CHLORINE MONOXIDE IN THE ANTARCTIC SPRING VORTEX .2. A COMPARISON OF MEASURED AND MODELED DIURNAL CYCLING OVER MCMURDO STATION, 1993

We have derived chlorine monoxide (ClO) mixing ratio profiles within t he Antarctic vortex on an hourly basis from ground-based measurements of pressure-broadened emission line spectra. This data set has provide d the first opportunity for a detailed comparison between the output o f a photochemical model and the measured in situ diurnal behavior of C lO in the Antarctic spring stratosphere. We stress the importance of t he diurnal behavior in furnishing a short-term, crucial test of the ca talytic chlorine chemistry which determines longer-term ozone depletio n. We obtain excellent agreement between our measured and modeled diur nal change using the rate constants recommended in the 1994 Jet Propul sion Laboratory (JPL) evaluation, giving support to current understand ing of perturbed chlorine chemistry in the Antarctic spring vortex. We have furthermore found that we can use our data to narrow the listed 1994 JPL uncertainty range for the ClO dimer formation rate constant a nd the equilibrium constant between ClO dimer formation and thermal di ssociation. We show that the new limits we set on the dimer formation rate constant reduce the uncertainty in the daily rate of chlorine cat alyzed ozone loss calculated from observed ClO concentrations by simil ar to 40% at 186-196 K. We find that a modeled total ozone loss rate i ncluding both chemistry and vertical transport, based on our measureme nts, agrees rather well with the amount and the linear trend of ozone loss seen throughout September in coincident balloon measurements.