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

Abstract

Abstract. The heterogeneous freezing temperatures of supercooled drops were measured using an acoustic levitator. This technique allows one to freely suspend single drops in the air without any wall contact. Heterogeneous nucleation by two types of illite (illite IMt1 and illite NX) and a montmorillonite sample was investigated in the immersion mode. Drops of 1 mm in radius were monitored by a video camera while cooled down to −28 • C to simulate freezing within the tropospheric temperature range. The surface temperature of the drops was contact-free, determined with an infrared thermometer; the onset of freezing was indicated by a sudden increase of the drop surface temperature. For comparison, measurements with one particle type (illite NX) were additionally performed in the Mainz vertical wind tunnel with drops of 340 µm radius freely suspended. Immersion freezing was observed in a temperature range between −13 and −26 • C as a function of particle type and particle surface area immersed in the drops. Isothermal experiments in the wind tunnel indicated that after the cooling stage freezing still proceeds, at least during the investigated time period of 30 s. The results were evaluated by applying two descriptions of heterogeneous freezing, the stochastic and the singular model. Although the wind tunnel results do not support the time-independence of the freezing process both models are applicable for comparing the results from the two experimental techniques.

Intercomparing different devices for the investigation of ice nucleating particles using Snomax<sup>®</sup> as test substance

“…With an infrared thermometer the temperature of the freezing drops is measured directly and free of contact. As this requires a circular spot of approximately 1 mm in diameter, the investigated drops had sizes of 2.0 ± 0.1 mm in diameter (Diehl et al, 2014). Because of their rather large volume and missing ventilated heat transfer, the levitated drops cooled down rather slowly while exchanging heat with the ambient air in the cold chamber which was approximately −23 • C during the Snomax experiments.…”

A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of 17 ice nucleation measurement techniques

“…The freezing temperature of INPs either immersed in or in contact with levitated supercooled water droplets suspended in the air can also be determined by the change in light scattering with a charge-coupled device (CCD) camera using an electrodynamic balance (EDB; Hoffmann et al, 2013), an acoustic levitator (Diehl et al, 2014) or in a vertical wind tunnel (Szakáll et al, 2009). The advantage of these methods is the ability to provide, via high-resolution images, substratefree information for statistically representative ice nucleation processes on a single droplet basis.…”

Coarse and giant particles are ubiquitous in Saharan dust export regions and are radiatively significant over the Sahara

“…Spectral RI data, for which the real part represents scattering and the imaginary part represents absorption, are taken from a range of sources. For the full spectrum, RI data are available from the OPAC database (Hess et al, 1998) based on values from d' Almeida et al (1991) and Shettle and Fenn (1979), Volz (1973), and Balkanski et al (2007), assuming a 1.5 % hematite content, as well as the World Meteorological Organization (WMO, 1983) and Fouquart et al (1987). For the shortwave spectrum RI data are also available from Colarco et al (2014), and for the longwave spectrum data are available from Di Biagio et al (2017), from which we have selected the Mauritania subset as it is representative of the middle of the range for their North Africa samples.…”