• remote sensing;
  • bromine;
  • troposphere;
  • retrieval;
  • diurnal variation

[1] Spectroscopic measurements of BrO using direct sun and zenith-sky viewing geometries are combined in an optimal estimation retrieval algorithm to obtain tropospheric and stratospheric columns of BrO. Seventy-two twilight periods are investigated over Lauder, New Zealand (45.0°S, 169.7°E), between March 2001 and April 2003. A direct comparison between tropospheric and stratospheric columns retrieved at 80°, 84°, and 87° solar zenith angles (SZAs) from the spectroscopic measurements and those calculated by the three-dimensional chemical transport model SLIMCAT shows good agreement. The stratospheric Bry loading of 21 pptv from the SLIMCAT calculations is consistent with the ground-based measurements. The seasonal and diurnal variation of the stratospheric BrO columns evident from the ground-based measurement retrievals is well described by the SLIMCAT model. The tropospheric column retrievals illustrate a high variability with a mean value of 0.2 pptv if the troposphere is assumed to be well mixed. An upper limit of 0.9 pptv is established for the ubiquitous BrO tropospheric column at 80° under cloud free conditions.