DOAS Zenith sky observations: 2. Seasonal variation of BrO over Bremen (53degrees N) 1994-1995

Zenith sky observations of BrO over Bremen (53 degrees N) are reported for the period of September 1994 to January 1996. BrO differential slant column s between 90 degrees and 80 degrees solar zenith angle showed a strong seas onal variation between a winter maximum of 1.9.10(14) molec/cm(2) and a sum mer minimum of 0.6.10(14) molec/cm(2) The seasonal variation in BrO twiligh t values is shown to be inversely correlated with NO2 columns in agreement with current knowledge of gas phase chemistry of bromine. In contrast to mo del predictions, no significant difference between morning and evening BrO measurements was observed. During a 6 day polar vortex excursion to mid-lat itudes OClO could be measured above Bremen indicating chlorine activation i n the vortex air. No significant increase in BrO differential slant columns was detected during this time.