Observations of chlorine monoxide over Scott Base, Antarctica, during the ozone hole, 1996-2005

We report observations of chlorine monoxide, ClO, in the lower stratosphere, made from Scott Base
(77.85º S, 166.77º E) in springtime during each year, 1996-2005. The ClO amounts in the atmosphere are
retrieved from remote measurements of microwave emission spectra. ClO column densities of up to about 2.5 ×
1015 cm-2
are recorded during September, when chlorine is present in chemically active forms due to reactions on
the surface of Polar Stratospheric Cloud (PSC) particles. Maximum mixing ratios of ClO are approximately 2
ppbv. The annual average of ClO column density during the activation period is anticorrelated with similar
averages of ozone column measured at nearby Arrival Heights, with correlation coefficient of –0.81, and with
averages of ozone mass integrated over the entire polar region, with similar correlation coefficients. There was a
substantial decrease in ClO amounts during 2002-2004. There has been no systematic change in the timing of
chlorine deactivation attributable to secular change in the Antarctic vortex