HR: 0800h
AN: A21C-0758    [Abstracts]
TI: Internal Structure in Supercooled Water Aerosols and Their Role in the Formation of Ice Clouds
AU: * Khalizov, A F
EM: khalizov@sciborg.uwaterloo.ca
AF: University of Waterloo, Waterloo Centre for Atmospheric Sciences, Waterloo, ON N2L3G1 Canada
AU: Earle, M E
EM: meearle@sciborg.uwaterloo.ca
AF: University of Waterloo, Waterloo Centre for Atmospheric Sciences, Waterloo, ON N2L3G1 Canada
AU: Zasetsky, A Y
EM: azaset@sciborg.uwaterloo.ca
AF: University of Waterloo, Waterloo Centre for Atmospheric Sciences, Waterloo, ON N2L3G1 Canada
AU: Sloan, J J
EM: sloanj@UWaterloo.CA
AF: University of Waterloo, Waterloo Centre for Atmospheric Sciences, Waterloo, ON N2L3G1 Canada
AB: The chemistry and physics of processes occurring in supercooled cloud droplets is a sensitive function of the structure of the liquid. Such properties as uptake and diffusion coefficients, viscosity and ice nucleation behaviour all change rapidly with the amount of supercooling in samples of pure water. Direct measurements of these properties can only be made in micron-sized droplets. This presentation reports an experimental study of the nanostructure of supercooled water droplets in the temperature range 240-294 K. The experiments were done in a cryogenic flow tube coupled to an FTIR spectrometer. The flowtube permits us to create and manipulate aerosol particles having a desired size distribution at precisely known cryogenic temperatures. The size distribution and internal structure of the supercooled liquid droplets are obtained from the infrared extinction spectra of the flowing aerosols. These measurements show that ordered ice-like nanoclusters form inside droplets of supercooled water. The size and number of these clusters increase with decreasing temperature; they can occupy up to 40% of the droplet volume near freezing. The size distribution of the particles in a supercooled water aerosol is also observed to change dramatically upon freezing. This demonstrates directly the importance of mass transfer through the vapour phase - a process that also occurs during formation of ice clouds. The efficient transfer of mass from liquid droplets to nascent ice crystals occurs because supercooled water has a substantial vapour pressure, even at its freezing temperature. In our experiments, when a water aerosol freezes, we observe the rapid formation of large ice particles with size distributions that are generally not lognormal and frequently are multi-modal. This complicates the interpretation of the freezing kinetics significantly, necessitating the use of an aerosol microphysics model that takes into account the mass transfer as well as the nucleation and freezing rates. We will report our observations of the formation of dynamic nanoclusters and our measurements of freezing rates and mass transfer properties in supercooled water droplets for temperatures down to about 234 K, which is the ultimate freezing temperature of the water aerosol droplets used in this study.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0317 Chemical kinetic and photochemical properties
DE: 0320 Cloud physics and chemistry
DE: 0360 Transmission and scattering of radiation
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting