Contact Freezing Experiments and Temperature-Dependent Particle Viscosity Measurements Using a Low-Temperature Dual-Balance Electrodynamic Trap

KYLE MCMILLAN, Dharma Johnson, Upasna Rai, Jingchuan Chen, Ryan Davis, Sarah D. Brooks, Margaret Tolbert, University of Colorado Boulder

     Abstract Number: 257
     Working Group: Aerosol Physics

Abstract
The interaction of clouds with incoming shortwave solar radiation and outgoing longwave terrestrial radiation remains a significant source of uncertainty in current climate models. A major reason for this uncertainty is poorly constrained estimates of the ratio of supercooled liquid water droplets to ice crystals in mixed phase-clouds. A more accurate ice nucleation scheme, including a greater understanding of heterogeneous ice nucleation processes is essential in improving these estimates. Additionally, the existence of semi-solid and glassy particles and the dependence of their viscosities on temperature and relative humidity further complicate the ability of models to accurately determine both the radiative effects of atmospheric aerosols on climate and the role of the particle phase in atmospheric chemistry. Here, we have developed a dual-balance linear quadrupole electrodynamic trap that has been modified for use at sub-freezing temperatures. Importantly, this apparatus allows us to probe the behavior of the particle phase on freely floating droplets, removing the unwanted influence of external surfaces. Mechanism-specific freezing experiments have been carried out to determine both immersion and contact ice nucleation temperatures. Using silver iodide as the ice nucleating particle, results have shown substantial enhancement in freezing temperatures in both the immersion and contact modes relative to the homogeneous freezing limit, with contact freezing occurring at the highest temperature. Further, indirect particle viscosity measurements were carried out on sorbitol and 1:1 sorbitol:NaCl under dry conditions using the droplet coalescence method. As expected, particle viscosity showed a strong dependence on temperature, with viscosities increasing by nearly two orders of magnitude in both systems studied from room-temperature to just above freezing. These experiments not only further establish the use of low-temperature particle and droplet levitation experiments but also demonstrate the importance of such techniques in understanding the particle phase in the atmosphere.