New observations of a large concentration of ClO in the springtime lower stratosphere over Antarctica and its implications for ozone‐depleting chemistry

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Abstract
We have made new measurements of stratospheric chlorine monoxide (ClO) at McMurdo Station, Antarctica, during the austral spring of 1987. Rotational emission line spectroscopy, employing a ground‐based detector, was used to determine mixing ratio profiles over the range ∼17–45 km. A spectral band pass double that was used for similar measurements in 1986 allowed an improvement to be made in the definition of the anomalous low‐altitude stratospheric ClO layer associated with springtime ozone depletion. A peak mixing ratio of 1.6±0.4 parts per billion by volume (ppbv) (95% confidence level) was found at 19.5±1 km at midday during the period September 20–24, 1987. The observed peak mixing ratio and diurnal behavior are discussed in relation to chemical depletion theories. Calculations indicate that the large observed ClO concentration provides an efficient closure for a catalytic Cl cycle through the ClO dimer mechanism, yielding good agreement with various observed features of O3 depletion.