AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Laboratory Measurements of Contact Freezing by Clay Minerals
WILL CANTRELL, Jyoti Thapa, Joseph Niehaus, Michigan Technological University
Abstract Number: 346 Working Group: Aerosols, Clouds, and Climate
Abstract Pure water is unlikely to freeze unless it is supercooled by approximately 34 K. The presence of a solid surface within the water can reduce the necessary supercooling dramatically. Many mineral dusts will catalyze freezing at supercoolings of 25 to 30 K if they are immersed within the liquid while some biological materials require less than 10 K of supercooling to initiate freezing. If those same substances are presented at the air-water interface, the characteristic freezing temperature is higher. Though theories have been proposed for this enhancement, none have been proven due, in part, to the fact that quantitative data by which those theories could be tested has been lacking.
We have developed a technique to measure the fraction of particles which act as freezing nuclei in the contact mode, and have measured contact freezing efficiencies (both by number and surface area of the particles) for a series of clay minerals: illite, kaolinite, montmorillonite, nontronite-brown, and nontronite-green (all acquired from the Clay Mineral Society). Kaolinite has the highest activity in the contact mode; approximately one in one thousand kaolinite aerosol particles catalyzes contact freezing at -20 C, dropping to one in thirty thousand at -15 C. Nontronite-brown has a freezing efficiency comparable to kaolinite at -20 C, but is more sensitive to temperature. The remaining minerals are much less sensitive to temperature, exhibiting essentially no variation in the measured efficiency with temperature.
We will discuss these measurements in light of the theories put forward to explain contact freezing. In particular, we will use this series of clay minerals to evaluate the hypothesis that a substance's chemical composition contributes to its ability to serve as a contact nucleus.