American Association for Aerosol Research - Abstract Submission

AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA

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Measurement of the Optical Properties and Contact Freezing Ability of Supercooled Water Droplets Using Optical Tweezers

Hassan Beydoun, Kyle Gorkowski, Mark Aboff, Jim Walker, Jonathan P. Reid, RYAN SULLIVAN, Carnegie Mellon University

     Abstract Number: 483
     Working Group: Aerosols, Clouds, and Climate

Abstract
Supercooled water is a ubiquitous substance in the atmosphere due to the significant energy barrier to the nucleation of the ice phase. Heterogeneous ice nucleation is a poorly constrained process with the contact freezing mode remaining the most nebulous due to the experimental challenge of colliding a supercooled water droplet and a potential ice nucleus in a controlled and directly observable manner. A refrigerated optical tweezers system capable of trapping individual water droplets at controlled subzero temperatures has been developed to permit the investigation of a range of supercooled water properties. Preliminary findings indicate that this system can detect individual droplet-ice nuclei collisions via the quenching of the surface-sensitive whispering gallery modes that occur at discrete wavelengths of water’s Raman mode. This paves the way for a novel contact freezing measurement system that retrieves contact freezing efficiencies directly without requiring any assumptions regarding the number of collisions prior to freezing. The occurrence of droplet freezing is readily observed from the Raman spectrum and visual imaging. Together, this enables the precise determination of the freezing efficiency of various types of atmospherically-relevant mineral dust particles, by measuring the number of particle-droplet contact events required before ice nucleation occurs. Novel measurements of the refractive index of supercooled water have also been retrieved using this method. These results indicate that the increase in water’s refractive index with decreasing temperature has been greatly underestimated.