Mid-latitude observations of the seasonal variation of BrO .1. Zenith-sky measurements

Zenith-sky observations of the seasonal evolution of BrO slant columns above Aberdeen (57 degrees N) between November 1994 and April 1995 are presented. The measurements show maximum differential BrO slant columns (90 degrees-81 degrees) of 2.4 x 10(14) molecules cm(-2) in January, decreasing to 1 x 10(14) in spring. On a seasonal timescale, BrO slant columns are anticorrelated with NO2 slant columns, which is qualitatively consistent with BrONO2 being the main nighttime bromine reservoir. There is some evidence for a small (5 x 10(13) molecules cm(-2)) increase in sunrise BrO slant columns inside the polar vortex, following chlorine activation on PSCs, as shown by measurements of large OClO slant columns.