Study of the Decomposition of Traces of SF6 in the Lower Atmosphere: The Kinetic Results
For the last three decades, sulfurhexafluoride has accumulated into the atmosphere to a concentration of ≈4 pptv, due mostly to leaks and releases from electrical equipment. Recognized as a very persistent gas, with lifetime exceeding three millennium, and as the molecule having the largest global warming potential, it was included as the sixth gas in the Kyoto Protocol. Since 95% of all the SF6 mass is contained in the troposphere, a test program was conducted to study the chemical stability and reactivity of the inert SF6 molecule in the typical conditions of the lower atmosphere (< I5 km). It was focused on the almost exclusive decomposition path resulting from the dissociative capture of an electron. Chemical kinetics of SF6 in SF6/air mixtures (1-1000 ppmv) were studied using reaction cells and setups favoring the interaction of SF6 with low energy electrons (≈0,2 eV). These were generated in-situ by photo-emission from the UV-irradiated inner Al cell walls. The following experimental conditions were chosen to simulate those of the lower atmosphere: pressure (25-100 kPa), temperature (-40 to +25°C), humidity (0.05-3.5%). The O2 concentration was also varied between 2-21%. In all cases, the decomposition of SF6 was observed to proceed at very low rates to form exclusively SO2F2, the only byproduct observed. A simple reaction mechanism is derived from the ion-molecule reaction of SF6 with O2, the typical negative ion formed by electron capture in air at atmospheric pressure. In our experimental conditions as in the atmosphere, this reaction must compete with many other neutralization paths of the O2, but the SF6 destruction certainly occurs, due to its large electron affinity. Indeed, the dissociation of SF6 into SF5 is certainly irreversible in those situations. The decomposition kinetics of SF6 measured in tropospheric air in this study can certainly improve the chemical reaction model used to estimate its persistence lifetime.
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