10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Isothermal Immersion Freezing Experiments Involving Mineral Dust: The Role of INP Surface Area
ASSAF ZIPORI, Daniel Knopf, Yinon Rudich, Weizmann Institute of Science
Abstract Number: 330 Working Group: Unraveling the Many Facets of Ice Nucleating Particles and Their Interactions with Clouds
Abstract Ice nucleation in mixed phase clouds are still not well understood. This has many implications on the understanding of the radiative effects and rain formation in such clouds. Immersion freezing is considered to be the most important ice nucleation pathway in mixed-phase clouds, and recent field and modeling studies suggest a time-dependent ice nucleation process. Currently, two main approaches are used to describe immersion freezing: (1) The classical nucleation theory (CNT) that assumes a stochastic freezing process expressed by a heterogeneous ice nucleation rate coefficient (Jhet) that depends on surface area and time, and (2) a deterministic or singular (time independent) approach which is described by an ice active site density (Ns), which is determined by the surface area only. It has been claimed that CNT fails to describe apparent varying freezing rates for isothermal experiments for single INP type. However, a recent study suggests that ice nucleation can be explained by a combination of stochastic uncertainty, when observing too little freezing events, and variation of INP surface area among droplets. We extend the experimental immersion freezing data set to test the effect of varying INP surface on immersion freezing using sufficiently large numbers of freezing events to render stochastic uncertainty negligible. We present results of isothermal experiments using the WeIzmann Supercooled Droplets Observation on a Microchip (WISDOM). Using this setup, it is possible to analyze a large number of freezing events in each experiment, with a narrow droplet size distribution, thereby providing a good data set for analysis and parameterization. In addition, the INP efficiencies of minerals expressed as Jhet and Ns were also determined applying a constant cooling rate experiments. Those were followed by isothermal freezing experiments, where the temperature was held constant for up to three hours, for different temperatures. Jhet derived from constant cooling experiments were applied to describe the increase in the frozen fraction during the isothermal freezing experiments and for comparison with the Jhet derived directly from the isothermal freezing experiments. In the next step, the INP size distribution was examined in order to properly estimate the INP surface area distribution in the drops. By assuming a varying INP surface area distributed between the drops, the observed frozen fraction values in the isothermal freezing experiment could be described using a single value of Jhet for the applied temperature. These experiments enhance our descriptive understanding of ice nucleation parametrization and classical nucleation theory. This will improve our ability to model primary ice nucleation in clouds for applications in cloud and climate models, with important effects on Earth’s radiation budget and the hydrological cycle.