Measurement report: Radiative efficiencies of (CF3)2CFC, CF3OCFCF2, and CF3OCF2CF3

Absorption cross-sections of emerging greenhouse gases (GHG) 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 that falls between the multipass cell and reference cell. This cell-to-cell comparison method is free from the uncertainty in 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 Wm-2ppb-1 for heptafluoroisobutyronitrile (CF3)2CFCN; commercially referred to as Novec-4710), 0.328 ± 0.013 Wm-2ppb-1 for perfluoro methyl vinyl ether (CF3OCFCF2; PMVE), and 0.544 ± 0.022 Wm-2ppb-1 for 1,1,1,2,2-pentafluoro-2-(trifluoromethoxy)ethane (CF3OCF2CF3; PFMEE).

本研究采用高分辨率傅里叶变换红外光谱（High-resolution Fourier Transform Infrared Spectroscopy, HR-FTIR），对新兴温室气体（Greenhouse Gas, GHG）的吸收截面（absorption cross-sections）进行测量，以估算其辐射效率。在定量光谱分析中，吸收剂压力、温度及光程长（Optical Path Length, OPL）相关的比尔-朗伯参数被精准测定，且可溯源至一级标准。安装于HR-FTIR光谱仪的多程池的光程长，通过光谱学方法完成校准。研究发现，平均一氧化二氮（N₂O）吸收信号的比值处于2217.4–2219.0 cm⁻¹区间，实验光谱分辨率为0.026 cm⁻¹，由此推导得到多程池与参比池的光程长之比。相较于直接参考谱线强度的校准方法，该池间对比法无需承担参考谱线强度带来的不确定度，有效降低了校准过程的不确定度。利用经光程长校准的多程池（3.169 ± 0.079 m），本研究在低吸收剂压力下以2 cm⁻¹的光谱分辨率测量吸收截面，以10 cm⁻¹为间隔进行光谱积分，并结合新型窄带模型进行计算，最终得到各气体的辐射效率。经测定，四氟化碳（CF₄）、六氟化硫（SF₆）及三氟化氮（NF₃）的辐射效率分别为0.085 ± 0.002、0.573 ± 0.016及0.195 ± 0.008 W·m⁻²·ppb⁻¹，与此前已发表的研究结果一致。针对新兴温室气体，七氟异丁腈（(CF₃)₂CFCN，商业名称为Novec-4710）的辐射效率为0.201 ± 0.008 W·m⁻²·ppb⁻¹；全氟甲基乙烯基醚（CF₃OCFCF₂，PMVE）为0.328 ± 0.013 W·m⁻²·ppb⁻¹；1,1,1,2,2-五氟-2-(三氟甲氧基)乙烷（CF₃OCF₂CF₃，PFMEE）为0.544 ± 0.022 W·m⁻²·ppb⁻¹。