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°N) between November 1994 and April 1995 are presented. The measurements show maximum differential BrO slant columns (90°–81°) of 2.4 × 1014 molecules cm−2 in January, decreasing to 1 × 1014 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 × 1013 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.