A new method for operating a continuous flow diffusion type ice chamber to investigate immersion freezing: assessment and performance study

 
Poster PDF

Author

Gourihar Kulkarni — Pacific Northwest National Laboratory

Category

High-latitude clouds and aerosols

Description

Glaciation in mixed-phase clouds predominately occurs through immersion freezing mode where ice nucleating particles (INP) immersed within a supercooled droplet induces nucleation of ice. Currently, model representations of this process is a large source of uncertainty in simulating cloud radiative properties, and to constrain these estimates, immersion freezing efficiencies of INPs derived from the laboratory measurements are commonly used. In this study, PNNL ice chamber was operated in a non-standard style and evaluated measuring ice nucleation properties of four INP species: K-feldspar, illite-NX, natural soil, and ambient desert dust that had shown ice nucleation over a wide span of supercooled temperatures. The droplets were activated onto the individual INPs in the upper growth section of the chamber maintained at ~-20 °C and relative humidity with respect to water (RHw) ~115% and cooled at a constant cooling rate (0.5 °Cmin-1) within the lower section (evaporator) of the ice chamber down to homogeneous freezing limit. The high RHw conditions to maximize the droplet activation was achieved with longer and independently temperature controlled evaporator section. This mode of operation provided continuous immersion freezing spectra, without any discontinuity, and also allowed to calculate the nucleation rates to assess the single- or multi-component nature of nucleation sites on the INP surface. The ice nucleation property measured with the surface active site density (ns) metric was higher by about one order of magnitude compared to the results obtained under the standard mode chamber operation. However, the results were comparable with the literature data that are determined explicitly via immersion freezing mechanism. Further, a better agreement between surface area normalized freezing (R/A) rate and the heterogeneous nucleation (Jhet) rate was observed only after distribution in the form of a linear fit of temperature dependence contact angle was used indicating the diverse nature of INP test species. These results would help to reconcile different formulations of describing ice nucleating ability in cloud models to improve the prediction of ice formation in clouds.