Retrieval of stratospheric and tropospheric BrO profiles and columns using ground-based zenith-sky DOAS observations at Harestua, 60° N
1Institut d'Aéronomie Spatiale de Belgique (IASB-BIRA), Brussels, Belgium
2Institute for Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK
3Institute for Environmental Physics, University of Heidelberg, Heidelberg, Germany
4Service d'Aéronomie du CNRS, Verrières le Buisson, France
5Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge, UK
Abstract. A profiling algorithm based on the optimal estimation method is applied to ground-based zenith-sky UV-visible measurements from Harestua, Southern Norway (60° N, 11° E) in order to retrieve BrO vertical profiles. The sensitivity of the zenith-sky observations to the tropospheric BrO detection is increased by using for the spectral analysis a constant reference spectrum corresponding to clear-sky noon summer conditions. The information content and retrieval errors are characterized and it is shown that the retrieved stratospheric profiles and total columns are consistent with correlative balloon and satellite observations, respectively. Tropospheric BrO columns are derived from profiles retrieved at 80° solar zenith angle during sunrise and sunset for the 2000–2006 period. They show a marked seasonality with mean column value ranging from 1.52±0.51×1013 molec/cm2 in late winter/early spring to 0.92±0.31×1013 molec/cm2 in summer, which corresponds to 1.0±0.3 and 0.6±0.2 pptv, respectively, if we assume that BrO is uniformly mixed in the troposphere. These column values are also consistent with previous estimates made from balloon, satellite, and other ground-based observations. Daytime (10h30 local time) tropospheric BrO columns are compared to the p-TOMCAT 3-D tropospheric chemical transport model (CTM) for the 2002–2003 period. p-TOMCAT shows a good agreement with the retrieved columns except in late winter/early spring where an underestimation by the model is obtained. This feature could be explained by the non-inclusion of sea-ice bromine sources in the current version of p-TOMCAT, which can therefore not reproduce the possible transport from the polar region to Harestua of air-masses with enhanced BrO concentration due to bromine explosion events in late winter/early spring. The corresponding daytime stratospheric BrO columns are compared to the SLIMCAT 3-D stratospheric CTM. The model run used, which assumes 21.2 pptv for the Bry loading (15 pptv for long-lived bromine species + 6 extra pptv for very short-lived species (VSLS) added by a scaling of CH3Br), significantly underestimates the retrieved BrO columns. A sensitivity study shows that a good quantitative agreement can only be obtained if 8 pptv accounting for VSLS are added directly (and not by a scaling of CH3Br) to the SLIMCAT long-lived bromine species profile. This contribution of the VSLS to the total bromine loading is also consistent with recently published studies.