PARTITIONING BETWEEN CHLORINE RESERVOIR SPECIES DEDUCED FROM OBSERVATIONS IN THE ARCTIC WINTER STRATOSPHERE

Simultaneous observations of several chlorine source gases, as well as HCl and ClO, have been performed in the Arctic stratosphere on 1 and 9 February 1994, using ballooon-borne instrumentation as a contributio n to SESAME (Second European Stratospheric Arctic and Mid latitude Exp eriment). The observed mixing ratios of HCl and N2O show a clear antic orrelation. No severe loss of HCl was observed inside the vortex durin g our measurement. These measurements showed that during this period a t 20 km and above, HCl was either in excess, or at least as abundant, as ClONO2 and comprised between 50 and 70% of the available chlorine, Cl-y. On I February, measurements were made inside the polar vortex, T he air mass sampled on this day showed a clear signature of diabatic d escent, and also enhanced levels of ClO with a maximum of 230 pptv at 22.5 km. A 10 day backward trajectory analysis showed that these air m asses had passed a large region of low temperatures a few hours prior to the measurement. Temperatures along the back trajectory at the 475 K and 550 K levels (20.1 and 23.7 km respectively) were cold enough fo r heterogeneous chlorine activation to occur, in agreement with the ob served elevated CIO mixing ratios.