The chlorine chemistry under highly activated polar stratospheric spring conditions is critical to understanding the dramatic ozone losses that lead to the formation of the stratospheric ozone hole'. A particular flight pattern of the Geophysica high altitude aircraft during VINTERSOL-EUPLEX/SOLVE II on the 30th January 2003 in the Arctic was designed to test our theoretical understanding of the ClO/ClOOCl system. The outbound and inbound flight legs followed a calculated pattern such that the same air masses were sampled before and after sunset. During the inbound flight three encounters of the contrail from the outbound flight confirmed the success of the flight planning. In-situ ClO measurements were examined to study the conversion of ClO into its nighttime reservoir during sunset. Applying a trajectory Match technique, traditionally used to examine ozone-sonde concentrations, we examined this 3 hour period of ClO observations. 72 ClO Match pairs were identified with temperatures ranging between 200-206 K and SZAs between 84o and 95o. For a given Match pair the total active chlorine ClOx is assumed to be constant, but allowed to vary between Matches. This allowed the kinetic parameters controlling the ClO concentration - Keq, kf and J with differing SZA and temperatures to be closely examined. Due to the particular design of the flight pattern we are able to retrieve robust values for Keq based on ClO measurements alone. Hence our results for Keq are independent of the more uncertain dimer measurements (ClOOCl). We find values for Keq that are about a factor of 3-4 smaller than current JPL recommendations relatively independent of kf and J, as long as kf is larger than 70% of its recommended value. Derived values for J and ClOx are strongly anticorrelated. Hence, without dimer observations the retrieved J is very sensitive to assumptions about ClOx. However, using total available chlorine as an upper limit for ClOx, we find that the upper limits of the current JPL 2006 recommended range of uncertainty for J are required to explain the data of the self-Match fight. If measured values of ClOOCl are used to better constrain ClOx, even larger values of J are inferred. These results contrast with a recent laboratory measurement of the ClO dimer cross sections (Pope et al. 2007, J. Phys. Chem., in press), which leads to large differences between modeled and measured ClO. We shall explore additional chemical mechanisms that could account for observed ClO and ClOOCl during the self-Match flight, in light of the new laboratory cross section.