Towards online sampling and measurement of ice nucleating particles over a broad temperature and concentration range: Microfluidic device for Ice Nucleation analysis In cONtinuous flow (MINION)

• Applicants: Dr. Sarah Grawe; Dr.-Ing. Danny Reuter
• Subject Area: Atmospheric Science
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 549897546

Ice nucleating particles (INPs) are atmospheric aerosol particles that catalyze the ice nucleation process in cloud droplets at temperatures between 0 and -37 °C. The type and number of INPs affect the primary ice formation in clouds and their properties. Measurements of INP concentration, i.e., the number of INPs active per volume of air at a given temperature, are experimentally challenging due to large atmospheric variability of ~10 orders of magnitude. Due to this, there is a wide range of different instruments available to determine INP concentrations, each covering a specific concentration and temperature range according to the measurement principle. None of the currently available instruments detect both very rare high-temperature INP (> -10 °C) and more common low-temperature INP (< -25 °C). One way to extend the measurable concentration and temperature range is to study very large numbers of droplets in the nanoliter volume range. These can be generated in an oil matrix using microfluidic droplet generators and subsequently be cooled down until freezing. Several devices already exist that use this approach for INP measurements. These can be divided into two methods: First, there are instruments where a fixed number of droplets is generated and stored in a microfluidic chip. This chip is cooled while the freezing of the droplets is observed using a microscope and camera. Secondly, devices exist in which droplets are continuously generated and passed over a cooled surface in a microfluidic channel. Advantages over the first method are the unrestricted number of droplets and the potential of connecting to a particle-into-liquid sampler, which would allow quasi-online measurements of INP concentrations. In this project proposal, we therefore suggest an advancement of the second method. The new system MINION (Microfluidic device for Ice Nucleation analysis In cONtinuous flow) is characterized by a novel form of optical phase state detection, which replaces the time-consuming and often error-prone evaluation of camera images of the previously developed devices. This optical method can be easily and inexpensively multiplied, so that in combination with a suitable cooling system, high-resolution INP measurements can be performed between 0 °C and the homogeneous freezing limit. The measurable INP concentration range expands by at least 2 orders of magnitude to 6 orders of magnitude compared to existing instruments, allowing both high and low temperature INPs to be detected. In the proposed project, in addition to the development of MINION, we plan to perform a comprehensive characterization and measurements of atmospheric INP concentrations using filter samples from a wide range of geographic origins.

• DFG Programme: Research Grants