Kinetic and thermochemical studies of the ClO + ClO + M <=> Cl(2)O(2) + M reaction

Dichlorine peroxide (ClOOCl), chloryl chloride (ClCl(O)O) and chlorine chlorite (ClOClO): very accurateab initiostructures and actinic degradation

The structural parameters of the three most stable isomers with formula Cl₂O₂, dichlorine peroxide, chloryl chloride and chlorine chlorite, were determined by high-levelab initiotheory. The photodissociation pathways were investigated.

UV spectroscopic determination of the chlorine monoxide (ClO) / chlorine peroxide (ClOOCl) thermal equilibrium constant

The thermal equilibrium constant between the chlorine monoxide radical (ClO) and its dimer, chlorine peroxide (ClOOCl), was determined as a function of temperature between 228 and 301K in a discharge flow apparatus using broadband UV absorption spectroscopy. A third-law fit of the equilibrium values determined from the experimental data provides the expression K _eq = 2.16 × 10^-27 e ^{(8527±35 K/T)} cm³ molecule^-1 (1σ uncertainty). A second-law analysis of the data is in good agreement. From the slope of the van't Hoff plot in the third-law analysis, the enthalpy of formation for ClOOCl is calculated, Δ H f ° (298 K) = 130.0 ± 0.6 kJ mol^-1. The equilibrium constant results from this study suggest that the uncertainties in K _eq recommended in the most recent (year 2015) NASA JPL Data Evaluation can be significantly reduced.

Kinetics of the ClO + CH3O2 reaction over the temperature range T = 250-298 K

The kinetics of the potentially atmospherically important ClO + CH3O2 reaction (1) have been studied over the range T = 250-298 K at p = 760 Torr using laser flash photolysis radical generation, coupled with time resolved ultraviolet absorption spectroscopy, employing broad spectral monitoring using a charge coupled device detector array. ClO radicals were monitored unequivocally using this technique, and introduction of CH3O2 precursors ensured known initial methylperoxy radical concentrations. ClO temporal profiles were thereafter analysed to extract kinetic parameters for reaction (1). A detailed sensitivity analysis was also performed to examine any potential systematic variability in k1 as a function of kinetic or physical uncertainties. The kinetic data recorded in this work show good agreement with the most recent previous study of this reaction, reported by Leather et al. The current work reports an Arrhenius parameterisation for k1, given by: . This work therefore concurs with that of Leather et al. implying that the title reaction is potentially less significant in the atmosphere than inferred from preceding studies. However, reaction (1) is evidently a non-terminating radical reaction, whose effects upon atmospheric composition therefore need to be ascertained through atmospheric model studies.

Kinetics of the ClO + HO2 reaction over the temperature range T = 210–298 K

Temporal traces showing the effect of increasing added methanol (red → orange) to a Cl/Cl₂O photolysis system, showing the progressive reduction of [ClO]₀ with increased [CH₃OH] but an increased rate of ClO loss at lower [ClO]₀, indicative of the ClO + HO₂ reaction.

Kinetic studies of the BrO + ClO cross-reaction over the range T = 246-314 K

The kinetics of the atmospherically important gas phase radical reaction between BrO and ClO have been studied over the temperature range T = 246-314 K by means of laser flash photolysis coupled with UV absorption spectroscopy. Charge-coupled-device (CCD) detection allowed simultaneous monitoring of both free radicals and the OClO product using 'differential' spectroscopy, which minimised interference from underlying UV absorbing species. In this way, the total rate coefficient for BrO + ClO → products (1) was measured, along with that for the OClO producing channel of this process BrO + ClO → OClO + Br (1c). These reaction rate coefficients are described by the Arrhenius expressions: k1/cm(3) molecule(-1) s(-1) = (2.5 ± 2.2) × 10(-12) exp[(630 ± 240)/T] and k(1c)/cm(3) molecule(-1) s(-1) = (4.6 ± 3.0) × 10(-12) exp[(280 ± 180)/T], where errors are 2σ, statistical only. An extensive sensitivity analysis was performed to quantify the potential additional systematic uncertainties in this work arising from uncertainties in secondary chemistry, absorption cross-sections and precursor concentrations. This analysis identified the reactions of initial and secondarily generated bromine atoms (specifically Br + O3 and Br + Cl2O) as particularly important, along with the reversible combination of ClO with OClO forming Cl2O3. Potential uncertainty in this latter process was used to define the lowest temperature of the present study. Results from this work indicate larger absolute values for k1 and k(1c) than those reported in previous studies, but a weaker negative temperature dependence for k(1c) than previously observed, resulting in a branching ratio for with a positive temperature dependence, in disagreement with previous studies. is the principal source of OClO in the polar stratosphere and is commonly used in atmospheric models as an indicator of stratospheric bromine chemistry. Thus these measurements might lead to a reinterpretation of modelled stratospheric OClO, which has also been suggested by previous comparisons of observations with atmospheric model studies.