Abstract. Absorption cross sections of emerging greenhouse gases (GHGs) were measured to estimate the radiative efficiency using high-resolution Fourier transform infrared spectroscopy (HR-FTIR). For quantitative spectroscopy, the Beer–Lambert parameters of absorber pressure, temperature, and optical path length (OPL) were accurately determined to be traceable to the primary standards. The OPL of the multipass cell mounted on the HR-FTIR spectrometer was spectroscopically calibrated. A ratio of the averaged N2O absorptions was found to be in the range of 2217.4–2219.0 cm−1, with a spectral resolution of 0.026 cm−1, yielding a ratio of OPLs between the multipass cell and reference cell. This cell-to-cell comparison method is free from the uncertainty of the referring line strength, which reduced the calibration uncertainty compared with the direct line-strength referring method. With the OPL-calibrated multipass cell (3.169 ± 0.079 m), the absorption cross sections were measured at low absorber pressures with a spectral resolution of 2 cm−1, integrated at 10 cm−1 intervals, and multiplied by the new narrow band model to yield the radiative efficiencies. The radiative efficiency values of CF4, SF6, and NF3 were evaluated to be 0.085 ± 0.002, 0.573 ± 0.016, and 0.195 ± 0.008 W m−2 ppb−1, respectively, which are consistent with previously reported values. For the emerging GHGs, the radiative efficiency values were determined to be 0.201 ± 0.008 W m−2 ppb−1 for heptafluoroisobutyronitrile (CF3)2CFCN; commercially referred to as Novec-4710), 0.328 ± 0.013 W m−2 ppb−1 for perfluoro methyl vinyl ether (CF3OCFCF2; PMVE), and 0.544 ± 0.022 W m−2 ppb−1 for 1,1,1,2,2-pentafluoro-2-(trifluoromethoxy)ethane (CF3OCF2CF3; PFMEE).

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Abstract. Absorption cross-sections (ACS) of emerging greenhouse gases (GHG) were measured to estimate the radiative efficiencies (RE) using high-resolution Fourier transform infrared spectroscopy (HR-FTIR). For quantitative spectroscopy, the Beer–Lambert parameters of absorber pressure, temperature, and optical pass length (OPL) have been accurately determined to be traceable to the primary standards. The OPL of the multipass cell (MP) mounted on the HR-FTIR spectrometer was spectroscopically calibrated. A ratio of the averaged N2O absorptions was measured in the range of 2,217.4–2,219.0 cm-1 with a spectral resolution of 0.026 cm-1, yielding a ratio of OPLs between the MP and reference cell (RC). This cell-to-cell comparison (CC) method does not include the uncertainty in the referring line strength, thereby reducing the calibration uncertainty compared with a direct line-strength referring method. With the OPL-calibrated MP (3.169 ± 0.079 m), the ACSs were measured at low absorber pressures with a spectral resolution of 2 cm-1, integrated at 10 cm-1 intervals, and multiplied by the new narrow band model to yield the RE values. The RE values of the classic GHGs CF4, SF6, and NF3 were evaluated to be 0.085 ± 0.002, 0.573 ± 0.016, and 0.195 ± 0.008 W m-2 ppb-1, respectively, which are consistent with previously reported values. For the emerging GHGs, the RE values were determined to be 0.201 ± 0.008 W m-2 ppb-1 for (CF3)2CFCN (heptafluoroisobutyronitrile, commercially referred to as Novec-4710), 0.328 ± 0.013 W m-2 ppb-1 for CF3OCFCF2 (perfluoro methyl vinyl ether, PMVE), and 0.544 ± 0.022 W m-2 ppb-1 for CF3OCF2CF3 (1,1,1,2,2-pentafluoro-2-(trifluoromethoxy)ethane, PFMEE).