Ice-nucleating particle quantification with a large volume drop assay using infrared thermometry on the IR-NIPI

Ice-nucleating particles (INPs) initiate the glaciation of mixed-phase clouds and therefore influence the radiative properties of clouds and the water cycle (Vergara-Temprado et al., 2018). However, INPs are rare in nature and their detection in field studies can be challenging. In particular, quantification of the very low INP concentrations relevant for the Hallet-Mossop ice multiplication regime is extremely challenging (and not possible with online instruments such as Continuous Flow Diffusion Chambers). Here we present a new large volume, offline immersion mode drop assay, the InfraRed Nucleation by Immersed Particle Instrument (IR-NIPI), for the detection of low concentrations of INPs. The IR-NIPI uses infrared (IR) emissions to detect the freezing temperature of 50 μL droplets within a 96 well plate (Harrison et al., 2018). The advantage of using infrared emissions is that it enables the system to monitor the temperature of the individual droplets. We present the development of a calibration technique for the IR-camera where the ice-liquid equilibrium temperature (i.e. 0 ˚C when the activity of water is ~1) is used to provide a convenient point of calibration for each well. We also tested this temperature calibration using ~100 μm K-feldspar mineral chips immersed in 50 μL droplets on the IR-NIPI and in 1 μL droplets on a well characterised cold stage (μL-NIPI). This test showed good agreement between the two techniques and suggested no bias in freezing temperatures between the different instruments. A proxy for mineral dust (NX-illite) and an atmospheric aerosol sample collected in the city of Leeds were also tested in the IR-NIPI system. The atmospheric INP concentrations were consistent with results obtained from the μL-NIPI instrument and the NX-illite data presented is in agreement with literature data. However, we also highlight instances where the INP concentration from IR-NIPI (and other large volume assays) are not self consistent and are not consistent with other techniques. This phenomena is unexplained at present.

Harrison, A. D., Whale, T. F., Rutledge, R., Lamb, S., Tarn, M. D., Porter, G. C., Adams, M. P., McQuaid, J. B., Morris, G. J., and Murray, B. J.: An instrument for quantifying heterogeneous ice nucleation in multiwell plates using infrared emissions to detect freezing, Atmos. Meas. Tech., 11, 5629-5641, 2018.

Vergara-Temprado, J., Miltenberger, A. K., Furtado, K., Grosvenor, D. P., Shipway, B. J., Hill, A. A., Wilkinson, J. M., Field, P. R., Murray, B. J., and Carslaw, K. S.: Strong control of Southern Ocean cloud reflectivity by ice-nucleating particles, Proc. Nat. Acad. Sci., 115, 2687-2692, 10.1073/pnas.1721627115, 2018.