Rate coefficients for the reaction of O(¹D) with the atmospherically long-lived greenhouse gases NF₃, SF₅CF₃, CHF₃, C₂F₆, c-C₄F₈, <i>n</i>-C₅F₁₂, and <i>n</i>-C₆F₁₄

• Main Authors: B. D. Hall, J. B. Burkholder, M. Baasandorj
• Format: Article
• Language: English
• Published: Copernicus Publications 2012-12-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/12/11753/2012/acp-12-11753-2012.pdf

The contribution of atmospherically persistent (long-lived) greenhouse gases to the radiative forcing of Earth has increased over the past several decades. The impact of highly fluorinated, saturated compounds, in particular perfluorinated compounds, on climate change is a concern because of their long atmospheric lifetimes, which are primarily determined by stratospheric loss processes, as well as their strong absorption in the infrared "window" region. A potentially key stratospheric loss process for these compounds is their gas-phase reaction with electronically excited oxygen atoms, O(¹D). Therefore, accurate reaction rate coefficient data is desired for input to climate change models. In this work, rate coefficients, <i>k</i>, were measured for the reaction of O(¹D) with several key long-lived greenhouse gases, namely NF₃, SF₅CF₃, CHF₃ (HFC-23), C₂F₆, c-C₄F₈, <i>n</i>-C₅F₁₂, and <i>n</i>-C₆F₁₄. Room temperature rate coefficients for the total reaction, <i>k</i>_Tot, corresponding to loss of O(¹D), and reactive channel, <i>k</i>_R, corresponding to the loss of the reactant compound, were measured for NF₃ and SF₅CF₃ using competitive reaction and relative rate methods, respectively. <i>k</i>_R was measured for the CHF₃ reaction and improved upper-limits were determined for the perfluorinated compounds included in this study. For NF₃, <i>k</i>_Tot was determined to be (2.55 ± 0.38) &times; 10^&minus;11 cm³ molecule⁻¹ s⁻¹ and <i>k</i>_R, which was measured using CF₃Cl, N₂O, CF₂ClCF₂Cl (CFC-114), and CF₃CFCl₂ (CFC-114a) as reference compounds, was determined to be (2.21 ± 0.33) &times; 10^&minus;11 cm³ molecule⁻¹ s⁻¹. For SF₅CF₃, <i>k</i>_Tot = (3.24 ± 0.50) &times; 10^&minus;13 cm³ molecule⁻¹ s⁻¹ and <i>k</i>_R < 5.8 &times; 10^&times;14 cm³ molecule⁻¹ s⁻¹ were measured, where <i>k</i>_R is a factor of three lower than the current recommendation of <i>k</i>_Tot for use in atmospheric modeling. For CHF₃ <i>k</i>_R was determined to be (2.35 ± 0.35) &times; 10^&minus;12 cm³ molecule⁻¹ s⁻¹, which corresponds to a reactive channel yield of 0.26 ± 0.04, and resolves a large discrepancy among previously reported values. The quoted uncertainties are 2σ and include estimated systematic errors. Upper-limits for <i>k</i>_R for the C₂F₆, c-C₄F₈, <i>n</i>-C₅F₁₂, and <i>n</i>-C₆F₁₄ reactions were determined to be 3.0, 3.5, 5.0, and 16 (in units of 10^&minus;14 cm³ molecule⁻¹ s⁻¹), respectively. The results from this work are compared with results from previous studies. As part of this work, infrared absorption band strengths for NF₃ and SF₅CF₃ were measured and found to be in good agreement with recently reported values.