HR: 17:45h
AN: A14C-07    [Abstracts]
TI: Crystallization of Cubic Ice in Liquid Water and Aqueous Solutions
AU: * Murray, B J
EM: bmurray@chem.ubc.ca
AF: University of British Columbia, Department of Chemistry 2336 Main Mall , Vancouver, BC V6T 1Z1 Canada
AU: Knopf, D A
EM: knopf@chem.ubc.ca
AF: University of British Columbia, Department of Chemistry 2336 Main Mall , Vancouver, BC V6T 1Z1 Canada
AU: Bertram, A K
EM: bertram@chem.ubc.ca
AF: University of British Columbia, Department of Chemistry 2336 Main Mall , Vancouver, BC V6T 1Z1 Canada
AB: It is often assumed that the only natural phase of ice that forms on Earth is that of hexagonal ice (ice Ih). However, the rare observation of haloes around the sun at 28$^{o}$ and cloud particles of cubic habit indicates that the metastable crystalline form of ice, cubic ice (ice Ic), may form in Earth's atmosphere. The conditions used to produce ice Ic in previous studies are most likely never experienced on Earth outside the laboratory, since they usually involve extremely fast cooling rates. Here we show, using X-ray diffraction, that cubic ice forms when micrometer-sized pure water and aqueous solution droplets freeze homogeneously using a modest cooling rate ( 10 K min$^{-1}$). In fact, ice Ic is the exclusive product when NaCl, (NH$_{4}$)$_{2}$SO$_{4}$, (NH$_{4}$)$_{3}$H(SO$_{4}$)$_{2}$ and HNO$_{3}$ solution droplets freeze at temperatures below 190 K, and pure water droplets freeze to nearly 50% ice Ic at 235 K. These results offers significant insights into the crystallization of ice, a process that occurs widely in nature and strongly suggest that cubic ice will form in the atmosphere. Recently, Murphy (GRL, Art. No. 2230, 2003) has shown that the formation of cubic ice in cirrus clouds may alter the particle size distribution sufficiently to increase dehydration in the tropopause region.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting