Retrieved tropospheric and stratospheric BrO columns over Lauder, New Zealand

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 (45.0degreesS, 169.7degreesE), between March 2001 and April 2003. A direct comparison between tropospheric and stratospheric columns retrieved at 80degrees, 84degrees, and 87degrees solar zenith angles (SZAs) from the spectroscopic measurements and those calculated by the three-dimensional chemical transport model SLIMCAT shows good agreement. The stratospheric Br-y 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 80degrees under cloud free conditions.