THE POSSIBLE DETECTION OF OBRO IN THE STRATOSPHERE

The analysis of spectral residua recorded at night by the balloon-born e AMON (Absorption par Minoritaires Ozone et Nor) UV-visible spectrome ter during five stratospheric flights at middle and high latitudes sho ws that some absorption features remain in the 475 - 550 nm range, whi le the Rayleigh, aerosol, ozone, and NO2, contributions are subtracted . The data reduction relating to these spectral lines is presented for the flight of February 26, 1997, at Kiruna (Sweden) inside the polar vortex. A good agreement exists between these unknown absorption featu res and those attributed to OBrO during recent laboratory measurements . The results of measurements from the other AMON flights are also pre sented. Assuming a OBrO cross section maximum similar to that of OClO, an upper limit for the OBrO mixing ratio is found to be around 20 ppt v at midlatitude, implying that OBrO would be the principal bromine sp ecies at night in the middle stratosphere. At high latitude the OBrO m ixing ratio decreases, particularly in the presence of OClO (also meas ured by AMON). The results are contradictory to current knowledge and, if confirmed, could argue for major revision of the assumed bromine c hemistry in the stratosphere.