Retrievals of chlorine chemistry kinetic parameters from Antarctic ClO microwave radiometer measurements

摘要

pstrongAbstract./strong Key kinetic parameters governing the partitioning of chlorine species in the Antarctic polar stratosphere were retrieved from 28 days of chlorine monoxide (ClO) microwave radiometer measurements made during the late winter/early spring of 2005 at Scott Base (77.85?° S, 166.75?° E). During day-time the loss of the ClO dimer chlorine peroxide (ClOOCl) occurs mainly by photolysis. Some time after sunrise, a photochemical equilibrium is established and the ClO/ClOOCl partitioning is determined by the ratio of the photolysis frequency, iJ/i, and the dimer formation rate, ik/isubf/sub. The values of iJ/i and ik/isubf/sub from laboratory studies remain uncertain to a considerable extent, and as a complement to these ongoing studies, the goal of this work is to provide a constraint on that uncertainty based on observations of ClO profiles in the Antarctic. First an optimal estimation technique was used to derive iJ/i/ik/isubf/sub ratios for a range of iK/isubeq/sub values. The optimal estimation forward model was a photochemical box model that takes iJ/i, ik/isubf/sub, and iK/isubeq/sub as inputs, together with a priori profiles of activated chlorine (ClOsubx/sub = ClO+2??ClOOCl), profiles of ozone, temperature, and pressure. JPL06 kinetics are used as a priori in the optimal estimation and for all other chemistry in the forward model. Using the more recent JPL09 kinetics results in insignificant differences in the retrieved value of iJ/i/ksubf/sub. A complementary approach was used to derive the optimal kinetic parameters; the full parameter space of iJ/i, ik/isubf/sub, iK/isubeq/sub and ClOsubx/sub was sampled to find the minimum in differences between measured and modelled ClO profiles. Furthermore, values of iK/isubeq/sub up to 2.0 times larger than recommended by JPL06 were explored to test the sensitivity of the iJ/i/ik/isubf/sub ratio to changes in iK/isubeq/sub. The results show that the retrieved iJ/i/ik/isubf/sub ratios bracket the range of 1.23 to 1.97 times the iJ/i/ik/isubf/sub value recommended by JPL06 over the range of iK/isubeq/sub values considered. The retrieved iJ/i/ik/isubf/sub ratios lie in the lower half of the large uncertainty range of iJ/i/ik/isubf/sub recommended by JPL06 and towards the upper portion of the smaller uncertainty range recommended by JPL09./p.