DOAS ZENITH SKY OBSERVATIONS .1. BRO MEASUREMENTS OVER BREMEN (53-DEGREES N) 1993-1994

Observations of stratospheric BrO over Bremen (53 degrees N) are repor ted for winter and early spring periods of 1993 and 1993/94. The BrO w as observed by ground-based near-UV absorption spectroscopy of sunligh t scattered in the zenith. Differential slant column densities for sol ar zenith angles 90 degrees/80 degrees in the range of 9x10(13) (detec tion limit) to 4.5x10(14) molecules/cm(2) having a high day-to-day var iability were found. For the majority of the measurements no significa nt difference was observed between the morning and evening behaviour o f BrO. Exceptions are the morning measurements from the winter of 1992 /93 where an accelerated production of BrO was observed. We believe th e latter best to be explained by the early morning rapid photolysis of elevated amounts of photo-labile Br-reservoirs formed during the nigh t. The largest differential slant column densities of BrO were measure d in December 1993 when the temperatures at 30 hPa dropped below 205 K . This might be an indication of heterogeneous conversion of bromine c ompounds on sulfate and other aerosols.