Near-UV Extinction of Submicron Thickness Ice Films in the 290-350 nm Range at 258 K

Ice clouds affect the energy balance of the atmosphere through absorption, reflection, and scattering of solar radiation. We have developed a new experimental technique to simultaneously measure thin ice film extinction and its thickness (about 0.06-0.21 μm) by combining Brewster angle cavity ring-down spectroscopy and quartz crystal microbalance. The ice film serves as a proxy for ice clouds. Thin ice films were formed by water vapor deposition on a silica surface at 258 K. The average extinction cross sections of ice films were determined to be about 6.6 × 10^-23, 8.1 × 10^-23, 5.3 × 10^-23, 5.6 × 10^-23, 5.2 × 10^-23, 5.1 × 10^-23, and 3.9 × 10^-23 cm²/molecule at wavelengths of 290, 300, 310, 320, 330, 340, and 350 nm at 258 K, respectively. Atmospheric implications of the results are discussed.