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A profiling algorithm based on the optimal estimation method is applied toground-based zenith-sky UV-visible measurements from Harestua, SouthernNorway (60° N, 11° E) in order to retrieve BrO vertical profiles. Thesensitivity of the zenith-sky observations to the tropospheric BrO detectionis increased by using for the spectral analysis a fixed referencespectrum corresponding to clear-sky noon summer conditions. The informationcontent and retrieval errors are characterized and it is shown that theretrieved stratospheric profiles and total columns are consistent withcorrelative balloon and satellite observations, respectively. TroposphericBrO columns are derived from profiles retrieved at 80° solar zenithangle during sunrise and sunset for the 2000–2006 period. They show a markedseasonality with mean column value ranging from 1.52±0.62×1013 molec/cm² in late winter/earlyspring to 0.92±0.38×1013 molec/cm² in summer, whichcorresponds to 1.0±0.4 and 0.6±0.2 pptv, respectively, if we assume that BrO is uniformly mixed in thetroposphere. These column values are also consistent with previous estimatesmade from balloon, satellite, and other ground-based observations. Daytime(10:30 LT) tropospheric BrO columns are compared to the p-TOMCAT 3-Dtropospheric chemical transport model (CTM) for the 2002–2003 period.p-TOMCAT shows a good agreement with the retrieved columns except in latewinter/early spring where an underestimation by the model is obtained. Thisfinding could be explained by the non-inclusion of sea-ice bromine sourcesin the current version of p-TOMCAT. Therefore the model cannot reproduce thepossible transport of air-masses with enhanced BrO concentration due to bromineexplosion events from the polar region to Harestua. The daytime stratospheric BrO columns arecompared to the SLIMCAT stratospheric 3-D-CTM. The model run used in this study, which assumes21.2 pptv for the Bry loading (15 pptv for long-lived bromine species and additional6 pptv for very short-lived species (VSLS) added by a scaling of CH3Br), significantlyunderestimates the retrieved BrO columns. A sensitivity study shows that a good agreementcan only be obtained if 6 to 8 pptv accounting for VSLS are added directly (and not bya scaling of CH3Br) to the SLIMCAT long-lived bromine species profile. This contributionof the VSLS to the total bromine loading is also consistent with recently published studies

Retrieval of stratospheric and tropospheric BrO profiles and columns using ground-based zenith-sky DOAS observations at Harestua, 60° N