An instrument for quantifying heterogeneous ice nucleation in multiwell plates using infrared emissions to detect freezing

Low concentrations of ice nucleating particles (INPs) are thought to be important for the properties of mixed phase clouds, but their detection is challenging. While instruments to quantify INPs online can provide relatively high time resolution data, they typically cannot quantify very low INP concentrations. Furthermore, typical online instruments tend to report data at a single defined set of conditions. Hence, there is a need for instruments where INP concentrations of less than 0.01 L-1 can be routinely and efficiently determined. The use of larger volumes of suspension in drop assays increases the sensitivity of an experiment to rarer INPs or rarer active sites due to the increase in aerosol or surface area of particulates per droplet. Here we describe and characterise the InfraRed-Nucleation by Immersed Particles Instrument (IR-NIPI), a new immersion freezing assay that makes use of IR emissions to determine the freezing temperature of individual 50μL droplets each contained in a well of a 96-well plate. Using an IR camera allows the temperature of individual aliquots to be monitored. Freezing temperatures are determined by detecting the sharp rise in well temperature associated with the release of heat caused by freezing. In this paper we first present the calibration of the IR temperature measurement, which makes use of the freezing period after initial nucleation when wells warm and their temperature is determined by the ice-liquid equilibrium temperature, i.e. 0°C when the water activity is ~1. We then tested the temperature calibration using ~100 μm chips of K-feldspar, by immersing these chips in 1 μL droplets on an established cold stage (μL-NIPI) as well as in 50 μL droplets on IR-NIPI; the results were consistent with one another indicating no bias in the reported freezing temperature. In addition we present measurements of the efficiency of the mineral dust NX-illite and a sample of atmospheric aerosol collected on a filter in the city of Leeds. NX-illite results are consistent with literature data and the atmospheric INP concentrations were in good agreement with the results from the μL-NIPI instrument. This demonstrates the utility of this approach, which offers a relatively high throughput of sample analysis and access to low INP concentrations.