We report aircraft measurements from the Stratosphere-Troposphere Experiments by Aircraft Measurements (STREAM) II campaign, performed during February 1995 from Kiruna, northern Sweden, near 67 ¿N latitude. We have measured trace species, e.g., O3, nitrogen compounds, HCl, hydrocarbons, CO, ice particles, and aerosols, to characterize the chemical conditions of the lowermost stratosphere at altitudes up to ~12.5 km. From the observation of anomalously high CO/C2H6 ratios, caused by enhanced C2H6 breakdown, we derive that heterogeneous chlorine activation has occurred. This has liberated a diurnal mean Cl concentration up to 1.0¿104 atoms cm-3, which efficiently destroys ozone. We also infer that much of the Cl activation has taken place on ice crystals above the tropopause. The measured crystal number densities were typical of thin cirrus. Model calculations suggest that heterogeneous chlorine conversion on ice crystals, in addition to that on liquid aerosols, may contribute significantly to ozone destruction in the middle- and high-latitude lowermost stratosphere. ¿ 1999 American Geophysical Union