This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
UV spectroscopic determination of the chlorine monoxide (ClO) / chlorine peroxide (ClOOCl) thermal equilibrium constant
J. Eric Klobasand David M. WilmouthJ. Eric Klobas and David M. Wilmouth J. Eric Klobasand David M. Wilmouth
Harvard John A. Paulson School of Engineering and Applied Sciences and Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA
Harvard John A. Paulson School of Engineering and Applied Sciences and Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA
Received: 22 Oct 2018 – Accepted for review: 27 Oct 2018 – Discussion started: 29 Oct 2018
Abstract. The thermal equilibrium constant between the chlorine monoxide radical (ClO) and its dimer, chlorine peroxide (ClOOCl), was determined as a function of temperature between 228–301 K 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: Keq = 2.16 × 10−27 e(8533 ± 25 K/T) cm3 molecule−1. A second law analysis of the data deviates minimally: Keq = (2.06 ± 1.27) × 10−27 e(8546 ± 123 K/T) cm3 molecule−1. From the slope of the van't Hoff plot in the third law analysis, the enthalpy of formation for ClOOCl is calculated, ∆H◦f (298 K) = 129.9 ± 0.6 kJ mol−1. The equilibrium constant results from this study suggest that the uncertainties in Keq recommended in the most recent (year 2015) NASA JPL Data Evaluation can be significantly reduced.
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Short summary
The thermal equilibrium constant governing the partitioning of the chlorine monoxide radical (ClO) and its dimer, chlorine peroxide (ClOOCl), was measured by broadband UV spectroscopy in the temperature range of 228–301 K. Uncertainty in the value of this equilibrium constant produces significant uncertainty in model determinations of expected polar ozone loss extent. The key results of this study indicate that the currently-recommended uncertainty for this reaction may be reduced.
The thermal equilibrium constant governing the partitioning of the chlorine monoxide radical...