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.