UV Absorption Spectrum of the ClO Dimer (Cl₂O₂) between 200 and 420 nm

Abstract

The UV photolysis of Cl(2)O(2) (dichlorine peroxide) is a key step in the catalytic destruction of polar stratospheric ozone. In this study, the gas-phase UV absorption spectrum of Cl(2)O(2) was measured using diode array spectroscopy and absolute cross sections, sigma, are reported for the wavelength range 200-420 nm. Pulsed laser photolysis of Cl(2)O at 248 nm or Cl(2)/Cl(2)O mixtures at 351 nm at low temperature (200-228 K) and high pressure (approximately 700 Torr, He) was used to produce ClO radicals and subsequently Cl(2)O(2) via the termolecular ClO self-reaction. The Cl(2)O(2) spectrum was obtained from spectra recorded following the completion of the gas-phase ClO radical chemistry. The spectral analysis used observed isosbestic points at 271, 312.9, and 408.5 nm combined with reaction stoichiometry and chlorine mass balance to determine the Cl(2)O(2) spectrum. The Cl(2)O(2) UV absorption spectrum peaks at 244.5 nm with a cross section of 7.6(-0.5)(+0.8) x 10(-18) cm(2) molecule(-1) where the quoted error limits are 2sigma and include estimated systematic errors. The Cl(2)O(2) absorption cross sections obtained for wavelengths in the range 300-420 nm are in good agreement with the Cl(2)O(2) spectrum reported previously by Burkholder et al. (J. Phys. Chem. A 1990, 94, 687) and significantly higher than the values reported by Pope et al. (J. Phys. Chem. A 2007, 111, 4322). A possible explanation for the discrepancy in the Cl(2)O(2) cross section values with the Pope et al. study is discussed. Representative, atmospheric photolysis rate coefficients are calculated and a range of uncertainty estimated based on the determination of sigma(Cl(2)O(2))(lambda) in this work. Although improvements in our fundamental understanding of the photochemistry of Cl(2)O(2) are still desired, this work indicates that major revisions in current atmospheric chemical mechanisms are not required to simulate observed polar ozone depletion.

Spatial, temporal, and vertical variability of polar stratospheric ozone loss in the Arctic winters 2004/2005–2009/2010

“…The absorption crosssections of Cl 2 O 2 are taken from Burkholder et al (1990) and are extrapolated to 450 nm. They are in very good agreement with the most recent Cl 2 O 2 spectrum measurements by Papanastasiou et al (2009). Monthly varying H 2 SO 4 fields leading to the formation of liquid aerosols in the CTM are computed from the outputs of a 2-D-model long-term simulation, which considers the impacts of volcanic eruptions.…”

A Match-based approach to the estimation of polar stratospheric ozone loss using Aura Microwave Limb Sounder observations

“…Such mechanisms were proposed in response to laboratory measurements of chlorine dimer cross sections (Pope et al, 2007) that appeared to challenge the generally accepted understanding of polar ozone loss. These mechanisms have, however, increasingly been shown to be unlikely (World Meteorological Organization, 2014), and more recent laboratory measurements of the same cross sections (Papanastasiou et al, 2009) are more in line with earlier findings than with the Pope et al value. Rex et al (1998) used ozonesonde-based matches to estimate day/night ozone loss partitioning during 1 January to 9 February 1992, finding no significant nighttime loss.…”

Variability in Antarctic ozone loss in the last decade (2004–2013): high-resolution simulations compared to Aura MLS observations

“…The kinetics data are taken from Sander et al (2011) but the Cl 2 O 2 photolysis cross sections from Burkholder et al (1990), with a log-linear extrapolation up to 450 nm (Stimpfle et al, 2004). These cross sections are in good agreement with the recent Cl 2 O 2 measurements by Papanastasiou et al (2009), which form the basis for the JPL 2011 recommendations (i.e., Sander et al, 2011). For each Antarctic winter the model was run from 1 May to 31 October.…”