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 nominal 0–3 and 0–7 μm particle diameters, respectively. Aqueous suspensions with ATD concentrations from 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. Depending on ATD concentration, heterogeneous ice nucleation occurred at temperatures up to 256 K or down to the onset of homogeneous ice nucleation (237 K). The measured heat release in the DSC was modeled with Classical Nucleation Theory (CNT), using experimentally determined ATD size and emulsion droplet volume distributions to quantify the heterogeneous surface present in the droplets. The observed dependence of the heterogeneous freezing temperatures on ATD concentrations could not be achieved with one constant contact angle but was reproduced by a model that describes heterogeneous ice nucleation occurring on active sites with variable ability to nucleate ice. This model implies that only a very minor fraction of the ATD surface is indeed effective as ice nuclei (IN). Keywords Heterogeneous nucleation, ice nucleation, mineral dust, DSC