Heterogeneous freezing of droplets with immersed surface modified mineral dust particles

Our understanding of the physical and chemical processes underlying heterogeneous ice is limited. For example, the question remains how surface modifications (e.g., due to atmospheric aging processes) influence the ability of aerosol particles to act as ice nuclei (IN). In the framework of the international measurement campaign FROST II (FReezing Of duST), which took place in April 2009, the heterogeneous freezing of droplets with immersed surface-modified and size-segregated mineral dust particles was investigated at the laminar diffusion cloud chamber LACIS (Leipzig Aerosol Cloud Interaction Simulator). The following measurements were performed: LACIS and CFDC (Continuous Flow thermal gradient Diffusion Chamber) were used to analyze the immersion freezing behavior of the surface modified mineral dust particles in different temperature regimes. The ability to act as IN in the deposition nucleation mode was quantified by the PINC (Portable Ice Nucleation Chamber), FINCH (Fast Ice Nucleus Chamber) and the CFDC instrument. AMS (Aerosol Mass Spectrometer) and ATOFMS (Aerosol Time-Of-Flight Mass Spectrometer) measurements were applied to determine particle composition. The hygroscopic growth and the critical super-saturations required for droplet activation were determined by means of an H-TDMA (Humidity-Tandem Differential Mobility Analyzer) and a CCN counter (Cloud Condensation Nucleus counter, Droplet Measurement Technologies), respectively. Here we concentrate on the results from investigations performed with LACIS during FROST II and thereafter. The quasi monodisperse 300 nm Arizona Test Dust (ATD) particles serving as mineral dust surrogate were chemically and physically treated by coating with sulphuric acid (H₂SO₄, three different coating thicknesses) and ammonium sulphate ((NH₄)₂SO₄). The different aerosol particle types were also thermally treated with a thermodenuder operating at 250°C, to remove volatile material from the aerosol particles. The H₂SO₄ coating modified the particles by reacting with the particle material, forming e.g. soluble sulfates and changing surface properties. The AMS showed free H₂SO₄ only for thick H₂SO₄ coatings. Uncoated particles and those coated with thin coatings of H₂SO₄ showed almost no hygroscopic growth. Particles coated with thicker coatings of H₂SO₄ and of (NH₄)₂SO₄ grew noticeably above 95% relative humidity. All investigated particles were found to activate at atmospherically relevant super-saturations. All surface modifications applied resulted in lowering the IN ability, with the deposition nucleation being more sensitive than the immersion freezing mode. Considering the immersion freezing behavior, pure ATD particles and particles coated with thin coatings of H₂SO₄ were more efficient IN than particles with thick H₂SO₄ or (NH₄)₂SO₄ coatings. Thermal treatments of the particles led to further decrease of the IN ability except for the uncoated dust particles and those coated with (NH₄)₂SO₄. In order to specify the temperature-dependent immersion freezing, two parameterizations based on either stochastic or singular hypothesis were performed. Additionally, measurements were done where the nucleation time for the immersion freezing was varied inside LACIS.