Abstract
Simultaneous observations of several chlorine source gases, as well asHCl and ClO, have been performed in the Arctic stratosphere on 1 and 9February 1994, using balloon-borne instrumentation as a contribution toSESAME (Second European Stratospheric Arctic and Mid latitude Experiment).The observed mixing ratios of HCl and N2O show a clearanticorrelation. No severe loss of HCl was observed inside the vortex duringour measurement. These measurements showed that during this period at 20 kmand above, HCl was either in excess, or at least as abundant, asClONO2 and comprised between 50 and 70% of theavailable chlorine, Cly. On 1 February, measurements were madeinside the polar vortex. The air mass sampled on this day showed a clearsignature of diabatic descent, and also enhanced levels of ClO with amaximum of 230 pptv at 22.5 km. A 10 day backward trajectory analysis showedthat these air masses had passed a large region of low temperatures a fewhours prior to the measurement. Temperatures along the back trajectory atthe 475 K and 550 K levels (20.1 and 23.7 km respectively) were cold enoughfor heterogeneous chlorine activation to occur, in agreement with theobserved elevated ClO mixing ratios.
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