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 (T_BT) and droplet temperature (T_well) 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 T_well≈0.97 T_BT + (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 N_INP(T) spectra.