Spectroscopic measurements of BrO using direct sun and zenith-sky viewinggeometries are combined in an optimal estimation retrieval algorithm to obtaintropospheric and stratospheric columns of BrO. Seventy-two twilight periods areinvestigated over Lauder, New Zealand (45.0 S, 169.7 E), between March 2001 and April2003. A direct comparison between tropospheric and stratospheric columns retrieved at80 , 84 , and 87 solar zenith angles (SZAs) from the spectroscopic measurements andthose calculated by the three-dimensional chemical transport model SLIMCAT showsgood agreement. The stratospheric Bry loading of 21 pptv from the SLIMCAT calculationsis consistent with the ground-based measurements. The seasonal and diurnal variationof the stratospheric BrO columns evident from the ground-based measurement retrievals iswell described by the SLIMCAT model. The tropospheric column retrievals illustratea high variability with a mean value of 0.2 pptv if the troposphere is assumed to be wellmixed. An upper limit of 0.9 pptv is established for the ubiquitous BrO troposphericcolumn at 80 under cloud free conditions.