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

Abstract

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 NF3, SF5CF3, CHF3 (HFC-23), C2F6, c-C4F8, n-C5F12, and n-C6F14. 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 NF3 and SF5CF3 using competitive reaction and relative rate methods, respectively. kR was measured for the CHF3 reaction and improved upper-limits were determined for the perfluorinated compounds included in this study. For NF3, kTot was determined to be (2.55 ± 0.38) × 10−11 cm3 molecule−1 s−1 and kR, which was measured using CF3Cl, N2O, CF2ClCF2Cl (CFC-114), and CF3CFCl2 (CFC-114a) as reference compounds, was determined to be (2.21 ± 0.33) × 10−11 cm3 molecule−1 s−1. For SF5CF3, kTot = (3.24 ± 0.50) × 10−13 cm3 molecule−1 s−1 and kR ×14 cm3 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 CHF3 kR was determined to be (2.35 ± 0.35) × 10−12 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 kR for the C2F6, c-C4F8, n-C5F12, and n-C6F14 reactions were determined to be 3.0, 3.5, 5.0, and 16 (in units of 10−14 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.