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

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

(Parte 1 de 5)

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

Dimitrios K. Papanastasiou,†,‡ Vassileios C. Papadimitriou,†,‡,§ David W. Fahey,†,‡ and James B. Burkholder*,†

Earth System Research Laboratory, Chemical Sciences DiVision, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, Colorado 80305-3328, and CooperatiVe Institute for Research in EnVironmental Sciences, UniVersity of Colorado, Boulder, Colorado 80309

ReceiVed: July 10, 2009; ReVised Manuscript ReceiVed: September 28, 2009

The UV photolysisof Cl2O2 (dichlorineperoxide) is a key step in the catalytic destructionof polar stratospheric ozone. In this study, the gas-phase UV absorption spectrum of Cl2O2 was measured using diode array spectroscopy and absolute cross sections, σ, are reported for the wavelength range 200-420 nm. Pulsed laser photolysis of Cl2O at 248 nm or Cl2/Cl2O mixtures at 351 nm at low temperature (200-228 K) and high pressure (∼700 Torr, He) was used to produce ClO radicals and subsequently Cl2O2 via the termolecular ClO self-reaction. The Cl2O2 spectrum was obtained from spectra recorded following the completion of the gasphase 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 Cl2O2 spectrum. The where the quoted error limits are 2σ and include estimated systematic errors. The Cl2O2 absorption cross sections obtained for wavelengths in the range 300-420 nm are in good agreement with the Cl2O2 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, 1, 4322). A possible explanation for the discrepancy in the Cl2O2 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 σCl2O2(λ) in this work. Although improvements in our fundamental understanding of the photochemistry of Cl2O2 are still desired, this work indicates that major revisions in current atmospheric chemical mechanisms are not required to simulate observed polar ozone depletion.

1. Introduction

Ozone depletion in the Arctic and Antarctic stratosphere during winter and spring is well-established from a variety of satellite, airborne, and ground-based observations.1 In the Antarctic, ozone depletion is extensive and leads to what is commonly called an “ozone hole”. It is recognized that halogen chemistry involving Cl- and Br-containing species is the principalcause of ozone depletionobservedin the polar regions.2 The link between ozone depletion and emission of man-made halogen compounds, e.g., chlorofluorocarbons (CFCs) and bromofluorocarbons (halons), led to the Montreal Protocol and its subsequent amendments and adjustments which regulate global production and consumption of these ozone depleting compounds. Polar ozone depletion alters the composition of Earth’s atmosphere and has long-term implications through its impact on climate.Atmosphericmodels constrainedby observations of ozone-depleting compounds predict that the polar and global ozone abundance levels will not return to pre-1980s values (i.e., before the Antarctic stratospheric ozone hole) until later than 2050.3 Accurate models of current and future atmospheric ozone depletion and recovery require a thorough understanding of the relevant gas-phase and heterogeneous halogen chemistry under the conditions found in the polar regions.

The majority of polar stratospheric ozone loss in current atmospheric model calculations4 is from the “ClO dimer” catalytic ozone destruction cycle that was first proposed by Molina and Molina5

where Cl2O2 (dichlorine peroxide, ClOOCl) is referred to as the ClO dimer. A discussion of other catalytic ozone destruction cycles is given elsewhere.2 Each of the elementary reactions in the ClO dimer cycle has been studied extensively over the last several decades using a variety of laboratory techniques and are consideredreasonablywell established.6 Atmosphericmodel calculations show that in the polar stratosphere the rate of Cl2O2 UV photolysis, reaction 2, is critical in determining the rate

* Corresponding author, James.B.Burkholder@noaa.gov. † Earth System Research Laboratory, Chemical Sciences Division,

National Oceanic and Atmospheric Administration.

‡ Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder.

§ Current address: Laboratory of Photochemistry and Chemical Kinetics,

Department of Chemistry, University of Crete, Vassilika Vouton, 71003, Heraklion, Crete, Greece.

10.1021/jp9065345 C: $40.75 2009 American Chemical Society Published on Web 1/06/2009

and extent of ozone depletion. The Cl2O2 photolysis rate is determined from the wavelength, λ, dependence of the overlap of its absorptionspectrumwith the solar flux, which is a function of solar zenith angle, SZA. For photolysis of Cl2O2 the most important wavelength region is >300 nm, the actinic region.

The photolysis quantum yield for Cl2O2 in this wavelength region is thought to be unity although only limited studies are currentlyavailable.7,8 AlthoughUV photolysisof Cl2O2 has been clearly shown to be critically important in the ClO dimer catalytic ozone destruction cycle, there exist considerable discrepancies in its UV absorption spectrum, particularly in the most critically important wavelength region. An objective of the present study was to resolve discrepancies in the Cl2O2 UV absorption spectrum using laboratory measurements capable of determining the Cl2O2 spectrum in this critically important wavelength region.

There are a number of experimental and theoretical studies for the UV absorption spectrum of Cl2O2 currently available in the literature including a previous study from our laboratory.9

A summary of the reported Cl2O2 spectra is shown in Figure 1.

It is well established that the Cl2O2 UV absorption spectrum is continuous with a maximum near 245 nm. However, in the wavelength region λ >300 nm the reported spectra are in poor agreement, which leads to significant levels of uncertainty in the calculated Cl2O2 atmospheric photolysis rate. The recent

Pope et al.10 study reported Cl2O2 absorption cross sections at λ >300 nm that are significantly lower than those reported in all previous studies. With Cl2O2 photolysis rates derived from the Pope et al.10 spectrum,models producevalues of ClO, Cl2O2, and ozone and its loss rate that are in poor agreement with in situ and remote-sensing observations.1 The Pope et al.10 study, which if correct, would, therefore, have significant implications for the current understanding of halogen chemistry in the polar stratosphere. Furthermore, if Pope et al.10 were correct, it would mean that the earlier Burkholder et al.9 study conducted at NOAA was in error outside the stated uncertainty limits.

Therefore,an objective of this study was to remeasure the Cl2O2 spectrum with a technique that was independent of that used by Burkholder et al.9

In this paper, we present a detailed study of the Cl2O2 UV absorption spectrum and report absorption cross sections over the wavelength range 200-420 nm. The Cl2O2 UV absorption spectrum was measured using complementary experimental techniques and several sources of Cl2O2 in order to minimize possible systematic errors. The primary experimental method used was pulsed laser photolysis of Cl2Oo rC l2O/Cl2 mixtures, at low temperature and high pressure, to produce ClO radicals and subsequently Cl2O2 via reaction 1, combined with diode array spectroscopy. The Cl2O2 cross sections obtained in this work at λ >300 nm are in good agreement with the values reported previously by Burkholder et al.,9 substantially higher than the values reported by Pope et al.,10 and greater than the values currently recommended by NASA/JPL6 for use in atmospheric models. This work therefore yields larger Cl2O2 atmospheric photolysis rates and more efficient catalytic ozone destructionin the polar stratospherethan those obtainedby using the current recommendation. Representative polar stratospheric photolysisrate coefficients,J, are calculatedand the uncertainties imposed by the estimated Cl2O2 cross section uncertainty are discussed.

2. Experimental Details

The UV absorption spectrum of Cl2O2 was determined from data obtained using the apparatus shown in Figure 2. The primary experimental method used pulsed UV laser photolysis

(PLP) of gas-phase Cl2Oo rC l2O/Cl2 mixtures to produce ClO radicals and subsequently Cl2O2 via the termolecular ClO selfreaction, reaction 1, at low temperatures (200-228 K) and high pressure (∼700 Torr). Diode array (DA) spectroscopy was used to measure UV absorption spectra of the reaction mixture that contained Cl2O2,C l2O, and Cl2 after completion of the ClO radical gas-phase chemistry and the formation of Cl2O2. Sequential pulsed photolysis of the sample and the recorded

UV absorption spectra were used to deduce the Cl2O2 UV absorption spectrum and its absolute cross section values. In the sequence of photolysis steps the partitioning between Cl2O2,

Cl2O, and Cl2 changed while the total chlorine content remained nearly constant (mass balance). Absolute absorption cross sections for Cl2O2 were determined at and near the peak of its UV spectrum, 244.5 nm, and other wavelengths in the range

200-420 nm on the basis of three observed isosbestic points, reactionstoichiometry,and chlorinemass balance.(An isosbestic point is defined as the waVelength where the total absorbance does not change during a chemical reaction.) A second experimental method employed in this study utilized a cold trapping (CT) technique recently developed by Pope et al.10 in which bulk Cl2O2 samples are collected in the solid phase.

Thermal desorption (TD) of the samples containing Cl2O2 collected using different trapping temperatures enabled a

Figure 1. Comparison of Cl2O2 UV absorption spectra and cross section values currently available in the literature. The figure illustrates the large scatter in Cl2O2 cross section values, particularly in the actinic region. The data for gas-phase Cl2O2 spectra recorded at temperatures in the range 195-296 K were taken from Burkholder et al.9 (blue line),

DeMore and Tschuikow-Roux13 (brown line), Huder and DeMore16

(light blue line, σCl O (λ >310 nm) based on extrapolation), Cox and Hayman15 (dotted line), and Pope et al.10 (green line). Cross section values at discrete wavelengths are from Bloss et al.17 (triangle), Molina et al.7 (diamond), and Chen et al.19 (squares). The data from Molina et al. and Chen et al. are plotted assuming a Cl2O2 photolysis quantum yield of unity. The Cl2O2 spectrum in a Ne matrix at 10 K from von Hobe et al.14 (long dashed line) and the NASA/JPL6 recommendation

(red line) for atmospheric modeling are included for comparison purposes. The earlier studies by Basco and Hunt24 and Molina and Molina5 have since been shown9 to have incorrectly identified the UV spectrum of Cl2O2 and are therefore not included.

13712 J. Phys. Chem. A, Vol. 113, No. 49, 2009 Papanastasiou et al.

qualitative measurement of the Cl2O2 absorption spectrum wavelength dependence and an evaluation of the influence of the trapped Cl2 impurity on the measured spectra. A description of the experimental setups and procedures used in this study is given below. 2.1. Diode Array Spectrometer. UV absorption spectra of gas-phase species were measured by passing a collimated beam froma3 0WD 2 lamp through a temperature-regulated reactor/ absorption cell. Two different absorption cells were used as shown in Figure 2 with pathlengths of ∼90 cm. The D2 beam exiting the reactor was focused onto the entrance slit of a 0.5 m spectrometer equipped with a 1024 pixel diode array detector. The spectrometer used an entrance slit width of 150 µm and a 150 grooves m-1 holographic grating. Spectra were measured over the wavelength range 190-530 nm at a resolution of ∼1 nm. Emission lines from an Hg pen-ray lamp were used to calibrate the wavelength scale. Typical detector exposure times were 0.3 s, and spectra were obtained from the average of 10 detector readings with a total acquisition time of ∼10 s.

Absorption,A, spectrawere calculatedusing Beer-Lambert’s law

where I(λ) and I0(λ) are the measured signals at wavelength λ with and without absorbing species present in the absorption cell, respectively, σi is the absorption cross section of species i at wavelength λ, L is the absorption cell path length, and [x]i is the concentration of species i. The peak-to-peak noise level of an absorption spectrum was typically ∼5 × 10-4 absorbance units. Over the duration of an experiment that typically lasted

15 min or less, the drift of the D2 lamp intensity was <0.001 absorbance units at all wavelengths.

Absorptionspectra were measuredin pairs using two separate

D2 lamps as shown in Figure 2 to help minimize the influence of second order and scattered light effects. One D2 lamp was used to measure absorption spectra over the entire wavelength range of the diode array spectrometer, 190-530 nm. For the other D2 lamp, a long-pass filter was used in the beam path to remove short wavelength radiation and minimize second-order and scattered light effects in the spectrometer. Different combinations of band-pass and long-pass filters were used over the course of our study to confirm the accuracyof the absorption spectra. The majority of the experiments were performed using a WG280 (λ >280 nm) long-pass filter. To improve optical system stability, ease of use, and speed with which spectra were recorded, two separate fixed D2 lamp setups were used rather than changing optical elements in the beam path during an experiment. The signal intensities from the two lamps were balanced by inserting a neutral density filter in the beam path of the unfiltered lamp. Rapid switching between lamps, a delay of ∼1 s, was accomplished using automated beam blocks. Absorption spectra were obtained by combining the two spectra by averaging the signals in the overlapping wavelength region. The agreementin the overlappingwavelengthregionwas always better than 2%. 2.2. Photolysis Sources. An excimer laser operating at 248

(KrF) or 351 (XeF) nm was used as the photolysis source in the pulsed laser photolysis diode array, PLP-DA, experiments, method 1. Laser fluences in the reaction cell were in the range

Figure 2. Schematic of experimental apparatus used to study the UV Cl2O2 absorption spectrum: (a) pulsed laser photolysis diode array (PLP-DA) setup where M is mirror, RM is a positive position movable dielectric mirror, F is an optical filter, ND is a neutral density filter, BS is a broad band beam splitter, and B is beam block; (b) cold trap thermal desorption (CT-TD) setup where TC is thermocouple.

UV Absorption Spectrum of the ClO Dimer J. Phys. Chem. A, Vol. 113, No. 49, 2009 13713

4-7m Jc m-2 pulse-1 for 248 nm and 2-4m Jc m-2 pulse-1 for 351 nm as measured with a power meter at the exit of the reactor. The photolysis beam made a single pass through the reactor and nearly filled the reactor volume. For the cold trap thermal desorption, CT-TD, experiments, method 2, the photolysis source was an excimer laser operating at 351 nm, with two passes through the reactor, or a continuous wave 150 W Xe arc lamp. The beam from the Xe arc lamp reflected off a UV cold mirror and passed through a UG5 band-pass filter (220 nm < λ < 430 nm) and a WG320 long-pass filter (λ >320 nm) before entering the photolysis cell. The effective wavelength range used for photolysis was 320-430 nm, and the photolysis beam made a single pass through the reactor. The Xe arc lamp light source was used in the majorityof the CT-DT experiments.

2.3. Production of Cl2O2. Several chemical schemes were

used over the course of the study to produce gas-phase Cl2O2. The chemical schemes used different photolytic sources to produce the ClO radical in the gas phase and were used to evaluate the influence of secondary chemistry and spectrally interfering species. All experiments were performed at low temperature (200-228 K) and high pressure (∼700 Torr, He) to optimize the production of Cl2O2 via reaction 1 and minimize ClO loss via its bimolecular self-reaction

(Thermochemical parameters, rate coefficients, quantum yields, and absorption cross section data quoted in this paper are taken from Sander et al.6 unless noted otherwise.) At the lowtemperature and high-pressure conditions used in our experiments reaction 5 was a minor loss for ClO radicals compared

of Cl2O2 such that the ClO concentration at equilibrium is small (<4 × 1012 molecules cm-3) and below our detection limit in experiments performed at temperatures <210 K. The ClO radical precursor used in the majority of our 248

nm pulsed photolysis experiments was Cl2O photolysis

(Parte 1 de 5)