Measurement of Ice Nuclei using the FRankfurt Ice Nuclei Deposition FreezinG Experiment (FRIDGE)

FRIDGE has two separate applications that address different pathways of the freezing process: It can be used as an isothermal diffusion chamber to measure deposition/condensation freezing INP or as a droplet freezing assay for immersion freezing INP.

Isothermal diffusion chamber

First, atmospheric (or laboratory) aerosol particles are collected by electrostatic precipitation onto silicon wafers (Fig. 1). The programmable semiautomatic aerosol collector for up to seven samples can be controlled remotely and is miniaturized to allow operation onboard unmanned aircrafts. In a second step, samples are analyzed in the FRIDGE isothermal vacuum diffusion chamber. During a measurement a sample disk is exposed to supersaturated water vapor at a temperature typically below -20°C. Activated ice crystals will grow to macroscopic size and will be counted by a CCD camera (Fig. 2 and 3). INP concentrations obtained by this method are representative for deposition nucleation and condensation freezing. It is possible to analyze the chemical composition and morphology of individual INPs by electron microscopy (in collaboration with TU Darmstadt).

Droplet freezing assay

In this case, atmospheric (or laboratory) aerosols are collected on PTFE membrane filters. Aerosols are washed off the filters by aqueous extraction. 50 – 100 droplets of typically 0.5 µm are placed homogeneously on a silanized wafer by a pipette (Fig. 4). Temperature is decreased constantly by 1 K/min until every droplet is frozen. This method gives the immersion mode freezing INP concentration.

Figure 1: Electrostatic aerosol collector	Figure 2: The INP counter FRIDGE under open condition with the cold stage and Si wafer in the central lower part and CCD camera (black) in the upper part.
Figure 3: Ice crystals grown by deposition / condensation freezing on a Si wafer inside FRIDGE (@-30°C, 101% RH)	Figure 4: Droplets of liquid (clear) and frozen (opaque) water on the FRIDGE immersion drop freezing assay.