Correlative Spectroscopic Ellipsometry and Cryo-Scanning Electron Microscopy Measurements of Cirrus-Analog Ice Crystals

We report multi-technique measurements of static and dynamic ice crystals grown in a novel low-pressure and low-temperature environmental chamber. The objective of this new experimental configuration is to compare lab-grown ice crystals with those that have been observed in cirrus by satellite remote sensing, aircraft in-situ sensors, and balloon-capture. The observations of natural cirrus crystals are essential for fundamental understanding and system modeling, but these critical observations are also limited by sensor resolution, imprecise environmental characterization, and sampling insufficiency. In these new lab experiments, environmental conditions can be controlled and measured precisely, and the effects of varying temperature, pressure, humidity, and nucleation mode can be probed independently. Ice nucleation, growth and sublimation experiments have long been fruitfully undertaken by many researchers, however this approach is novel for several reasons: 1) the environment is specifically generated to match cirrus conditions, and 2) multiple microscopic and nanoscale measurement and imaging techniques are applied to provide a highly precise and quantitative view of the ice crystals as they evolve (i.e. “HD in 4D”).

Ice crystals are nucleated, grown, sublimated, and measured under a range of environmental condition sets approximating natural cirrus environments: total air pressure between 100 mb and 400 mb; temperature between -30°C to -70°C; water vapor saturation from 15% to 200% with respect to ice; and several mineral substrates and ice nucleation modes. Dry gas and vapor pressures are mixed, balanced, and measured using mass flow control and dynamic vacuum pressure adjustments. The stainless steel and aluminum environmental chamber has a volume of ~10 L and includes an in-situ spectroscopic ellipsometry beam path (Horiba Inc.) with a peltier-cooled sample plane in the center of the chamber. The ellipsometer produces a broad-spectrum beam (400 nm-1000nm wavelength) of linearly polarized light (variable spot sizes) that reflects from the ice surface into the ellipsometric polarization analyzer. The resulting measurement of polarization results in highly precise measurements (sub-nanometer resolution) of ice thickness perpendicular to the substrate surface, as well as additional information about surface roughness. Ellipsometrer measurement performance is best at thin thicknesses less than ~10 microns. Each successive non-destructive measurement requires no more than ~30 seconds and can define highly precise deposition/sublimation rates. Images and videos of growing ice crystals (to ~5 micron horizontal resolution) are also simultaneously recorded by long-working distance stereo-microscopy (Olympus SZ). Following a growth or sublimation experiment, the ice sample can be captured, hermetically sealed, and cryo-transferred into the TCNJ cryo-SEM for nano-scale imaging (Hitachi SU5000 and Quorum Inc.), including 3D texture analysis (Alicona MeX).

Experimental results are still in their early stages – ellipsometry, stereomicroscopy, and cryo-SEM systems are all fully functional, with optimization of workflow and exploration of parameter space currently underway. Ultimately, we aim to gain new insights on the relative influences of temperature, humidity, pressure, and nucleation on the complex array of growth rates, particle-scale morphologies, and sub-micron surface textures that make up natural cirrus clouds.