Abstract
We are presenting a new approach to analyze the freezing behavior of aqueous droplets containing ice nucleating particles. The freezing chip consists of an etched and sputtered (15 × 15 × 1) mm gold-plated silicon or pure gold chip, enabling the formation of droplets with defined diameters between 20 and 80 µm. Several applications like an automated process control and an automated image evaluation were implemented to improve the quality of heterogeneous freezing experiments. To show the functionality of the setup, we compared freezing temperatures of aqueous droplets containing ice nucleating particles (i.e., microcline, birch pollen washing water, juniper pollen, and Snomax® solution) measured with our setup, with literature data. The ice nucleation active surface/mass site density (ns/m) of microcline, juniper pollen, and birch pollen washing water are shown to be in good agreement with literature data. Minor variations can be explained by slight differences in composition and droplet generation technique. The nm values of Snomax® differ by up to one order of magnitude at higher subzero temperatures when compared with fresh samples but are in agreement when compared with reported data of aged Snomax® samples.
Highlights
The influence of clouds on the Earth’s climate system, weather phenomena, and hydrological cycle is well-investigated [1,2,3,4,5]
Aerosol particles play a crucial role by acting as cloud condensation nuclei (CCN) for liquid droplets or as ice nucleating particles (INPs) for the formation of ice particles
By placing 2 μL of the sample with a pipette on the chip and reabsorbing the suspension into the pipette, a thin film of suspension is applied on the freezing chip
Summary
The influence of clouds on the Earth’s climate system, weather phenomena, and hydrological cycle is well-investigated [1,2,3,4,5]. Cloud microphysics determine cloud albedo in the visible and infrared spectral ranges, cloud lifetime, and precipitation properties [6] In all these processes, aerosol particles play a crucial role by acting as cloud condensation nuclei (CCN) for liquid droplets or as ice nucleating particles (INPs) for the formation of ice particles. The vial-shaking technique has been used in our laboratory for several years and enables the investigation of ice nucleation activity (INA) of droplets with diameters down to 20 μm (4 pL volume) in a wide temperature range down to −38 ◦ C without any restrictions [14,26,27]. Our new approach for droplet-freezing experiments aims to reduce these issues as e.g., a nonuniform droplet size distribution, while still providing the ability to create small droplets and to investigate them in a wide temperature range
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