Atmos. Chem. Phys., 19, 6205–6215, 2019 https://doi.org/10.5194/acp-19-6205-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. UV spectroscopic determination of the chlorine monoxide (ClO) = chlorine peroxide (ClOOCl) thermal equilibrium constant J. Eric Klobas1,2 and David M. Wilmouth1,2 1Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA 2Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA Correspondence: J. Eric Klobas (
[email protected]) Received: 22 October 2018 – Discussion started: 29 October 2018 Revised: 19 April 2019 – Accepted: 24 April 2019 – Published: 10 May 2019 Abstract. The thermal equilibrium constant between the net V 2O3 ! 3O2 (R5) chlorine monoxide radical (ClO) and its dimer, chlorine peroxide (ClOOCl), was determined as a function of tem- Within this cycle, the equilibrium governing the partitioning perature between 228 and 301 K in a discharge flow ap- of ClO and ClOOCl in Reaction (R1) is defined as follows. paratus using broadband UV absorption spectroscopy. A [ClOOCl] third-law fit of the equilibrium values determined from Keq D (1) [ClO]2 the experimental data provides the expression Keq D 2:16 × 10−27e.8527±35 K=T / cm3 molecule−1 (1σ uncertainty). A This thermal equilibrium is a key parameter that deter- second-law analysis of the data is in good agreement. From mines the nighttime partitioning of active chlorine in the the slope of the van’t Hoff plot in the third-law analy- winter–spring polar vortex.