Rate coefficients for the reaction of O( 1D) with the atmospherically long-lived greenhouse gases NF 3, SF 5CF 3, CHF 3, C 2F 6, c-C 4F 8, n-C 5F 12, and n-C 6F 14
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, k, were measured for the reaction of O( 1D) with several key long-lived greenhouse gases, namely NF 3, SF 5CF 3, CHF 3 (HFC-23), C 2F 6, c-C 4F 8, n-C 5F 12, and n-C 6F 14. Room temperature rate coefficients for the total reaction, kTot, corresponding to loss of O( 1D), and reactive channel, kR, corresponding to the loss of the reactant compound, were measured for NF 3 and SF 5CF 3 using competitive reaction and relative rate methods, respectively. kR was measured for the CHF 3 reaction and improved upper-limits were determined for the perfluorinated compounds included in this study. For NF 3, kTot was determined to be (2.55 ± 0.38) × 10 −11 cm 3 molecule −1 s −1 and kR, which was measured using CF 3Cl, N 2O, CF 2ClCF 2Cl (CFC-114), and CF 3CFCl 2 (CFC-114a) as reference compounds, was determined to be (2.21 ± 0.33) × 10 −11 cm 3 molecule −1 s −1. For SF 5CF 3, kTot = (3.24 ± 0.50) × 10 −13 cm 3 molecule −1 s −1 and kR < 5.8 × 10 ×14 cm 3 molecule −1 s −1 were measured, where kR is a factor of three lower than the current recommendation of kTot for use in atmospheric modeling. For CHF 3kR was determined to be (2.35 ± 0.35) × 10 −12 cm 3 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 kR for the C 2F 6, c-C 4F 8, n-C 5F 12, and n-C 6F 14 reactions were determined to be 3.0, 3.5, 5.0, and 16 (in units of 10 −14 cm 3 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 NF 3 and SF 5CF 3 were measured and found to be in good agreement with recently reported values.