Efficiency of Immersion Mode Ice Nucleation on Surrogates of Mineral Dust

Abstract

A differential scanning calorimeter (DSC) was used to explore heterogeneous ice nucleation of emulsified aqueous suspensions of two Arizona test dust (ATD) samples with particle diameters of nominally 0-3 and 0-7 µm, respectively. Aqueous suspensions with ATD concentrations of 0.01-20 wt% have been investigated. The DSC thermograms exhibit a homogeneous and a heterogeneous freezing peak whose intensity ratios vary with the ATD concentration in the aqueous suspensions. Homogeneous freezing temperatures are in good agreement with recent measurements by other techniques. Depending on ATD concentration, heterogeneous ice nucleation occurred at temperatures as high as 256 K or down to the onset of homogeneous ice nucleation (237 K). For ATD-induced ice formation Classical Nucleation Theory (CNT) offers a suitable framework to parameterize nucleation rates as a function of temperature, experimentally determined ATD size, and emulsion droplet volume distributions. The latter two quantities serve to estimate the total heterogeneous surface area present in a droplet, whereas the suitability of an individual heterogeneous site to trigger nucleation is described by the compatibility function (or contact angle) in CNT. The intensity ratio of homogeneous to heterogeneous freezing peaks is in good agreement with the assumption that the ATD particles are randomly distributed amongst the emulsion droplets. The observed dependence of the heterogeneous freezing temperatures on ATD concentrations cannot be described by assuming a constant contact angle for all ATD particles, but requires the ice nucleation efficiency of ATD particles to be (log)normally distributed amongst the particles. Best quantitative agreement is reached when explicitly assuming that high-compatibility sites are rare and that therefore larger particles have on average more and better active sites than smaller ones. This analysis suggests that a particle has to have a diameter of at least 0.1 µm to exhibit on average one active site.

Ice nucleation on mineral dust particles: Onset conditions, nucleation rates and contact angles

“…J het is defined as the number of nucleation events per unit surface area of solid material per unit time. Note that J het is referred to as both a rate [ Pruppacher and Klett , 1997; Martin , 2000; Hung et al , 2003; Parsons et al , 2004a, 2004b; Archuleta et al , 2005; Pant et al , 2006] and a rate coefficient [ Dymarska et al , 2006; Marcolli et al , 2007] in the literature. The heterogeneous nucleation rate is related to the onset data through : where ω is the number of ice crystals nucleated, A s is the total mineral dust surface area available for heterogeneous nucleation, and t is the observation time.…”

An Empirical Parameterization of Heterogeneous Ice Nucleation for Multiple Chemical Species of Aerosol

“…Larger IN particles have more and better active sites than smaller ones, which is why they are more likely to have a highernucleating efficiency. The natural singular character of IN emerges from the probability distribution of nucleation efficiencies, and hence of characteristic freezing temperatures, among IN particles (e.g., Marcolli et al 2007). This distribution of nucleation efficiencies arises from that of the "contact angle" (i.e., suitability of an individual site to trigger nucleation) among active sites on IN particles.…”

“…The number mixing ratio of aerosols in group X is n X . This minimum value of IN diameter (0.1 m) has been introduced because central (aerosol) residual particles of snow crystals have been observed to be usually larger than that size (e.g., Pruppacher and Klett 1997;Chen et al 1998;Prenni et al 2007a; see also Marcolli et al 2007). Here, ⍀ X,1,* is the component of ⍀ X due to aerosols with diameters between 0.1 and 1 m in the background-troposphere scenario.…”

“…However, the first-order de-scription of the nucleation that they observed is provided by the singular hypothesis according to their analysis (e.g., Vali 1994Vali , p. 1852. Marcolli et al (2007) have confirmed this picture with laboratory studies of immersion freezing. Marcolli et al inferred a distribution of rare but efficient active sites over the IN surface where ice embryos can form, as invoked by the singular hypothesis (see section 2).…”

Impact of heterogeneous ice nuclei on homogeneous freezing events in cirrus clouds

“…As a consequence, the freezing properties of these mixtures are very different; each component could have its own characteristic freezing threshold depending on chemical and/or microphysical properties. It is know from laboratory studies that the size of IN also plays a crucial role; larger particles activate as ice crystals at lower relative humidities than smaller particles [ Pruppacher and Klett , 1997; Archuleta et al , 2005; Kanji and Abbatt , 2006; Marcolli et al , 2007]. A priori, it is not clear how these different heterogeneous IN properties will affect homogeneous nucleation occurring later within the same environment.…”