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Recent balloon-borne spectroscopic observations of stratospheric OBrO absorption features [Renard et al., 1998] gave rise to speculations whether OBrO is an important nighttime reservoir for stratospheric bromine. We have analyzed direct moon light spectra collected during 6 Transall flights for OBrO absorption signatures occurring between 470 and 550 nm. OBrO absorptions were not detected by stratospheric measurements at mid- or high latitude, both early or late in the night, made in early winter, or spring. Using the recently measured OBrO absorption cross section (σOBrO = (1.8±0.5)×10−17 cm²) by Knight et al. [2000], or the previously assumed OBrO cross section (σOBrO =1.5×10−17 cm²) by Renard et al. [1998] [further on all the results based on the latter cross section are given in () for direct comparison], our measurements imply an upper limit for stratospheric OBrO of < 6.4 (7.7) ppt (typically 2.5 (3) ppt), and as low as 1.5 (1.8) ppt for an assumed Gaussian profile that maximises at 28 km. Although our result is closer in magnitude to the results of Renard et al. [1998], it does not confirm their findings, but it points more to the results of a modeling study that reject sizeable amounts (< 0.01 ppt) of stratospheric OBrO being formed by known gas-phase reactions even using extreme assumptions in favor of OBrO at night [Chipperfield et al., 1998]. |
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