Chlorine monoxide in the Antarctic spring vortex: 1. Evolution of midday vertical profiles over McMurdo Station, 1993

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Abstract
We have obtained a prolonged record of emission spectra from chlorine monoxide in the vicinity of McMurdo Station, Antarctica, during formation of the austral spring ozone hole of 1993. These spectra have been processed to obtain vertical mixing ratio profiles by deconvolution of pressure‐broadened line shapes. The resulting profiles give a detailed evolution for both altitude distribution and mixing ratio of ClO during development of a major ozone hole event. In early September, very strong emission was observed from pressure‐broadened low‐altitude ClO. Deconvolutions show that this came from an unusually thick layer, extending well above 20 km in altitude. This layer decreased steadily in thickness through September, accompanied by a shift of the peak mixing ratio from ∼21 km altitude in early September to ∼17–18 km by the end of the month, indicating an apparent descent rate of order 100 meters per day, although we argue that the true descent rate is probably lower than the apparent rate. A brief, significant decrease in ClO content occured in late September when the inner vortex edge (defined by the magnitude of Ertel's potential vorticity = 5.2·10−5at ∼19–20 km) approached McMurdo, signifying that a strong gradient in ClO exists near the inner vortex edge. A rapid and apparently final deactivation of chlorine in the lower stratosphere was observed to start about October 1–2. The findings of initially large values of ClO well above 20 km are consistent with observation of polar stratospheric cloud formation in this range during the austral winter of 1993, and with observations showing increased ozone depletion above 20 km relative to previous years.