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The heterogeneous freezing temperatures of supercooled drops were measuredusing an acoustic levitator. This technique allows one to freely suspend singledrops in the air without any wall contact. Heterogeneous nucleation by twotypes of illite (illite IMt1 and illite NX) and a montmorillonite sample wasinvestigated in the immersion mode. Drops of 1 mm in radius were monitoredby a video camera while cooled down to −28 °C to simulatefreezing within the tropospheric temperature range. The surface temperatureof the drops was contact-free, determined with an infrared thermometer; theonset of freezing was indicated by a sudden increase of the drop surfacetemperature. For comparison, measurements with one particle type (illiteNX) were additionally performed in the Mainz vertical wind tunnel with dropsof 340 μm radius freely suspended. Immersion freezing was observed ina temperature range between−13 and −26 °C as a function ofparticle type and particle surface area immersed in the drops. Isothermalexperiments in the wind tunnel indicated that after the cooling stagefreezing still proceeds, at least during the investigated time period of 30 s. The results were evaluated by applying two descriptions of heterogeneousfreezing, the stochastic and the singular model. Although the wind tunnelresults do not support the time-independence of the freezing process bothmodels are applicable for comparing the results from the two experimentaltechniques

atmospheric chemistry and physics

Particle surface area dependence of mineral dust in immersion freezing mode: investigations with freely suspended drops in an acoustic levitator and a vertical wind tunnel