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

New measurements of stratospheric chlorine monoxide (ClO) were made 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 about 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 + or - 0.4 parts per billion by volume (ppbv) (95 percent confidence level) was found at 19.5 + or - 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.