Micro-PINGUIN: Microtiter plate-based ice nucleation detection in gallium with an infra-red camera
Ice nucleation particles play a crucial role in atmospheric processes e.g., they can trigger ice formation in clouds and thus influence their lifetime and optical properties. The quantification and characterization of these particles require reliable and precise measurement techniques. In this publication, we present a novel droplet freezing instrument to measure immersion freezing of biotic and abiotic ice-nucleating particles within the temperature range of 0 {degrees}C to -25 {degrees}C. Immersion freezing of the samples is investigated using 384-well PCR plates with a sample volume of 30 {micro}l. Nucleation events are detected with high precision using a thermal camera that records the increase in infrared emission due to the latent heat release. To maximize the thermal contact between the PCR plate and the surrounding cooling unit, we use a gallium bath as a mount for the PCR plate. The combination of good thermal connectivity and precise temperature recording enables accurate ({+/-} 0.81 {degrees}C at -10 {degrees}C) and reproducible ({+/-} 0.20 {degrees}C) detection of the nucleation temperatures. For comparison with already existing instruments, the new ice nucleation instrument, "micro-PINGUIN", was characterized using Snomax(R) (hereafter Snomax) and Illite NX suspensions. The results are in agreement with what has been reported in the literature for the already existing instruments. Consequently, the results that will be produced using the micro-PINGUIN are of good quality and can be compared to the results produced by other validated instruments for the study of immersion freezing of various ice nucleating particles. Further, we investigated the reproducibility of experiments using Snomax suspensions and found poor reproducibility when suspensions were prepared freshly even if the same batch of Snomax is used. This could be attributed to substrate heterogeneity, aging effects, and dilution errors. The reproducibility of the measurements is greatly improved for Snomax suspensions that are prepared in advance and stored frozen in aliquots. Thus, we suggest the use of suspensions frozen in aliquots for further reproducibility measurements and intercomparison studies.