Abstract
Abstract. In this paper, the effect of the morphological modification of aerosol particles with respect to heterogeneous ice nucleation is comprehensively investigated for laboratory-generated hematite particles as a model substrate for atmospheric dust particles. The surface-area-scaled ice nucleation efficiencies of monodisperse cubic hematite particles and milled hematite particles were measured with a series of expansion cooling experiments using the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud simulation chamber. Complementary offline characterization of physico-chemical properties of both hematite subsets were also carried out with scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, dynamic light scattering (DLS), and an electro-kinetic particle charge detector to further constrain droplet-freezing measurements of hematite particles. Additionally, an empirical parameterization derived from our laboratory measurements was implemented in the single-column version of the Community Atmospheric Model version 5 (CAM5) to investigate the model sensitivity in simulated ice crystal number concentration on different ice nucleation efficiencies. From an experimental perspective, our results show that the immersion mode ice nucleation efficiency of milled hematite particles is almost an order of magnitude higher at −35.2 °C < T < −33.5 °C than that of the cubic hematite particles, indicating a substantial effect of morphological irregularities on immersion mode freezing. Our modeling results similarly show that the increased droplet-freezing rates of milled hematite particles lead to about one order magnitude higher ice crystal number in the upper troposphere than cubic hematite particles. Overall, our results suggest that the surface irregularities and associated active sites lead to greater ice activation through droplet freezing.
Highlights
The chemical and physical surface structure of an aerosol particle can greatly influence its microphysical characteristics such as hygroscopicity (e.g., Twohy and Anderson, 2008; Shilling et al, 2007), chemical reactivity (e.g., Di Cosimo et al, 1998), optical properties (Linke et al, 2006), and ice nucleation efficiency (e.g., Schill and Tolbert, 2013; Chapter 9 of Pruppacher and Klett, 1997)
Chemical homogeneity within single particles was confirmed by energy dispersive X-ray (EDX) spectroscopy composition analysis, and EDX spectra of cubic and milled hematite particles were statistically similar at the 90 % confidence limit
Laboratory and modeling studies were performed to examine the role of surface morphology upon ice nucleation on hematite particles that heterogeneously freeze at −35.2 ◦C < T < −33.5 ◦C
Summary
The chemical and physical surface structure of an aerosol particle can greatly influence its microphysical characteristics such as hygroscopicity (e.g., Twohy and Anderson, 2008; Shilling et al, 2007), chemical reactivity (e.g., Di Cosimo et al, 1998), optical properties (Linke et al, 2006), and ice nucleation efficiency (e.g., Schill and Tolbert, 2013; Chapter 9 of Pruppacher and Klett, 1997). It has been discussed that the surface’s physical and chemical properties may play a major role in the initiation of ice crystals in mixed-phase clouds and ice clouds (Mason, 1971; Cziczo et al, 2013), yet our current understanding of ice nucleation properties of various aerosols is still rudimentary, in part due to the fact that changes in composition, size, and Published by Copernicus Publications on behalf of the European Geosciences Union. Hiranuma et al.: Influence of surface morphology surface morphology are often inherently related during an aerosol’s atmospheric lifetime and aging process (Hiranuma et al, 2013)
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