First profile measurements of tropospheric BrO

Tropospheric BrO profiles (about 0.6+/-0.2 ppt, and 2.0+/-0.8 ppt at profil e maximum) were measured for the first time. Our measurements add new infor mation to recent speculations - based on indirect evidence - of BrO possibl y being ubiquitous in the free troposphere [Harder. et al., 1998; Friess et al., 1999; Van Roozendael et al., 1999; Pundt et al., 2000]. Our study rel ies on a detailed comparison of BrO slant column densities (BrO-SCD) measur ed in the troposphere from the LPMA/DOAS (Laboratoire de Physique Moleculai re et Applications and Differential Optical Absorption Spectroscopy) gondol a by direct Sun absorption, and BrO-SCD values subsequently measured in the lowermost stratosphere during balloon ascent. The difference in total atmo spheric BrO-SCDs measured in the troposphere and lowermost stratosphere - a fter a suitable correction for the change in BrO due to photochemistry and the observation geometry - is then attributed to tropospheric BrO.