Mutual combination of ClO radicals

Abstract

The mutual combination reactionis proposed as the rate-limiting step in the removal of C10 radicals a t moderate pressures. The third-order rate constants measured a t room temperature were kl(Ar) = 3.51 f 0.14 X lo9 lz/mo12.sec, kl(He) = 2.8 X lo9 l2/molZ.sec, and k l ( O 2 ) 0 7.9 X lo9 12/mo12-sec. There is also an independent second-order reaction2C10 -products for which hs = 8 X lo6 I/mol.sec. A new absorption spectrum has been observed in the ultraviolet and attributed to ClzOz. The extinction coefficient for C1202 has been measured a t six wavelengths, and, between 292 and 232 nm, it increases from 0.4 X lo3 to 2.9 X lo3 1/ mol-cm. In the presence of the chlorine atom scavengers OClO or ClzO, ClzOz exists in equilibrium with C10. The equilibrium constant Kel = 3.1 f 0.1 X lo6 l/mol a t 298 K, and, with AS? estimated to be -133 f 11 J/K-mol, AH: = -69 f 3 kJ/mol and AHP(Clz0z) = 136 f 3 kJ/mol.

Understanding the kinetics of the ClO dimer cycle

“…No laboratory measurements of product yields are available for wavelengths longer than 308 nm, which represents a considerable gap in laboratory confirmation of ozone loss by the ClO+ClO cycle. The absorption cross section of Cl 2 O 2 , σ ClOOCl , has been determined in a number of laboratory studies (Basco and Hunt, 1979;Molina and Molina, 1987;Permien et al, 1988;Cox and Hayman, 1988;DeMore and Tschuikow-Roux, 1990;Burkholder et al, 1990;Huder and DeMore, 1995;Pope et al, 2007). The spectra obtained by Basco and Hunt (1979) and Molina and Molina (1987) are significantly different in shape from the other studies and have been proposed to be influenced by Cl 2 O 3 and possibly other impurities.…”

“…A large number of laboratory studies have addressed K eq (Basco and Hunt, 1979;Cox and Hayman, 1988;Nickolaisen et al, 1994;Ellermann et al, 1995;Plenge et al, 2005), k rec Trolier et al, 1990;Nickolaisen et al, 1994;Bloss et al, 2001;Boakes et al, 2005) and σ ClOOCl (Basco and Hunt, 1979;Molina and Molina, 1987;Permien et al, 1988;Cox and Hayman, 1988;DeMore and Tschuikow-Roux, 1990;Burkholder et al, 1990;Huder and DeMore, 1995;Pope et al, 2007). For each of these parameters, large discrepancies exist that often cannot be explained by the reported uncertainty limits.…”

“…(3) Figure 1a shows various laboratory measurements of K eq at different temperatures, and the temperature dependence resulting from the analysis of these data described above. Basco and Hunt (1979) and Ellermann et al (1995) only measured K eq at 298 K so that no robust deterimnation of the temperature dependence of K eq and thus r H • can be obtained from these studies. Even though concerns have been raised about their reliability (Crowley, 2006), they are included for completeness and seem to be in good agreement with Cox and Hayman (1988).…”

“…We start by using statistical thermodynamics to constrain K eq , and exploit the result together with corresponding thermodynamic properties to successively constrain k diss and k rec with the help of unimolecular rate theory as developed by Troe (1977aTroe ( , b, 1979 The equilibrium of Reaction (R1) and its temperature dependence have been addressed in numerous studies. Laboratory measurements of K eq have been carried out by Basco and Hunt (1979), Cox and Hayman (1988), Nickolaisen et al (1994) and Ellermann et al (1995). Avallone and Toohey (2001) and von Hobe et al (2005) …”

Evaluation of stratospheric chlorine chemistry for the Arctic spring 2005 using modelled and measured OClO column densities

“…However, the change in absorption cross section causes simply an offset relative to the original model and does not explain the diurnal behaviour of the OClO. (Basco and Hunt, 1979;Cox and Hayman, 1988;Horowitz et al, 1994;Nickolaisen et al, 1994;Plenge et al, 2005;Bröske and Zabel, 2006). The JPL 2002 recommendations are an average of Cox and Hayman (1988) and Nickolaisen et al (1994), which were then extrapolated to the temperatures prevailing in the atmosphere.…”