[1] The recently developed immersion mode cooling chamber has been used as an extension of the Zurich ice nucleation chamber (ZINC) in order to measure the ice nucleation efficiency of size-selected kaolinite particles in the immersion mode. Particles with selected diameters of 200, 400, and 800 nm have been activated as cloud condensation nuclei in order to obtain water droplets with single immersed particles. After continuous cooling of the droplets to the experimental temperature in ZINC, the frozen fraction of the droplets was measured with a recently developed depolarization detector, the ice optical detector (IODE). Temperatures below −30°C were necessary to freeze 50% of the droplets throughout the investigated size range. Although not very strong, a size dependence of the freezing efficiency has been observed. The median freezing temperature increases from −35°C for 200 nm kaolinite particles to −33°C for 800 nm particles. An experiment with 200 nm ammonium sulfate particles in the same temperature range resulted in no significant frozen fraction of the droplets. This suggests that the ice crystals observed in experiments with kaolinite particles nucleated heterogeneously. The temperature-dependent frozen fraction of water droplets has been fitted with different theoretical models. Comparison of the resulting fit curves with the data suggests that including the concept of active sites in the description of the ice nucleus surface is more appropriate than the approximation of a constant contact angle.Citation: Lüönd, F., O. Stetzer, A. Welti, and U. Lohmann (2010), Experimental study on the ice nucleation ability of size-selected kaolinite particles in the immersion mode,

The recently developed Zurich Ice Nucleation Chamber (ZINC) was used to explore ice nucleation of size-selected mineral dust particles at temperatures between −20 • C and −55 • C. Four different mineral dust species have been tested: montmorillonite, kaolinite, illite and Arizona test dust (ATD). The selected particle diameters are 100 nm, 200 nm, 400 nm and 800 nm. Relative humidities with respect to ice (RH i ) required to activate 1% of the dust particles as ice nuclei (IN) are reported as a function of temperature. An explicit size dependence of the ice formation efficiency has been observed for all dust types. 800 nm particles required the lowest RH i to activate. Deposition nucleation below water saturation was found only below −30 • C or −35 • C dependent on particle size. Minimum RH i for 1% activation were 105% for illite, kaolinite and montmorillonite at −40 • C, respectively 110% for ATD at −45 • C. In addition, a possible parameterisation for the measured activation spectra is proposed, which could be used in modeling studies.

The time dependence of immersion freezing was studied for temperatures between 236 K and 243 K. Droplets with single immersed, size-selected 400 nm and 800 nm kaolinite particles were produced at 300 K, cooled down to supercooled temperatures, and the fraction of frozen droplets with increasing residence time was detected. To simulate the conditions of immersion freezing in mixed-phase clouds we used the Zurich Ice Nucleation Chamber (ZINC) and its vertical extension, the Immersion Mode Cooling chAmber (IMCA). We observed that the frozen fraction of droplets increased with increasing residence time in the chamber. This suggests that there is a time dependence of immersion freezing and supports the importance of a stochastic component in the ice nucleation process. The rate at which droplets freeze was observed to decrease towards higher temperatures and smaller particle sizes. Comparison of the laboratory data with four different ice nucleation models, three based on classical nucleation theory with different representations of the particle surface properties and one singular, suggest that the classical, stochastic approach combined with a distribution of contact angles is able to reproduce the ice nucleation observed in these experiments most accurately. Using the models to calculate the increase in frozen fraction at typical mixed-phase cloud temperatures over an extended period of time, yields an equivalent effect of −1 K temperature shift for an increase in times scale by one order of magnitude. This suggests that temperature is more important than time

[1] The low-temperature aerosol and cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe was used to investigate the effect of sulfuric acid coating on the ice nucleation efficiency of soot aerosol particles from a spark discharge generator. The uncoated (sulfuric acid-coated) soot aerosol showed a nearly lognormal size distribution with number concentrations of 300-5000 cm À3 (2500-56,000 cm À3 ), count median diameters of 70-140 nm (90-200 nm), and geometric standard deviation of 1.3-1.4 (1.5-1.6). The volume fraction of the sulfuric acid coating to the total aerosol volume concentration ranged from 21 to 81%. Ice activation was investigated in dynamic expansion experiments simulating cloud cooling rates between about À0.6 and À3.5 K min À1. At temperatures between 186 and $235 K, uncoated soot particles acted as deposition nuclei at very low ice saturation ratios between 1.1 and 1.3. Above 235 K, ice nucleation only occurred after approaching liquid saturation. Coating with sulfuric acid significantly increased the ice nucleation thresholds of soot aerosol to saturation ratios increasing from $1.3 at 230 K to $1.5 at 185 K. This immersion mode of freezing nucleates ice well below the thresholds for homogeneous freezing of pure sulfuric acid solution droplets measured in previous AIDA experiments. A case study indicated that in contrast to the homogeneous freezing the nucleation rate of the immersion freezing mechanism depends only weakly on relative humidity and thereby the solute concentration. These results show that it is important to know the mixing state of soot and sulfuric acid aerosol particles in order to properly assess their role in cirrus formation. Citation: Möhler, O., et al. (2005), Effect of sulfuric acid coating on heterogeneous ice nucleation by soot aerosol particles,

Abstract. The homogeneous freezing of supercooled H2SO4/H2O solution droplets was investigated in the aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe. 24 freezing experiments were performed at temperatures between 189 and 235 K with aerosol particles in the diameter range 0.05 to 1 µm. Individual experiments started at homogeneous temperatures and ice saturation ratios between 0.9 and 0.95. Cloud cooling rates up to -2.8 K min-1 were simulated dynamically in the chamber by expansion cooling using a mechanical pump. Depending on the cooling rate and starting temperature, freezing threshold relative humidities were exceeded after expansion time periods between about 1 and 10 min. The onset of ice formation was measured with three independent methods showing good agreement among each other. Ice saturation ratios measured at the onset of ice formation increased from about 1.4 at 231 K  to about 1.75 at 189 K. The experimental data set including thermodynamic parameters as well as physical and chemical aerosol analysis provides a good basis for microphysical model applications.

Measuring systems for atmospheric ice nuclei are undergoing development anew and are beginning to meet the needs for studies of aerosol effects on ice-containing clouds. U nderstanding and predicting the formation of ice in clouds and its possible relation to the changing state of atmospheric composition (aerosols and gas phase) remain enigmatic. Such knowledge and capabilities are critical to quantifying the role of aerosols and their changing compositions on clouds, precipitation, and climate (Denman et al. 2007;Levin and Cotton 2009). This challenge is a major motivation for renewed attempts to measure ice nucleation processes in general, and to design and deploy new portable systems for measuring ice nuclei (IN), the particles that are considered the only means for initiation of the ice phase at temperatures warmer than about −36°C in the atmosphere. The fundamental desire to understand ice nucleation remains the same as when such research began in earnest more than 60 yr ago. The search to identify atmospheric ice nuclei lapsed during the 1970s-80s

A new instrument to study ice nucleation, the Zurich Ice Nucleation Chamber (ZINC), has been constructed. It is a continuous flow diffusion chamber following the design by Rogers (1988) but has a flat parallel plate geometry. The instrument can operate at temperatures as low as 236 K with the current setup with ice supersaturations of up to 50%. The typical sample flow is 1 lpm with a total flow of 10 lpm using twice 4.5 lpm for sheath flows. FLU-ENT simulations were performed and are presented to discuss the flow, temperature, and humidity profiles within the main chamber. Activation experiments with silver iodide particles were used to validate the instrument against literature data. We report the onset of freezing for an activated fraction of 2% of all particles. The data exhibit an almost linear trend between 257 K (111.5% RHi) and 237 K (119% RHi) with very good agreement with literature data.

Abstract.The new portable ice nucleation chamber (PINC) developed by the Institute for Atmospheric and Climate Sciences of ETH Zurich was operated during two measurement campaigns at the high alpine research station Jungfraujoch situated at 3580 m a.s.l, in March and June 2009. During this time of the year, a high probability of Saharan dust events (SDE) at the Jungfraujoch has been observed. We used an impactor with a cutoff size of 1 µm aerodynamic diameter and operated the system at −31 • C and relative humidities of 127 % and 91 % with respect to ice and water, respectively. Investigation of the ambient number concentration of ice nuclei (IN) in the deposition nucleation mode and during a SDE in the free troposphere is reported. The results discussed in this paper are the first continuous IN measurements over a period of several days at the Jungfraujoch. The average IN concentration found during the campaign in March was 8 particles per liter whereas during the campaign in June, the average number concentration was higher up to 14 particles per liter. Two SDEs were detected on 15 and 16 June 2009. Our measurements show that the SDEs had IN number concentration up to several hundreds per liter. We found the best correlation between the number concentration of the larger particle fraction measured by an optical particle counter and the IN number concentration during a Saharan dust event. This correlation factor is higher for particles larger than 0.5 µm meaning that a higher concentration of larger particles induced higher IN number concentration. No correlation could be found between the black carbon mass concentration and the variations in IN number concentration.