| 20 Aug 2026
Temperature uncertainty characterisation of the FrESH droplet-freezing assay
Abstract. Reliable offline measurements of ice-nucleating particles (INPs) with droplet-freezing assays require accurate assignment of droplet temperature. We present FrESH (Freezing Experiment Setup Helsinki), a dual-plate immersion-freezing instrument developed for high-throughput analysis of filter-collected aerosol samples. FrESH uses standard 96-well polymerase chain reaction plates cooled in an ethanol bath with optical detection of freezing.
The relationship between bath temperature (TBT) and droplet temperature (Twell) was characterised using five PT100 sensors at fixed temperature and during cooling ramps for two chiller models. Wells were consistently warmer than the bath, and the bath-to-well relationship was approximately linear over the range from 0 to -35 °C, with plate-averaged relations of the form Twell≈0.97 TBT + (0.2–0.4) °C. Root-mean-square residuals of the linear fits were generally around 0.1 °C and remained below 0.2 °C in the sensor-resolved summaries. Analysis of routine FrESH measurement data revealed persistent spatial freezing patterns across the plates, consistent with position-dependent temperature variability. Combining the identified sources of uncertainty gives an estimated droplet-temperature uncertainty of 0.43–0.44 °C (1σ) over the operational range. This temperature characterisation is used to assign droplet temperatures and associated uncertainties in FrESH-derived NINP(T) spectra.
This manuscript presents a temperature uncertainty characterization of the FrESH droplet-freezing assay. Accurate temperature assignment is undoubtedly important for offline measurements of ice-nucleating particles. The authors have also performed a careful set of static and dynamic temperature measurements and constructed an uncertainty budget for the instrument.
However, I have a major concern regarding the novelty and scientific contribution of the present manuscript. In its current form, the work is largely limited to temperature characterization of a specific droplet-freezing instrument. This is primarily an instrumental and engineering characterization problem. I do not see a sufficiently substantial technical innovation or scientific advance that would justify publication as a standalone study. A substantially stronger manuscript would require additional experimental content. In particular, an intercomparison with an established instrument and/or a scientific application demonstrating the benefit of the improved temperature characterization would considerably strengthen the work. Therefore, I cannot recommend publication in its present form.
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