Heterogeneous freezing of ice on atmospheric aerosols containing ash, soot, and soil

Abstract

[1] Nucleation of ice crystals in the atmosphere often occurs through heterogeneous freezing processes in which an atmospheric aerosol acts as the ice nuclei (IN). Depending on the ambient conditions and composition of the available IN, heterogeneous nucleation may occur through one of several freezing mechanisms, including contact and immersion. In this study, an optical microscope apparatus equipped with a cooling stage and a digital camera was used to observe the freezing events of individual droplet-IN samples. For each experiment, a particular IN was either placed in contact with the surface or immersed in the bulk of an ultra pure water droplet. Using volcanic ash as the IN, we observed that inside-out contact freezing occurred at warmer temperatures than immersion freezing. We also conducted contact freezing experiments using three representative aerosol types as the potential IN. The most effective contact freezing IN was Pahokee Peat soil with an average freezing temperature of À10.5°C, followed by volcanic ash (À11.2°C), and finally soot (À25.6°C). In addition, we have used classical nucleation theory to derive the heterogeneous nucleation rates for the IN compositions explored.

Deposition and immersion-mode nucleation of ice by three distinct samples of volcanic ash

“…The results of these studies, combined with previous studies on large, 250-300 µm ash particles from the 1980 Mt. St. Helens eruption (Fornea et al, 2009), suggest that there is one characteristic ice nucleation efficiency for all ash, even when accounting for frozen fractions and surface area . While such behavior would allow for a great simplification in models, these results represent only two volcanoes..…”

A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of 17 ice nucleation measurement techniques

“…More recently, other optical microscopy techniques coupled with a unique method of encapsulating particles into droplets followed by cooling (Iannone et al, 2011) or using the hydrophobic squalene/water emulsion were introduced to the community. Using a similar approach, substrate-supported cooling studies have been applied to determine the freezing temperature in the contact mode (e.g., Fornea et al, 2009;Niehaus et al, 2014), or of deposition nucleation (e.g., Kanji and Abbatt, 2006;Bingemer et al, 2012;Dymarska et al, 2006). The microscopy-coupled substrate-supported freezing devices are advantageous for visualizing the consequences of specific ice nucleation modes in controlled and simulated environments.…”

“…Previous laboratory experiments have taken diverse approaches in an attempt to mimic ice nucleation and freezing processes. These heterogeneous ice formation processes include deposition nucleation, immersion freezing, condensation freezing, and contact freezing (Vali, 1985), inside-out contact freezing (i.e., freezing of an immersed INP in contact with the droplet surface from the inside; Durant and Shaw, 2005;Fornea et al, 2009) and surface condensation freezing (i.e., freezing of supercooled water or residual aqueous solution trapped on particle surfaces, e.g., by the inverse Kelvin effect; Christenson, 2013;Hiranuma et al, 2014a;Marcolli, 2014;Welti et al, 2014;Wex et al, 2014). Without INPs, pure cloud water droplets or solution within particles can be supercooled to below −37 • C before freezing (Koop et al, 2000;Murray et al, 2010;Rosenfeld and Woodley, 2000).…”

Ice nucleation by fertile soil dusts: relative importance of mineral and biogenic components

“…Four mechanisms have been hypothesised to account for the heterogeneous nucleation of ice in mixed-phase clouds: deposition, contact, immersion and condensation, each of which has been defined by Vali (1985), although other mechanisms may also be possible (Fornea et al, 2009;Durant and Shaw, 2005). Vali (1985) defines immersion freezing as…”