Microanalysis Techniques to Study Atmospheric Ice Nucleation and Ice Crystal Growth
The prediction of how ice crystals form represents one of the great conundrums in the atmospheric sciences with important implications for the hydrological cycle and climate. Ice-nucleating particles (INPs), typically consisting of sub-and supermicrometer-sized aerosol particles which can be inorganic, organic, biogenic, or biological, initiate heterogeneous ice nucleation processes leading to ice crystal formation. Heterogeneous ice nucleation commences on the nanoscale at the substrate surface and depends on ambient temperature and humidity. Microanalysis techniques are uniquely suited to examine the physicochemical features of the INPs under relevant atmospheric conditions thereby advancing our understanding of the principal processes that promote ice nucleation. In the atmosphere ice crystals experience growth and sublimation resulting in complex microscopic morphologies which, in turn, impact the ice crystal's properties. This chapter provides an overview of microanalysis techniques employed in either a multimodal or in situ manner, which shed light on the atmospheric heterogeneous ice nucleation process and how these techniques are applied to explore the complex morphologies of ice crystals. The first section introduces the various atmospheric ice nucleation pathways, provides a brief outline of the underlying nucleation theory demonstrating that nucleation proceeds on the nanoscale, and describes the different ice crystal habits of growth. Section two provides an overview of the microanalysis techniques and experiments to study INPs or ice-nucleating substrates including multimodal instrument approaches followed by in situ ice nucleation studies. Examples of techniques include Raman, atomic force, electron, and X-ray microscopy with discussion of the techniques’ unique capabilities to examine the physical and chemical properties of the ice-nucleating substrate. The third section presents electron microscopy studies of ice crystals during growth and sublimation displaying the morphological complexities of ice crystals. Lastly, section four discusses typical experimental requirements including sample sizes, radiation effects, and the role of standard INPs.