Measurements of perfluoro-n-hexane and perfluoro-2-methylpentane infrared absorption cross-sections from 298 to 350 K

Abstract

Perfluoro-n-hexane and perfluoro-2-methylpentane are fully fluorinated alkanes used as non-ozone-depleting alternatives to chlorofluorocarbons and hydrochlorofluorocarbons. These compounds are long-lived and potent greenhouse gases due to their strong CF bonds and infrared absorption in the atmospheric window. Infrared absorption cross-sections are required to quantify the climate impact of these compounds via the radiative efficiency and global warming potential. To our knowledge, there are only two experimental measurements for perfluoro-n-hexane, and there are no experimental measurements for perfluoro-2-methylpentane in the infrared. In this work, we provide a set of absorption cross-sections in the range 515–1500 cm⁻¹, at 0.1 cm⁻¹ resolution from 298 to 350 K for each compound. We calculate the absorption cross-section between 0 and 515 cm⁻¹ using density functional theory with various basis sets. The 6-31,G(d,p) basis set with the B3LYP functional is found to provide the best results. Using both measurements and calculations combined, we calculate the radiative efficiency and global warming potential for each compound. No significant temperature dependence was observed in these quantities. The average radiative efficiency derived from all cross-sections is 0.48 ± 0.06 W m⁻² ppbv⁻¹ for perfluoro-n-hexane and 0.46 ± 0.06 W m⁻² ppbv⁻¹ for perfluoro-2-methylpentane. The average 100-year global warming potential derived from all cross-sections is 9590 ± 1260 for perfluoro-n-hexane and 9220 ± 1210 for perfluoro-2-methylpentane.