived from three-dimensional analyses, including vertical cross sections, constant pressure, and constant potential temperature surfaces, iteratively adjusted for consistency. Only radiosonde observations were used in all of the analyses to provide an objective test of the subjective analysis methods and of the assumptions that potential vorticity, like ozone, is a quasi-conservative stratospheric tracer and that both quantities would be negatively correlated with carbon monoxide, a tropospheric tracer. The resulting correlations are remarkably high and the data is of sufficient quality to permit quantitative evaluations of the deformational growth of the tropopause fold, including the effective eddy diffusion coefficient, the compensating entrainment velocity, and the mass of air and ozone entrained. ¿American Geophysical Union 1987