Mid-infrared Extinction Spectra and Optical Constants of Supercooled Water Droplets

Complex refractive indices of supercooled liquid water have been retrieved at 269, 258, 252, and 238 K in
the 4500−1100 cm-1 wavenumber regime from series of infrared extinction spectra of micron-sized water
droplets. The spectra collection was recorded during expansion experiments in the large coolable aerosol
chamber AIDA of Forschungszentrum Karlsruhe. A Mie inversion technique was applied to derive the low-temperature refractive index data sets by iteratively adjusting the room-temperature optical constants of liquid
water until obtaining the best agreement between measured and calculated infrared spectra of the supercooled
water droplets. The new optical constants, revealing significant temperature-induced spectral variations in
comparison with the room-temperature refractive indices, proved to be in good agreement with data sets
obtained in a recent study. A detailed analysis was performed to elaborate potential inaccuracies in the retrieval
results when deriving optical constants from particle extinction spectra using an iterative procedure.

在269K、258K、252K及238K的温度下，通过对微米级水滴红外消光光谱系列的分析，成功获取了超冷却液态水的复杂折射率数据。该光谱数据收集是在弗劳恩霍夫卡尔斯鲁厄研究中心的大型可调冷气溶胶室AIDA的膨胀实验期间进行的。通过应用米氏逆变换技术，通过迭代调整室温下液态水的光学常数，直至获得超冷却水滴测得与计算红外光谱的最佳一致性，从而推导出低温折射率数据集。与室温折射率相比，新的光学常数揭示了显著的温度诱导光谱变化，并且与最近研究中获得的数据集表现出良好的一致性。对从粒子消光光谱中推导光学常数时可能出现的潜在误差进行了详细分析。