Observation of a Heterogeneous Source of OClO from the Reaction of ClO Radicals on Ice

Abstract

Experiments presented in this contribution demonstrate a heterogeneous source of several chlorine oxides, in particular OClO and ClClO 2 , from ClO radicals passed over water-ice surfaces at low pressures. ClO radicals were generated in a flow system and reaction products were monitored by UV-vis absorption spectroscopy and time-of-flight mass spectrometry using electron-impact and resonance-enhanced multiphoton ionization, respectively. In all experiments an efficient release of OClO into the gas phase was observed upon ice eVaporation. No such gas-phase products directly formed in the initial interaction of ClO with the ice surface were detected. To explain these findings, it is proposed that a ClO‚H 2 O complex is formed in a rapidly established equilibrium between ClO monomers and gas-phase H 2 O. The existence of this complex as well as a surface-enhanced ClO-recombination process is assumed to be responsible for the observed efficient reactive uptake of ClO radicals onto the ice surface. Several possible reaction pathways resulting in the formation of the experimentally observed products are presented. As an alternative pathway, H 2 O-facilitated gas phase disproportionation of ClO yielding hypochlorous and chlorous acid and subsequent deposition on the surface is considered. The observation of OClO evolving from an ice surface previously exposed to ClO radicals, as well as the lack of any symmetric ClOOCl dimer formation in the presence of high water mixing ratios, carries some possible atmospheric implications: First, there is currently a missing source of OClO in the chemically perturbed polar vortex. The ClO + BrO reaction is presently believed to be the only source of OClO in the stratosphere, although several studies show this reaction system to severely underestimate OClO production in this atmospheric subsystem. It is suggested that this experimentally observed heterogeneous source of OClO could carry implications for the total atmospheric OClO budget. Second, the ClO‚H 2 O complex could directly or indirectly affect the ClOOCl formation rate and thus strongly impact homogeneous chlorine chemistry.

Atmospheric origins of perchlorate on Mars and in the Atacama

“…A role for water ice as a reaction surface for adsorbed species may be a possible path to perchlorate chemistry. ClO radicals are readily produced by atmospheric oxidation of chlorine volatiles, and ClO can interact on ice to produce OClO when the ice sublimates [ McKeachie et al , 2004]. Chlorine dioxide (the OClO molecule is not to be confused with Cl‐O‐O, which is the chlorine peroxy radical, unfortunately also sometimes called “chlorine dioxide”) is a possible source of perchlorate because it can react with O 3 or O to make chlorine trioxide [ Wayne et al , 1995]: …”

Photodissociation cross sections of ClOOCl at 248.4 and 266 nm

“…To check the effect of the mass filter at different m/z setting, we repeated the experiment in which we detected the daughter ions of ClOOCl. After the rescaling, the overall consistency among the scaled spectra and the molecular beam results at 266 and 308 nm is significantly improved, except that the spectrum by McKeachie et al 14 16 prepared a pure ClOOCl sample in a neon matrix, the light scattering from the solid matrix limited their sensitivity in the absorbance determination; as a result, the weak absorption of ClOOCl at Ͼ300 nm could not been determined precisely. After correction for the ion flight times, a daughter ion should exhibit the same arrival time as its parent ion.…”

Characterization of two types of intermolecular interactions on halogen monoxide monohydrates

“…In addition, new laboratory data of absorption cross section of ClOOCl have revealed that the photolysis rate of this crucial molecule should be much slower than it was previously assumed 11. As a consequence of these new data, there is a renewed open debate about the chemical processes responsible for the “ozone hole”12, 13 and several investigations have been conducted in the last years to complete the understanding of this chemistry 14–16…”