Study of the Decomposition of Traces of SF₆ in the Lower Atmosphere: The Kinetic Results

Abstract

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.