Immersion freezing of clay minerals and bacterial ice nuclei

Abstract

The immersion mode ice nucleation efficiency of clay minerals and biological aerosols has been investigated using the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber. Both monodisperse and polydisperse populations of (1) various clay dust samples as well as (2) Snomax® (a proxy for bacterial ice nucleators) and (3) hematite are examined in the temperature range between −4°C and −35°C. The temperature dependence of ice formation inferred by the INAS (Ice Nucleation Active Surface-Site) density is investigated and discussed as a function of cooling rate and by comparing to predicted nucleation rates (i.e., classical nucleation theory with θ-probability density function nucleation scheme). To date, we observe that maintaining constant AIDA temperature does not trigger any new ice formation during the immersion freezing experiments with clay dust samples and Snomax®, implying strong temperature dependency (and weak time dependency) within our time scales and conditions of experiments. Ice residuals collected through a newly developed PCVI (Pumped ounter-flow Virtual Impactor) with the 50% cut size diameter of 10 to 20 μm have also been examined by electron microscope analyses to seek the chemical and physical identity of ice nuclei in clay minerals. In addition to the AIDA results, complementary measurements with mobile ice nucleation counters are also presented.