Ice nucleation abilities of surface collected mineral dust particles from the
Sahara (SD) and Asia (AD) are investigated for the temperature (T) range
253–233 K and for supersaturated relative humidity (RH) conditions in the
immersion freezing regime. The dust particles were also coated with a proxy
of secondary organic aerosol (SOA) from the dark ozonolysis of
α-pinene to better understand the influence of atmospheric coatings on
the immersion freezing ability of mineral dust particles. The measurements
are conducted on polydisperse particles in the size range
0.01–3 µm with three different ice nucleation chambers. Two of the
chambers follow the continuous flow diffusion chamber (CFDC) principle
(Portable Ice Nucleation Chamber, PINC) and the Colorado State University
CFDC (CSU-CFDC), whereas the third was the Aerosol Interactions and Dynamics
in the Atmosphere (AIDA) cloud expansion chamber. From observed activated
fractions (AFs) and ice nucleation active site (INAS)
densities, it is concluded within experimental uncertainties that
there is no significant difference between the ice nucleation ability of the
particular SD and AD samples examined. A small bias towards higher
INAS densities for uncoated versus SOA-coated dusts is found but
this is well within the 1σ (66 % prediction bands) region of the
average fit to the data, which captures 75 % of the INAS densities observed in this study. Furthermore, no systematic differences are
observed between SOA-coated and uncoated dusts in both SD and AD cases,
regardless of coating thickness (3–60 nm). The results suggest that any
differences observed are within the uncertainty of the measurements or
differences in cloud chamber parameters such as size fraction of particles
sampled, and residence time, as well as assumptions in using INAS densities to compare polydisperse aerosol measurements which may show
variable composition with particle size. Coatings with similar properties to
that of the SOA in this work and with coating thickness up to 60 nm are not
expected to impede or enhance the immersion mode ice nucleation ability of
mineral dust particles.</p
