A droplet freezing technique similar to was used to determine the freezing temperature spectrum of aqueous suspensions. We have previously used this technique to study the ice-nucleating properties of inorganic, organic, and environmental INS samples . Hydrophobic glass slides (Hampton Research HR3-239) were rinsed with ultrapure water, dried with nitrogen gas, and placed on a cold stage (Grant Asymptote EF600 cryocooler). A micropipette was used to place twenty 1 μL drops of sample onto each slide (Figure ). A second 1 μL drop of 0.01 M KNO3 solution was then added to each sample droplet, bringing the total volume of each droplet to 2 μL, and the overall concentration of KNO3 in each droplet to 0.005 M. The 0.01 M KNO3 solution was prepared using ultrapure water. KNO3 was added to combat the effects of any loss of K + ions during the density gradient centrifugation on the freezing properties of K-feldspar . KNO3 was added to all samples for consistency. One slide of 20 droplets was prepared for each sample. A chamber with a digital camera was placed over the cold stage to isolate the droplets from the ambient air. A small flow of N2 gas was passed through the chamber to prevent condensation during cooling. The cold stage temperature was decreased at a rate of 3 °C min -1 until all sample droplets were frozen while the digital camera attached to the chamber recorded video of the freezing process. The freezing temperature of each droplet was determined by processing the recorded video and cold stage temperature data via a MATLAB script . The cold stage temperature measurements had an uncertainty of approximately ± 0.25 °C according to the manufacturer specifications. This uncertainty was verified by measuring the melting points of water and dodecane and comparing the measured values with literature values . Because KNO3 was added to all droplets, all freezing