Analysis of OBrO, IO, and OIO absorption signature in UV-visible spectra measured at night and at sunrise by stratospheric balloon-borne instruments

Abstract : Absorption bands of OBrO, IO, and OIO in the visible region have been investigated in the data of the AMON (''Absorption par les Minoritaires Ozone et Nox'') and SALOMON (''Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et Nox'') balloon-borne spectrometers used to obtain measurements in the nighttime stratosphere, since 1992 and 1998 respectively. The absorption features initially detected in AMON residual spectra and attributed to OBrO are also observable in SALOMON data with better accuracy. New estimates of OBrO cross-section amplitudes taking into account recent laboratory measurements are used for the OBrO retrieval. A consequence is that previously published OBrO concentration and mixing ratio values are revised downwards of around 40%. Further tests are performed to assess the consistency of the OBrO detection. No correlation exists between OBrO and NO 2 vertical profiles which practically rules out the possibility for the structures ascribed to OBrO absorption to be due to remaining NO 2 contributions. It is shown that variability of OBrO quantities at high latitudes obtained from various AMON and SALOMON flights is possibly linked to the chemical processes involving the production of OClO. At midlatitudes, the exceptional and unexpected conditions of the April 28, 1999 SALOMON flight allow us to observe the drop in OBrO concentrations just after sunrise. As expected, if previous studies of stratospheric iodine species are considered, IO and OIO absorption lines are never detected in the residual spectra. The presence of unknown structures in the residual spectra in the IO and OIO absorption regions is obvious and tends to distort the retrievals. The possibility that these remaining features result from a temperature dependence effect or uncertainties of O 3 and/or NO 2 cross-sections is suggested. Thus, more accurate laboratory measurements and sets of cross-sections for low temperature are needed.