Monday, 11 January 2016
The most important reaction cycle controlling the rate of polar ozone loss is the ClOOCl mechanism, first introduced by Molina and Molina in 1987, in which chlorine peroxide is photolyzed to produce Cl atoms, which destroy stratospheric ozone; the ClO products then self-react to re-form ClOOCl. Importantly, the photolysis reaction is the rate-limiting step in the ClOOCl catalytic sequence, and thus the absorption cross sections and the quantum yield of Cl from ClOOCl photolysis are critical for constraining ozone loss rates in the atmosphere. Significant challenges have been encountered in many past laboratory studies aimed at accurately determining the cross sections of ClOOCl due to difficulty in isolating the spectrum from other absorbing species, either added or produced when chemically forming ClOOCl. In particular, the uncertainty in correcting ClOOCl laboratory spectra for the presence of molecular chlorine, which has an unstructured absorption that peaks at approximately 330 nm, has led to reported ClOOCl cross sections that differed by more than an order of magnitude in some of the most atmospherically relevant wavelength regions. Several recent published studies have served to better constrain the ClOOCl cross section and quantum yield values, but significant uncertainties remain. While the UV cross sections set the photolysis rate, which controls ClOOCl concentrations during the daytime, the equilibrium constant establishes the concentrations of ClOOCl during nighttime. The lab-based, JPL-11 recommended equilibrium constant includes high error bars at atmospherically relevant temperatures (~75% at 200 K) and does not agree well with equilibrium constants empirically determined from in situ atmospheric data. Here we present results of an analysis of available ClOOCl ultraviolet cross section, quantum yield, and equilibrium constant data and present new laboratory spectroscopic results.
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