The abundance of reactive chlorine in the lower stratosphere is observed to increase sharply with exposure to temperatures below about 195 K, a temperature which is near the nitric acid trihydrate (NAT) equilibrium condensation point. Measurements from the NASA ER-2 aircraft and a model of chemistry along back trajectories are used to examine the mechanism for this apparent temperature threshold in chlorine activation. The flight of July 28, 1994, from the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) campaign in the southern hemisphere is studied because it provides measurements in an ongoing activation episode. Isentropic back trajectories from the aircraft sampling points indicate that the sampled air was cooling at the rate of 20 to 30 K d-1 to temperatures below the NAT condensation point and had not been below the NAT condensation point prior to that for at least 10 days. Hence the observed amount of active chlorine should be kinetically limited by the recent parcel temperatures. The measurements show enhanced ClO and decreased HCl at temperatures below 195 K even in the absence of significant polar stratospheric cloud particle surface area. The model of chemistry along back trajectories, constrained by the ER-2 chemical and microphysical measurements, indicates that an initial inorganic chlorine (Cly) partitioning of approximately half HCl and half ClONO2 is consistent with the observations. At this initial Cly partitioning, the model using heterogeneous reactions on liquid sulfate and ternary solutions with the most recent sticking coefficient evaluations closely reproduces the latitude gradient and temperature threshold of chlorine activation observed in the data. The sudden increase in activation in the model is a result of the steep exponential temperature dependence of the sticking coefficient for HCl+ClONO2 on liquid aqueous solutions.¿ 1997 American Geophysical Union