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 4 de 5)

The spectrum peaks at 244.5 nm with a cross section value of 7.6 × 10-18 cm2 molecule-1. Toward shorter wavelengths the spectrum shows a minimum at ∼218 nm and the onset of another strong absorption feature whose maximum is outside the wavelength range of our measurements. The absorption features at longer wavelength appear as shoulders on the main absorptionband at 270 and 340 nm. The diffuseabsorptionband near 340 nm correlates with the Cl2 absorption spectrum.

Therefore, the accuracy of the Cl2O2 absorption spectrum in this wavelength region is dependent on accurately accounting for absorption by Cl2. Although the uncertainty in our measurements at wavelengths longer than 380 nm is large, >50%, the observationof an isosbesticpointat 408.5nm in our experiments indicates that Cl2O2 absorbs in this wavelength region. 3.4. Error Analysis. Systematic and absolute uncertainties in our measurements affect both the wavelength dependence of the Cl2O2 absorption spectrum and its absolute absorption cross sections. The high precision of our absorption measure- ments was the result of the stability of the D2 lamp intensity and optical setup. In the wavelength range 200-280 nm, the uncertainty in the Cl2O2 absorbance due to the measurement precision was small, <1%. At longer wavelengths where the absorptioncross sectionsfor Cl2O2 are smaller,the measurement precision makes a larger contribution to the overall uncertainty:

Figure 9 shows the Cl2O2 absorption spectrum obtained in this work along with the estimated uncertainty as a function of wavelength. The estimated uncertainty in the Cl2O2 absolute cross sections is 5-10% (2σ confidence level) at and around the peak of the Cl2O2 spectrum, near 245 nm. A parametrization for the estimated uncertainties in the actinic region is given in the Supporting Information. The estimated uncertainty includes the uncertainty associated with the spectral subtraction of Cl2O and Cl2, which does not contribute significantly to the overall uncertainty, <1%. The uncertainty in the isosbestic wavelength at 312.9 nm is estimated to be +0.3/-0.5 nm. The Cl2O2 spectrum obtained using the lower limit of the isosbestic wavelength in the spectral analysis would increase the Cl2O2 cross section in the wavelength region 330-350 nm as a result of smaller Cl2 subtraction. The increase would be 16% at 350 nm for an isosbestic wavelength of 312.4 nm and a decrease of

10% for an isosbestic wavelength of 313.2 nm.

The presence of unaccounted for impurities was examined experimentallyand determined to be negligible.The most likely impuritiesare OClO and Cl2O3 whileClClOOand otherchlorine oxides were also considered. OClO, which has a strong and readily detectable structured UV/vis absorption band, did not make a significant contribution to our measured spectra. We estimate that Cl2O3 made a negligible contribution, <1% at 265 nm, to our measured absorption spectra. ClClOO, an isomer of

Cl2O2, absorbs strongly in the UV. On the basis of our detection limit for ClClOO, ∼7 × 1011 molecules cm-3, we estimate that its contribution to the measured absorption spectra was <1% at all wavelengths.

The influence of secondary chemistry is difficult to quantitativelyevaluate.The high degree of consistencyfor the residual

Figure 9. The Cl2O2 absorption spectrum obtained in this work (heavy black line) including the estimated error limits as a function of wavelength given by the shaded region (see text of discussion of error analysis). The absorption cross sections at the isosbestic points (271, 312.9, and 408.5 nm) from this work are also shown (red circles). The heavy dashed line gives an exponential extrapolationof our Cl2O2 cross section data beyond the range of our measurements. The light gray shaded region shows the extrapolation of the estimated cross section uncertainty. See the Supporting Information for the cross section data and extrapolation parameters. The absorption cross section data from Burkholder et al.9 (blue line), Bloss et al.17 (triangle), and Pope et al.10 (green line) as well as the NASA/JPL recommendation6 (red line) are included for comparison. The data from Chen et al.19 (squares) at 308 and 351 nm are given assuming a Cl2O2 photolysis quantum yield of unity at these wavelengths.

UV Absorption Spectrum of the ClO Dimer J. Phys. Chem. A, Vol. 113, No. 49, 2009 13721

absorptionspectraobtainedin the PLP-DAexperimentssuggests however that it is small. The photolysis of Cl2O at 248 nm is reported to yield >90% Cl atoms. Therefore, a small yield of O atoms is possible. O atoms would react with the Cl2O that is present in large excess initially of two ClO radicals and loss of Cl2O lead to the same overall reaction stoichiometry as obtained if no O atoms were formed.

In subsequent photolysis steps O atoms could possibly react where the rate coefficient for this reaction is currently unknown.

The agreement between our results obtained using Cl2O (248 nm) and Cl2/Cl2O (351 nm) photolysis sources indicates the possible small yield of O atoms in reaction 6 did not influence the determination of the Cl2O2 spectrum significantly. In addition to the uncertainties associated with the absorption measurements and spectral analysis, the breakdown of the reaction stoichiometry due to loss of ClO or Cl2O2 influences our results. It is difficult to estimate this uncertainty, but it is probably <10%. The stoichiometry was shown to be valid for

Cl2 where we obtainedgood agreementfor its peak cross section with the recommended value. Unaccounted for loss of Cl2O2 or ClO would lead to an overestimationof the Cl2O2 concentration and consequently an underestimation of the peak Cl2O2 absorption cross section. Finally, the Cl2O2 absorption cross molecule-1 were the quoted uncertainties are at the 2σ level and include estimated systematic errors. 3.5. ComparisonwithPreviousStudies.Thereare a number of publishedstudiesof the Cl2O2 UV absorptionspectrum,9,10,13-16 its absolute cross section values,9,13,15,17 and photolysis products and quantum yields7,8,18,19 that we can compare with the present work. A summary of previously reported Cl2O2 UV absorption spectrum data is given in Figure 1 and summarized in Table 1.

The spectrum recommended by NASA/JPL6 for use in atmo- spheric modeling studies is included in Figures 1 and 9 for comparison purposes. The IUPAC20 evaluation panel currently recommends the Cl2O2 spectrum and extrapolation for the long wavelength region reported in Huder and DeMore.16 The discrepancies among the previous studies and comparison with the results from the present study are discussed here.

Although there have been numerous studies of the Cl2O2 UV absorption spectrum reported over the past 30 years, the overall level of agreement among the various data sets is rather poor. The scatter in the data is primarily associated with difficulties in preparing, purifying, and handling gas-phase samples of the transient Cl2O2 molecule and measuring its weak absorption cross sections in the actinic region. The scatter in the laboratory data translates directly into uncertainties in the calculated atmospheric photolysis rate for Cl2O2. The highest level of agreement among these studies is in and around the peak of the Cl2O2 spectrum near 245 nm, with differences of <10% in most cases. Note that several of the studies shown in Figure 1 do not include absolute cross section determinations and have been normalized to the peak cross section recommended by NASA/JPL. As shown in Figure 1, the largest discrepancies are in the long wavelength region, λ >300 nm, which is the region most important for calculating atmospheric photolysis rates. The studies of Cox and Hayman,15 Burkholder et al.,9 and

DeMore and Tschuikow-Roux13 report high Cl2O2 cross section values in the 250-300 nm range and may have been influenced by spectral impurities, the primary impurity being Cl2O, which was used as a radical precursor, while the Cox and Hayman15 and Burkholder et al.9 studies also show evidence for the possible interference from Cl2O3. For λ >300 nm, Pope et al.10 reportedthe lowest Cl2O2 cross sectionvalues,while Burkholder et al.9 has reported the highest values. The differences are large, nearly a factor of 10 at 350 nm, which results in substantial and significant differences in the calculated atmospheric photolysis rates. It is worth noting that the studies prior to Pope et al.10 also show discrepancies in this wavelength region, Figure

1, although the differences are much smaller. The Cl2O2 spectrum obtained in the present work is in excellent agreement with results from Burkholder et al. for λ >300 nm. Chen et al.19 used a newly developed experimental technique that combined pulsed laser photolysis in a molecular beam with mass spec- trometric detection of Cl2O2. Their experiments measured the

TABLE 1: Summary of Cl2O2 UV Absorption Spectrum Studies reference year temperature (K) technique wavelength (λ) (nm) 1020σ(λ) (cm2) a Absorption cross section value at the peak of the spectrum, λ ) 245 nm. Cross section data reported over the range of wavelengths given.

b Reported Cl2O2 absorption spectrum normalized to the recommended peak absorption cross section. c Cl2O2 absorption spectrum measured in a Ne matrix. d Cl2O2 absorption spectrum reported at λ >310 nm is reported in Huder and DeMore using a log-linear extrapolation of values measured at shorter wavelengths. e Cl2O2 cross section calculated here assuming a unit Cl2O2 photolysis quantum yield at these wavelengths. B, broad band photolysis; DA, diode array spectroscopy; DF, discharge flow; FTIR, Fourier transform infrared spectroscopy; PLP, pulsed laser photolysis; TA, transient absorption; MS, mass spectrometry, CT-TD, cold trap thermal desorption; MI, matrix isolation.

13722 J. Phys. Chem. A, Vol. 113, No. 49, 2009 Papanastasiou et al.

product of the Cl2O2 absorption cross section and its photolysis quantum yield, σCl O (λ)Φ(λ). The experimental method is independent of Cl2 and therefore not susceptible to uncertainties associated with possible Cl2 impurities as discussed below. Values of σCl O (λ)Φ(λ) reported for wavelengths of 308 and

351 nm are in excellent agreement with the present Cl2O2 cross section values, within 7%, when Φ(λ) is assumed to be unity.

The good agreement between the Chen et al.19 results and those obtained here provide additionalevidence that the methods used in the present work to account for Cl2 are valid. 3.5.1. Cl2 Spectral Interference. A source of uncertainty in the reported Cl2O2 spectra is associated with the uncertainties in corrections for Cl2 absorption. Cl2 is present as an impurity in all gas-phase Cl2O2 spectrum studies reported to date, and measured spectra must account for its contribution to obtain accurate cross section values at λ >300 nm. The point that differences in Cl2 subtraction alone can account for the majority of the discrepancies in the Cl2O2 cross section at λ >300 nm is illustrated in Figure 10 using experimental data obtained in this study. In this example, an absorption spectrum containing significant Cl2 absorption was taken from our CT-TD experiments. We compared the residual normalized spectra obtained following Cl2 subtraction with the highest (Burkholder et al.9), intermediate (Huder and DeMore16), and lowest (Pope et al.10)

Cl2O2 spectra reported in the wavelength range >300 nm. Subtracting various amounts of Cl2 on a subjective basis, using aC l2 reference spectrum recorded under identical conditions, we obtain the Cl2O2 spectrum obtained in this work and good agreement with the Cl2O2 spectrum reported by Burkholder et al.9 Increasing the amount of Cl2 subtraction yields reasonable agreement with the exponential extrapolation given by Huder and DeMore16 although systematic deviations are observed.

Increasing the amount of Cl2 subtraction further yields reason- able agreement with the Cl2O2 spectrum reported by Pope et al.,10 although the residual spectrum contains slightly negative values at the longer wavelengths. The difference in the amount of Cl2 subtraction needed to obtain the residuals that agree with the Burkholder et al.9 and Pope et al.10 spectra was only 4.5%.

This exercisehighlightsthe need for accuratemethodsto account for Cl2 in the determination of the Cl2O2 spectrum. Prior to the Pope et al.10 study, only Burkholder et al.9 and

DeMore and Tschiukow-Roux13 had reported absorption cross section data for wavelengths >320 nm. As shown in Figure 10, the uncertainty in the magnitude of the Cl2 corrections can account for the majority of the discrepancies in the reported

Cl2O2 spectra. Burkholderet al.9 used a discharge flow chemical titration method to determine the Cl2 contribution to their measured absorption spectra. They estimated the uncertainty in their Cl2O2 absorption cross section at 330 nm to be in the range 10-15%. DeMore and Tschiukow-Roux13 corrected their measured spectra for Cl2 absorption using a subjective iterative spectral subtraction approach. They report the uncertainty in their Cl2O2 cross section to be ∼30% at ∼350 nm. Pope et al.10 fit their measured absorption spectra, which contained both

Cl2O2 and Cl2, using two Gaussian profiles and a reference Cl2 absorption spectrum. The fitting procedure optimized the parameters of the Gaussian functions (peak location and width, in wavelength space) and the abundance of Cl2. On the basis of their measurement precision, they estimated the uncertainty in the Cl2O2 cross section at ∼350 nm to be ∼25%. The absolute uncertainties in their Cl2O2 absorption cross sections in the long wavelength region are most likely larger and dependent on the validity of the assumptions applied in the spectral fitting, which are difficult to quantify. In such a spectral analysis approach, a correlation between the UV absorption spectra of Cl2 and Cl2O2 would lead to systematic errors in the Cl2O2 spectrum obtained. In our work, we obtained UV absorption spectra using the CT-

TD method that containedCl2 and Cl2O2 that are consistentwith those reported by Pope et al.,10 who used the same experimental method. Therefore, the source of the discrepancies in the reported Cl2O2 spectra between the two studies is not the result of the spectral measurements themselves but lies in the interpretation of the measured spectra, as illustrated in Figure 10. In our work, we have also shown that absorption spectra obtained using the CT-TD method at trapping temperatures greaterthan 151 K are consistentwith our PLP-DAexperiments, Figure 5. In conclusion, the major source of the discrepancy between our work and that of Pope et al.10 is attributed to the uncertainty in the Cl2 subtraction. In the present study our methods were designed to quantitatively account for Cl2 interference by using the observed isosbestic wavelengths, reaction stoichiometry, and chlorine mass balance.

von Hobe et al.14 reported a Cl2O2 absorption spectrum measuredin a 10 K Ne matrix,shownin Figure1. Cl2O2 samples were prepared using the CT method, and samples purified by vacuum distillation were shown to contain negligible amounts of Cl2. The wavelength dependence of the spectrum at λ <300 nm is in good agreement with that from this work although the width of the absorption band is narrower due to the much lower temperature of the matrix measurement. In the long wavelength region, von Hobe et al. report Cl2O2 absorption at wavelengths out to 400 nm, which is consistent with our gas-phase observa- tions that Cl2O2 absorbs in this wavelength region. Their Cl2O2 spectrum (normalized to a peak cross section of 6.5 × 10-18 cm2 molecule-1) is a factor of 2 lower than the value reported in this work and a factor of 7 greater than the value reported by Pope et al.10 at 350 nm.

Figure 10. Illustration of the sensitivity of the Cl2O2 spectral analysis to the Cl2 spectral subtraction and the level of agreement that can be obtained with representative Cl2O2 spectra reported in the literature. The experimental spectrum was recorded using the CT-TD method and contains a large contribution from Cl2 as seen by the peak in the spectrum at 330 nm. An optimized agreement of residual spectra (lines) obtained by subtracting Cl2 absorption from the experimental spectrum with the Cl2O2 spectra reported by Burkholder et al. (O),9 Huder and DeMore (4),16 and Pope et al. (0)10 are shown.

UV Absorption Spectrum of the ClO Dimer J. Phys. Chem. A, Vol. 113, No. 49, 2009 13723

Absolute cross section values for Cl2O2 have been reported by Cox and Hayman,15 Burkholder et al.,9 DeMore and

Tschiukow-Roux,13 and Bloss et al.17 The peak absorption cross section values in these studies are systematically lower than the value obtained in this work, 7.6-0.5 although all the reported values fall within the combined 2σ error limits. The cross sections reported by Bloss et al.17 and Chen et al.19 (assuming a unity quantum yield) are in excellent agreement, within 7%, with the values obtained in this work.

For the actinic region, the Cl2O2 absorptionspectrumreported in this work is in good agreement with spectra reported by

Burkholder et al.9 and the values reported by Chen et al.19 for 308 and 351 nm. These studies used different experimental techniquesand obtainedconsistentCl2O2 cross section data. Our spectrum is systematically larger than that reported by DeMore and Tschiukow-Roux13 althoughthe two data sets overlapwithin the combined uncertainties. This work and the studies of Burkholder et al.,9 Chen et al.,19 and DeMore and Tschiukow-

Roux13 report Cl2O2 absorption cross section values obtained using different experimental techniques that are significantly greater than those reported in the Pope et al.10 study.

4. Atmospheric Implications

In the winter/spring Antarctic polar vortex, the ClO dimer catalytic ozone destruction cycle, reactions 1-4, accounts for the majority of calculated ozone depletion.4 In the Arctic polar stratosphere, the photochemistry of Cl2O2 is also important although the extent of ozone depletion is less due to the meteorological variability in the winter vortex. A critical parameter in determining the efficiency of the ClO dimer cycle is the atmospheric photolysis rate coefficient, J,o fC l2O2 which is calculated from the product of the Cl2O2 absorption cross section, its photolysis quantum yield, and the solar flux integrated over all wavelengths

whereσ(λ) is the absorptioncrosssectionof Cl2O2 at wavelength

and solar zenith angle (SZA, ). It is worth noting that the efficiency of the ClO dimer cycle also influences the efficiency of the ClO + BrO catalytic ozone destruction cycle

by altering the atmospheric abundance of ClO.

J(λ) values calculated for a SZA of 86° and altitude of 20 km, representative conditions for stratospheric measurements of ClO and Cl2O2 in the Arctic, with the Cl2O2 cross section data from this work and from severalpreviousstudies(Burkhold- er et al.,9 NASA/JPL,6 and Pope et al.10) is given in Figure 1. These studies were chosen because they represent the highest, currently recommended, and lowest Cl2O2 cross section data available for the long wavelength region. For the wavelength region longer than the experimentally available values an exponential extrapolation was used as shown in Figure 9. The wavelength-dependent solar fluxes were obtained using the NCAR online TUV calculator.21 J(λ) has a maximum between 330 and 340 nm and falls to nearly zero at 300 nm. The absolute magnitude of J(λ) and the decrease toward longer wavelength differ for the different Cl2O2 cross section data sets as expected. The Pope et al.10 cross section data set results in the lowest maximum value of J(λ). The data sets from this work and Burkholder et al.9 yield similar magnitude J(λ) values while the NASA/JPL6 values have a similar wavelength dependence but are systematically lower.

J(λ >350 nm) is significant for the Cl2O2 cross section data from this work, Burkholder et al.,9 and NASA/JPL.6 J(λ >420 nm), i.e., beyond the limit of our experimental data, makes a non-negligible, ∼10-30%, contribution to the total photolysis rate coefficient. The long wavelength contribution depends on the SZA and will also depend on the subjective method used to extrapolatethe cross sectiondata. Pope et al.10 report Cl2O2 cross section data that decreases significantly at wavelengths greater than 300 nm with reported values extending out to 360 nm. Including an extrapolation of their cross section data to longer wavelengths increases the calculated total photolysis rate coefficient only slightly. The differences in the photolysis rate coefficients calculated for the cross section data from Pope et al.10 and this work have significant implications for atmospheric model calculated ClOx abundance and ozone loss rates.

The uncertainties in the Cl2O2 absorption cross sections from this work, as a function of wavelength, are shown in Figure 9.

We extended the estimated uncertainty limits beyond 420 nm using an exponential extrapolation, shown in Figure 9, in order

Figure 1. Comparison of wavelength-dependent Cl2O2 atmospheric photolysis rate coefficients, J(λ), calculated for a solar zenith angle

(SZA) of 86° at an altitude of 20 km using the Cl2O2 UV absorption cross obtained in this work (heavy black) where the upper and lower limits of the shaded region were calculated using the estimated uncertainty limits for the Cl2O2 cross section data reported in this study and shown in Figure 9 (see text for details of the error analysis). J(λ) values calculated using previously reported Cl2O2 cross section data are included for comparison: Burkholder et al.9 (blue), NASA/JPL6

(red), and Pope et al.10 (green). Solar fluxes were calculated using the online NCAR TUV calculator.21

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