Resolution of an important discrepancy between remote and in-situ measurements of tropospheric BrO during Antarctic enhancements

Abstract. Tropospheric BrO was measured by a ground-based remote-sensing spectrometer at Halley in Antarctica, and BrO was measured by remote-sensing spectrometers in space using similar spectral regions and Differential Optical Absorption Spectroscopy (DOAS) analyses. Near-surface BrO was simultaneously measured at Halley by Chemical Ionisation Mass Spectrometry (CIMS), and in an earlier year near-surface BrO was measured at Halley over a long path by a DOAS spectrometer. During enhancement episodes, total amounts of tropospheric BrO from the ground-based remote-sensor were similar to those from space, but if we assume that the BrO was confined to the boundary layer they were very much larger than values measured by either near-surface technique. This large apparent discrepancy can be resolved if substantial amounts of BrO were in the free troposphere during most enhancement episodes. Amounts observed by the ground-based remote sensor at different elevation angles, and their formal inversions to vertical profiles, also show that much of the BrO was often in the free troposphere. This is consistent with the ~5 day lifetime of Bry, from the enhanced BrO observed during some Antarctic blizzards, and from aircraft measurements of BrO well above the surface in the Arctic.