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Volume 18, issue 3 | Copyright

Special issue: Anthropogenic dust and its climate impact

Atmos. Chem. Phys., 18, 1785-1804, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 07 Feb 2018

Research article | 07 Feb 2018

Characterization of individual ice residual particles by the single droplet freezing method: a case study in the Asian dust outflow region

Ayumi Iwata1 and Atsushi Matsuki2 Ayumi Iwata and Atsushi Matsuki
  • 1Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa, 920-1192, Japan
  • 2Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa, 920-1192, Japan

Abstract. In order to better characterize ice nucleating (IN) aerosol particles in the atmosphere, we investigated the chemical composition, mixing state, and morphology of atmospheric aerosols that nucleate ice under conditions relevant for mixed-phase clouds. Five standard mineral dust samples (quartz, K-feldspar, Na-feldspar, Arizona test dust, and Asian dust source particles) were compared with actual aerosol particles collected from the west coast of Japan (the city of Kanazawa) during Asian dust events in February and April 2016. Following droplet activation by particles deposited on a hydrophobic Si (silicon) wafer substrate under supersaturated air, individual IN particles were located using an optical microscope by gradually cooling the temperature to −30°C. For the aerosol samples, both the IN active particles and non-active particles were analyzed individually by atomic force microscopy (AFM), micro-Raman spectroscopy, and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX). Heterogeneous ice nucleation in all standard mineral dust samples tested in this study was observed at consistently higher temperatures (e.g., −22.2 to −24.2°C with K-feldspar) than the homogeneous freezing temperature (−36.5°C). Meanwhile, most of the IN active atmospheric particles formed ice below −28°C, i.e., at lower temperatures than the standard mineral dust samples of pure components. The most abundant IN active particles above −30°C were predominantly irregular solid particles that showed clay mineral characteristics (or mixtures of several mineral components). Other than clay, Ca-rich particles internally mixed with other components, such as sulfate, were also regarded as IN active particle types. Moreover, sea salt particles were predominantly found in the non-active fraction, and internal mixing with sea salt clearly acted as a significant inhibiting agent for the ice nucleation activity of mineral dust particles. Also, relatively pure or fresh calcite, Ca(NO3)2, and (NH4)2SO4 particles were more often found in the non-active fraction. In this study, we demonstrated the capability of the combined single droplet freezing method and thorough individual particle analysis to characterize the ice nucleation activity of atmospheric aerosols. We also found that dramatic changes in the particle mixing states during long-range transport had a complex effect on the ice nucleation activity of the host aerosol particles. A case study in the Asian dust outflow region highlighted the need to consider particle mixing states, which can dramatically influence ice nucleation activity.

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This study directly characterized the morphology, composition, and mixing states of individual ice residue particles by using an individual particle freezing method. We show that clay mineral particles are more important ice nuclei sampled during an Asian dust event and that the internal mixing of particles during long-range transport has a complex effects on the particles' ice nucleating activity.
This study directly characterized the morphology, composition, and mixing states of individual...