Characterization of individual ice nuclei collected during CARES and a new view on immersion freezing kinetics

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Description

We investigate particles collected during the Carbonaceous Aerosols and Radiative Effects Study CARES) campaign for their ability for cold cloud formation. A combination of micro-spectroscopic and optical single particle analytic methods is applied to relate individual particle physical and chemical properties with observed water uptake and ice nucleation. The most efficient ice nuclei (IN) from a particle population are identified and characterized. Single particle characterization is provided by computer-controlled scanning electron microscopy with energy dispersive analysis of X-rays and scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy. A vapor controlled cooling-stage coupled to a microscope system is applied to determine the onsets of water uptake, immersion freezing, and deposition ice nucleation as a function of temperature (T) as low as 200°K and relative humidity (RH) up to water saturation. These measurements reveal that the majority of particles collected during CARES are coated by organic material. The identified IN, active above the homogeneous ice nucleation threshold, are also coated by organics and are thus similar to the majority of the particles that do not nucleate ice. This suggests that highly abundant and chemically complex organic aerosol, typical of an urban environment, can initiate ice formation.

In another set of experiments we study immersion freezing of micrometer-sized water and aqueous solution droplets containing various IN such as humic acid compounds and biological and mineral dust particles. These compounds show significantly enhanced freezing T compared to homogeneous ice nucleation. The immersion freezing T follows solution water activity (aw) similar to the aw-based homogeneous ice nucleation description. That is, it follows the ice melting curve shifted by a constant value in aw. We find that along the experimentally determined freezing curve (as a function of aw and T) the nucleation rate coefficient, Jhet, in units per surface area and time is constant. Particle aw in equilibrium equals ambient RH, allowing straightforward implementation into cloud models. Changes in IN surface areas result in a corresponding change in freezing T and Jhet as expected from classical nucleation theory. The findings allow for a new and computationally low-demand description of immersion freezing by knowledge of RH and IN surface area only, independent of the type of solute and applicable to a variety of atmospheric conditions.