High concentrations of chlorine monoxide at low altitudes in the Antarctic spring stratosphere: secular variation

Abstract

We have reported the observation of millimetre-wave emission from abnormally high concentrations of chlorine monoxide, ClO, in the lower stratosphere over McMurdo Station, Antarctica during the period 1–22 September 1986 and demonstrated¹ its diurnal behaviour. Here we report on the secular variation of daytime ClO in the lower stratosphere with a time resolution of 4 or 5 days, during the period 1 September to 16 October. We show that the ClO concentration declines during the latter half of September and disappears by early October. As ClO is directly involved in a catalytic ozone destruction cycle, these observations give evidence that the Antarctic ozone hole² is caused by a chemical depletion mechanism which operates in the early spring. This is also consistent with models in which large quantities of ClO are maintained through chemical reactions^3–5 on the surface of polar stratospheric cloud particles which evaporate with the warming of the stratosphere.