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

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13724 J. Phys. Chem. A, Vol. 113, No. 49, 2009 Papanastasiou et al.

to quantify the impact on the overall Cl2O2 atmospheric photolysis rate coefficient. The J(λ) values obtained using the upper and lower limits of the cross section data in eq VI are shown in Figure 1. As expected, the uncertainty in J(λ >420 nm) is large due to the large uncertainty in the Cl2O2 absorption cross section at these wavelengths. It is also worth noting that including the extrapolated Cl2O2 cross section data as shown in Figure 9 leads to asymmetric uncertainty limits in J(λ). For a SZA of 86°, shown in Figure 1, J(λ >420 nm) accounts for ∼23% of the total photolysis rate coefficient. The range of uncertainty in J(λ) using the Cl2O2 cross section data reported here is approximately +50%/-20% at the 2σ confidence level.

The estimated uncertainty in J(λ <420 nm) is (18% at the 2σ confidence level. Thus, our estimated lower limit for the photolysis rate coefficient is only slightly dependent on the extrapolation of the Cl2O2 cross section data to wavelengths >420 nm. The estimatedupper limit for J is, however,dependent on the extrapolated Cl2O2 cross section data. The extrapolation method used here most likely yields a maximum to the upper limit of the Cl2O2 cross section values for wavelengths >420 nm. Including the upper limit extrapolated Cl2O2 cross section data increases the overall photolysis rate by ∼50%. Reduction of the uncertainty in the Cl2O2 atmospheric photolysis rate coefficient below the 50% level requires improved knowledge of the Cl2O2 absorption cross sections and photolysis quantum yields at wavelengths >420 nm.

Figure 12 shows the photolysis rate coefficients, J, integrated over the wavelength range 290-725 nm as a function of SZA for Cl2O2 cross section data from the present work, Burkholder et al.,9 NASA/JPL,6 and Pope et al.10 The J values obtained using the present Cl2O2 spectrum and that of Burkholder et al.9 agree very well, within ∼10%, which is within the combined uncertainties of the Cl2O2 cross section values for the two studies. The J values from the present work are ∼30% higher than NASA/JPL and a factor of 9 greater than those obtained using the Pope et al.10 data for SZA < 80°. At larger SZAs the differencesbetween this work and NASA/JPL6 and Pope et al.10 are greater. These differences have a significant impact on atmospheric model calculated ClOx abundance and ozone loss rates as shown in previous atmospheric model studies of polar stratospheric halogen chemistry.4,2,23 The uncertainty in the J(SZA) values, shown in Figure 12, increases with SZA from +50%/-20% at 86° to a maximum at ∼92°.

The present Cl2O2 absorption cross section data are in close agreement with the data previously reported by Burkholder et al.9 for λ >300 nm. We point out the similaritywith the previous Burkholder et al.9 study primarily because previous modeling studies of polar stratospheric in situ and remote sensing measurements have frequently included the Burkholder et al.9

Cl2O2 cross section data in their analysis. Therefore, we can draw meaningful conclusions here from the previous model studies by a direct comparison with the Burkholder et al.9 cross section data. von Hobe et al.23 recently provided a summary of

ClOx Arctic field measurements and an atmospheric model analysis using different photochemical input parameters includ- ing the Cl2O2 absorption cross section data from Burkholder et al.9 The available ClOx field measurement data sets, unfortunately, do not provide a consistent enough picture of ClOx in the Arctic to make an evaluation of Cl2O2 cross section data from field measurements alone. However, von Hobe et al.23 concluded that the “best” agreement between model and observations was obtained for Cl2O2 photolysis rates falling between the Burkholder et al.9 and the NASA/JPL6 recom- mendations, while using the Pope et al.10 Cl2O2 cross section data resulted in very poor agreement. If the Pope et al.10 cross section data are correct, it would therefore require including unknown chemical processes to explain the discrepancies between models and observations. In another study, Frieler et al.4 calculated ozone loss in the Arctic and Antarctic vortices using various photochemical input parameters. They obtained good quantitative agreement with field observations when using

Cl2O2 photolysis rates calculated using the Burkholder et al.9 data combined with increased levels of Brx loading, ∼6 ppt.

5. Concluding Remarks

The UV absorption spectrum of Cl2O2 was measured in the present study over the wavelength range 200-420 nm at

200-228 K. The present study was designed to quantitatively account for Cl2 spectral interference by using the observed isosbestic wavelengths, reaction stoichiometry, and chlorine mass balance. The Cl2O2 absorption cross section at the peak cm2 molecule-1, which is ∼17% larger than that currently recommended by NASA/JPL6 and IUPAC.20 The cross sections in the long wavelength region of the spectrum agree reasonably well with the values reported by Burkholder et al.9 but are in poor agreement with those reported in the recent study by Pope et al.10 The Cl2O2 absorption spectrum and cross section values obtained in this study yield atmospheric photolysis rate coef- ficients that are of similar magnitude to those calculated using the data from Burkholder et al.9 The stratospheric photolysis rate coefficients obtained from this work are ∼30% greater than those obtained using the NASA/JPL6 recommendedCl2O2 cross section values. The present results provide strong evidence that

Figure 12. The upper frame shows the integrated atmospheric photolysis rate coefficients, J, calculated for Cl2O2 as a function of solar zenith angle (SZA) using the UV absorption cross section data from this work (black). 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 values calculated using the Cl2O2 cross section data from Burkholder et al.9 (blue), NASA/JPL6 (red), and Pope et al.10 (green) are included for comparison. Solar fluxes were calculated using the online NCAR TUV calculator.21 The lower frame shows the same data relative to the values obtained using the NASA/

JPL recommended Cl2O2 cross section data.

UV Absorption Spectrum of the ClO Dimer J. Phys. Chem. A, Vol. 113, No. 49, 2009 13725 the recent study by Pope et al.10 underestimated the Cl2O2 absorption in the wavelength region 300-400 nm, the region most important for atmospheric photolysis rate calculations. It has been suggested that if the Pope et al.10 results were correct, then significant gaps in our current understanding of polar stratospheric chemistry and ozone depletion mechanisms exist. Although improvements in our fundamental understanding of the photochemistryof Cl2O2 are still desired, our work indicates that major revisions in current atmospheric chemical mecha- nisms are not required to simulate observed polar ozone depletion.

Acknowledgment.We thankSPARCfor organizingthe “The

Role of Halogen Chemistry in Polar Stratospheric Ozone Depletion” workshop and supporting discussion on this topic. This work was supported in part by NOAA’s Climate Goal and in part by NASA’s Atmospheric Composition, Upper Atmospheric Research Program.

Supporting Information Available: Tabulated Cl2O2 absorption cross section data obtained in this work, parameteriza- tion of the wavelength-dependent estimated uncertainties in the

Cl2O2 absorption cross section values, and simulations of isosbestic point behavior obtained using a chemical reaction mechanism. This material is available free of charge via the Internet at http://pubs.acs.org.

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