Dimitrios K. Papanastasiou

UV absorption spectrum of the ClO dimer (Cl2O2) between 200 and 420 nm.

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.

Nitryl chloride (ClNO2): UV/vis absorption spectrum between 210 and 296 K and O(3P) quantum yield at 193 and 248 nm.

Recent studies have shown that the UV/vis photolysis of nitryl chloride (ClNO2) can be a major source of reactive chlorine in the troposphere. The present work reports measurements of the ClNO2 absorption spectrum and its temperature dependence between 210 and 296 K over the wavelength range 200–475 nm using diode array spectroscopy. The room temperature spectrum obtained in this work was found to be in good agreement with the results from Ganske et al. (J. Geophys. Res. 1992, 97, 7651) over the wavelength range common to both studies (200–370 nm) but differs systematically from the currently recommended spectrum for use in atmospheric models. The present results lead to a decrease in the calculated atmospheric ClNO2 photolysis rate by 30%. Including the temperature dependence of the ClNO2 spectrum decreases the calculated atmospheric photolysis rate at lower temperatures (higher altitudes) even further. A parametrization of the wavelength and temperature dependence of the ClNO2 spectrum is presented. O(3P) quantum yields, Φ(ClNO2)(O), in the photolysis of ClNO2 at 193 and 248 nm were measured at 296 K using pulsed laser photolysis combined with atomic resonance fluorescence detection of O(3P) atoms. Φ(ClNO2)(O)(λ) was found to be 0.67 ± 0.12 and 0.15 ± 0.03 (2σ error limits, including estimated systematic errors) at 193 and 248 nm, respectively, indicating that multiple dissociation channels are active in the photolysis of ClNO2 at these wavelengths. The Φ(ClNO2)(O)(λ) values obtained in this work are discussed in light of previous ClNO2 photodissociation studies and the differences are discussed.

Oxalyl chloride, ClC(O)C(O)Cl: UV/vis spectrum and Cl atom photolysis quantum yields at 193, 248, and 351 nm.

Oxalyl chloride, (ClCO)(2), has been used as a Cl atom photolytic precursor in numerous laboratory kinetic and photochemical studies. In this study, the UV/vis absorption spectrum of (ClCO)(2) and the Cl atom quantum yields in its photolysis at 193, 248, and 351 nm are reported. The UV∕vis spectrum was measured between 200 and 450 nm at 296 K using diode array spectroscopy in conjunction with an absolute cross section obtained at 213.9 nm. Our results are in agreement with the spectrum reported by Baklanov and Krasnoperov [J. Phys. Chem. A 105, 97-103 (2001)], which was obtained at 11 discrete wavelengths between 193.3 and 390 nm. Cl atom quantum yields, Φ(λ), were measured using pulsed laser photolysis coupled with time resolved atomic resonance fluorescence detection of Cl. The UV photolysis of (ClCO)(2) has been shown in previous studies to occur via an impulsive three-body dissociation mechanism, (COCl)(2) + hv → ClCO* + Cl + CO (2), where the excited ClCO radical, ClCO*, either dissociates or stabilizes ClCO* → Cl + CO (3a), → ClCO (3b). ClCO is thermally unstable at the temperatures (253-298 K) and total pressures (13-128 Torr) used in our experiments ClCO + M → Cl + CO + M (4) leading to the formation of a secondary Cl atom that was resolvable in the Cl atom temporal profiles obtained in the 248 and 351 nm photolysis of (ClCO)(2). Φ(193 nm) was found to be 2.07 ± 0.37 independent of bath gas pressure (25.8-105.7 Torr, N(2)), i.e., the branching ratio for channel 2a or the direct formation of 2Cl + 2CO in the photolysis of (ClCO)(2) is >0.95. At 248 nm, the branching ratio for channel 2a was determined to be 0.79 ± 0.15, while the total Cl atom yield, i.e., following the completion of reaction (4), was found to be 1.98 ± 0.26 independent of bath gas pressure (15-70 Torr, N(2)). Φ(351 nm) was found to be pressure dependent between 7.8 and 122.4 Torr (He, N(2)). The low-pressure limit of the total Cl atom quantum yield, Φ(0)(351 nm), was 2.05 ± 0.24. As part of this work, rate coefficients for the thermal decomposition of ClCO were measured between 253 and 298 K at total pressures between 13 and 128 Torr (He and N(2) bath gases). The N(2) bath gas results were combined with the data reported in Nicovich et al. [J. Chem. Phys. 92, 3539-3544 (1990)] to yield k(4)(T, N(2)) = (4.7 ± 0.7) × 10(-10) exp [-(2987 ± 16)/T] cm(3) molecule(-1) s(-1), while the He bath gas data fit yielded k(4)(T, He) = (2.3 ± 2.1) × 10(-10) exp [-(2886 ± 218)/T] cm(3) molecule(-1) s(-1). The quoted uncertainties are at the 2σ level from the precision of the fit. In addition, the room temperature rate coefficient for the Cl + ClNO reaction was measured in this work to be (1.03 ± 0.10) × 10(-10) cm(3) molecule(-1) s(-1).

ClO radical yields in the reaction of O(1D) with Cl2, HCl, chloromethanes, and chlorofluoromethanes.

Absolute ClO radical product yields in the gas-phase reactions of O((1)D) with Cl(2), HCl, CCl(4), CHCl(3), CH(2)Cl(2), CH(3)Cl, CFCl(3), CF(2)Cl(2), CF(3)Cl, CHFCl(2), and CHF(2)Cl are reported. Product yields were measured using pulsed-laser photolysis of O(3) to produce O((1)D) in the presence of excess reactant combined with dual wavelength differential cavity ring-down spectroscopic detection of the ClO radical. ClO radical absorption cross sections for the A(2)Π(v = 10) ← X(2)Π(v = 0) transition band head near 280 nm were determined between 200 and 296 K as part of this work. The ClO product yields obtained at room temperature were Cl(2) (0.77 ± 0.10), HCl (0.20 ± 0.04), CCl(4) (0.79 ± 0.04), CHCl(3) (0.77 ± 0.04), CH(2)Cl(2) (0.73 ± 0.04), CH(3)Cl (0.46 ± 0.06), CFCl(3) (0.79 ± 0.04), CF(2)Cl(2) (0.76 ± 0.06), CF(3)Cl (0.82 ± 0.06), CHFCl(2) (0.73 ± 0.05), and CHF(2)Cl (0.56 ± 0.03), where the quoted error limits are 2σ of the measurement precision. ClO product yields in the O((1)D) + Cl(2) and CFCl(3) reactions were found to be independent of temperature between 200 and 296 K, within the precision of the measurements. The absolute ClO yields obtained in this study are compared with previously reported values determined using relative and indirect methods.