Chlorine activation and ozone destruction in the northern lowermost stratosphere

We report aircraft measurements from the Stratosphere-Troposphere Experimen ts by Aircraft Measurements (STREAM) II campaign, performed during February 1995 from Kiruna, northern Sweden, near 67 degrees N latitude. We have mea sured trace species, e.g., O-3, nitrogen compounds, HCl, hydrocarbons, CO, ice particles. and aerosols, to characterize the chemical conditions of the lowermost stratosphere at altitudes up to similar to 12.5 km. From the obs ervation of anomalously high CO/C2H6 ratios, caused by enhanced C2H6 breakd own, we derive that heterogeneous chlorine activation has occurred. This ha s liberated a diurnal mean Cl concentration up to 1.0 x 10(4) atoms cm(-3), which efficiently destroys ozone. We also infer that much of the Cl activa tion has taken place on ice crystals above the tropopause. The measured cry stal number densities were typical of thin cirrus. Model calculations sugge st that heterogeneous chlorine conversion on ice crystals, in addition to t hat on liquid aerosols, may contribute significantly to ozone destruction i n the middle- and high-latitude lowermost stratosphere.