Quantitative Determination of the Contact Freezing Probabilities of Various Ice Nuclei

Contact freezing, i.e. the initiation of freezing of supercooled liquid droplets by their contact with external ice nuclei has been known for a long time to be more effective than immersion freezing with the same nucleus being immersed within the droplet. It is difficult however to quantify the efficiency of contact freezing nuclei experimentally, as the rate of contacts between the external nuclei and the droplets is not easily accessible. We report here on quantitative temperature- dependent contact freezing probabilities for various mineral dust particles and some organic ice nuclei, obtained from laboratory experiments. Individual levitated droplets have been brought in contact with well characterized external aerosol particles and the contact freezing probabilities have been calculated from the measured freezing rates and contact rates, the latter being determined offline by counting the number of scavenged particles under and environmental scanning electron microscope.