AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Contact Freezing of Water by Simple Ionic Compounds
JOSEPH NIEHAUS, Will Cantrell, Michigan Technological University
Abstract Number: 244 Working Group: Aerosols, Clouds, and Climate
Abstract Heterogeneous freezing is responsible for the initial formation of ice in the lower to mid troposphere, where temperatures are rarely low enough for homogeneous freezing. Heterogeneous freezing at such temperatures is typically through the immersion/condensation mode, where the catalyzing substance is immersed within bulk water. Contact freezing, so-called because the catalyzing substance is at the air-water interface (i.e. in contact with it), is also possible. Published results (Niehaus et al., 2014; Hoffman et al., 2013) show that ice nucleating particles become active in the contact mode before the immersion mode. Particles which are effective ice nucleators in the contact mode are assumed to be good immersion ice nuclei as well. We will present results which contradict that assumption. In tests using water droplets at moderate supercoolings (-9 to -16 C), we see freezing initiated by particles composed of the ionic compounds NaCl, NaI, KCl, KI, NaOH, and KOH. Although soluble substances typically depress the freezing and melting point, if the water is previously supercooled then the impact of the particles with the surface of the liquid can trigger freezing. We have ruled out the endothermic cooling hypothesis (Knollenberg, 1969) by demonstrating that NaOH and KOH, which have exothermic heats of dissolution, cause freezing. The warmest temperature at which contact freezing occurs in these tests exhibit a composition dependence; for example, NaCl is a less efficient nucleator than NaI. The highest freezing temperature achievable by the compounds also scales with the density of the compound. We will discuss these experimental results in light of various mechanisms for contact freezing which have been proposed.
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Knollenberg, R.G., 1969. J. Atmos. Sci. 26, 125–129. doi:10.1175/1520-0469(1969)026<0125:TLCINM>2.0.CO;2
Niehaus, J., Bunker, K.W., China, S., Kostinski, A., Mazzoleni, C., Cantrell, W., 2014. J. Atmos. Oceanic Technol. 31, 913–922. doi:10.1175/JTECH-D-13-00156.1