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 reactions3–5 on the surface of polar stratospheric cloud particles which evaporate with the warming of the stratosphere.