Efficiency of immersion mode ice nucleation on surrogates of mineral dust C. Marcolli, S. Gedamke, T. Peter, and B. Zobrist Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland
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.
Citation: Marcolli, C., Gedamke, S., Peter, T., and Zobrist, B.: Efficiency of immersion mode ice nucleation on surrogates of mineral dust, Atmos. Chem. Phys., 7, 5081-5091, doi:10.5194/acp-7-5081-2007, 2007.