Snowflake Surface Chemistry

Snow crystals formed in the atmosphere are made in several shapes that are dependent on the temperature and supersaturation of their surrounding environment (Figure 1).  On the surface of all ice is a quasi-liquid layer (QLL, Figure 2) with a thickness that is temperature dependent.  The temperature at which the QLL disappears is an open source of debate, but what is important is how it affects the morphology of the snow crystal.  It is believed that changes in the QLL thickness, which is different at a given temperature depending on the crystal facet, is what causes the distinct morphological transitions in Figure 1 (Kuroda & Lacmann, J. Crystal Growth 1982).  Extensive work on snow crystals has recently been done by Libbrecht; however, there have to date been no direct measurements of the water supersaturation or impurity (e.g. acetic acid) concentrations at the point of crystal growth.  As a method of testing the hypothesis of Kuroda and Lacmann, a snow growth chamber was constructed.  Initially, crystals were grown on an electrode with an applied potential of 2kV (Figure 3); however, this was traded for a string hung down the center of the chamber which allowed growth of many crystals as a function of temperature and/or supersaturation at once (Figure 4).