Intercomparison of measured and modelled BrO slant column amounts for the Arctic winter and spring 1994/95

Diurnal variations of the slant column amount of BrO (BrO-SCD) measured at Kiruna/Esrange (67.9 degrees N, 21.1 degrees E) in winter/spring 1994/95 ar e compared with photochemical calculations using the SLIMCAT model. SLIMCAT assumes a total stratospheric bromine of 20 ppt, JPL-97 kinetics but no tr opospheric inorganic bromine. Observed local noon BrO-SCDs are underestimat ed by the model by 20% to 40%. Sensitivity test of existing uncertainties i n the stratospheric chemistry of bromine can not resolve the discrepancy. A tropospheric contribution to the total atmospheric BrO, corresponding to a vertical BrO column of 1 to 2 x 10(13)/cm(2), (or mixing ratios of 0.75 pp t to 2 ppt assuming an uniform tropospheric height profile) is suggested.