First measurements of ClOOCl in the stratosphere: The coupling of ClOOCl and ClO in the Arctic polar vortex

The first measurements of ClOOCl in the stratosphere have been acquired from a NASA ER-2 aircraft, deployed from Kiruna, Sweden (68degreesN, 21degreesE), during the joint SOLVE/THESEO-2000 mission of the winter of 1999/2000. ClOOCl is detected by thermal dissociation into two ClO fragments that are measured by the well-known technique of chemical conversion, vacuum ultraviolet resonance fluorescence. Ambient ClO is detected simultaneously. Observations of the ratio [ClOOCl]/[ClO](2) (estimated uncertainty of +/-25%, 1 sigma) are used with a time-dependent photochemical model, to test the model representation of the ratios of kinetic parameters J/k(Prod) and k(Loss)/k(Prod) for day and nighttime observations, respectively. Here, k(Prod) and k(Loss) are the rate constants for ClOOCl production and loss, respectively, and J is the photolysis rate of ClOOCl. The observations are in good agreement with J based upon the 2002 JPL recommended cross sections for ClOOCl [Sander et al., 2003], if the true value of k(Prod) is given by either the 2000 JPL recommendation [Sander et al., 2000] or the work of Trolier et al. [1990]. The larger values of k(Prod) given by Bloss et al. [2001] and the 2002 JPL recommendation are consistent with the observations only if J is increased by a significant amount. This is accomplished if J is calculated with the larger ClOOCl cross sections measured by Burkholder et al. [ 1990]. The J values for ClOOCl based on the Huder and DeMore [1995] cross sections are too small, by factors of similar to1.6 to 2.5 for all values of k(Prod), based on the observations. Nighttime results suggest that, for 190 < T < 200 K, the values for K-Eq (the equilibrium constant, equal to the ratio of k(Prod)/k(Loss)) of Cox and Hayman [1988] and Avallone and Toohey [2001] are in best agreement with the observations.