Abstract.A new process is presented by which water soluble organics might influence ice nucleation, ice growth, chemical reactions and water uptake of aerosols in the upper troposphere: the formation of glassy aerosol particles. Glasses are disordered amorphous (non-crystalline) solids that form when a liquid is cooled without crystallization until the viscosity increases exponentially and molecular diffusion practically ceases. The glass transition temperatures, T g , homogeneous ice nucleation temperatures, T hom , and ice melting temperatures, T m , of various aqueous inorganic, organic and multi-component solutions are investigated with a differential scanning calorimeter. The investigated solutes are: various polyols, glucose, raffinose, levoglucosan, an aromatic compound, sulfuric acid, ammonium bisulfate and mixtures of dicarboxylic acids (M5), of dicarboxylic acids and ammonium sulfate (M5AS), of two polyols, of glucose and ammonium nitrate, and of raffinose and M5AS. The results indicate that aqueous solutions of the investigated inorganic solutes show T g values that are too low to be of atmospheric importance. In contrast, aqueous organic and multicomponent solutions readily form glasses at low but atmospherically relevant temperatures (≤230 K). To apply the laboratory data to the atmospheric situation, the measured phase transition temperatures were transformed from a concentration to a water activity scale by extrapolating water activities determined between 252 K and 313 K to lower temperatures. The obtained state diagrams reveal that the higher the molar mass of the aqueous organic or multi-component solutes, the higher T g of their respective solutions at a given water activity. To a lesser extent, T g also depends on the hydrophilicity of the organic solutes. Therefore, aerosol particles containing larger ( 150 g mol −1 ) and more hydrophobic organic molecules are more likely to form glasses at interCorrespondence to: T. Koop (thomas.koop@uni-bielefeld.de) mediate to high relative humidities in the upper troposphere. Our results suggest that the water uptake of aerosols, heterogeneous chemical reactions in aerosol particles, as well as ice nucleation and ice crystal growth can be significantly impeded or even completely inhibited in organic-enriched aerosols at upper tropospheric temperatures with implications for cirrus cloud formation and upper tropospheric relative humidity.

We present measurements of water uptake and release by single micrometre-sized aqueous sucrose particles. The experiments were performed in an electrodynamic balance where the particles can be stored contact-free in a temperature and humidity controlled chamber for several days. Aqueous sucrose particles react to a change in ambient humidity by absorbing/desorbing water from the gas phase. This water absorption (desorption) results in an increasing (decreasing) droplet size and a decreasing (increasing) solute concentration. Optical techniques were employed to follow minute changes of the droplet's size, with a sensitivity of 0.2 nm, as a result of changes in temperature or humidity. We exposed several particles either to humidity cycles (between ∼2% and 90%) at 291 K or to constant relative humidity and temperature conditions over long periods of time (up to several days) at temperatures ranging from 203 to 291 K. In doing so, a retarded water uptake and release at low relative humidities and/or low temperatures was observed. Under the conditions studied here, the kinetics of this water absorption/desorption process is controlled entirely by liquid-phase diffusion of water molecules. Hence, it is possible to derive the translational diffusion coefficient of water molecules, D(H(2)O,) from these data by simulating the growth or shrinkage of a particle with a liquid-phase diffusion model. Values for D(H(2)O)-values as low as 10(-24) m(2) s(-1) are determined using data at temperatures down to 203 K deep in the glassy state. From the experiment and modelling we can infer strong concentration gradients within a single particle including a glassy skin in the outer shells of the particle. Such glassy skins practically isolate the liquid core of a particle from the surrounding gas phase, resulting in extremely long equilibration times for such particles, caused by the strongly non-linear relationship between concentration and D(H(2)O). We present a new parameterization of D(H(2)O) that facilitates describing the stability of aqueous food and pharmaceutical formulations in the glassy state, the processing of amorphous aerosol particles in spray-drying technology, and the suppression of heterogeneous chemical reactions in glassy atmospheric aerosol particles.

The heterogeneous ice nucleation rate coefficient (j het) of water droplets coated with a monolayer of 1-nonadecanol was determined from multiple freezing/melting cycles. Freezing was monitored optically with a microscope for droplet radii between 31 and 48 μm and with a differential scanning calorimeter for radii between 320 and 1100 μm. The combination of these two techniques allows the surface area of the 1-nonadecanol nucleating agent to be varied by more than a factor of 1000, showing that j het increases only by ∼5 orders of magnitude over a temperature range of 18 K. This is roughly 5 times less than the change in the ice nucleation rate coefficient for homogeneous ice freezing at around 238 K or for heterogeneous ice freezing in the presence of a solid ice nucleus, such as Al2O3. This temperature dependence of j het can be reconciled with the framework of classical nucleation theory, when assuming a reduced compatibility of the alcohol monolayer with the ice embryo as the temperature decreases. We attribute this finding to an enhanced ability of the alcohol monolayer to adapt to the ice structure close to the ice melting point due to larger thermal density fluctuations in the monolayer, which in turn makes the monolayer serve as a better ice nucleus.

Abstract.A differential scanning calorimeter (DSC) was used to explore heterogeneous ice nucleation of emulsified aqueous suspensions of two Arizona test dust (ATD) samples with particle diameters of nominally 0-3 and 0-7 µm, respectively. Aqueous suspensions with ATD concentrations of 0.01-20 wt% have been investigated. The DSC thermograms exhibit a homogeneous and a heterogeneous freezing peak whose intensity ratios vary with the ATD concentration in the aqueous suspensions. Homogeneous freezing temperatures are in good agreement with recent measurements by other techniques. Depending on ATD concentration, heterogeneous ice nucleation occurred at temperatures as high as 256 K or down to the onset of homogeneous ice nucleation (237 K). For ATD-induced ice formation Classical Nucleation Theory (CNT) offers a suitable framework to parameterize nucleation rates as a function of temperature, experimentally determined ATD size, and emulsion droplet volume distributions. The latter two quantities serve to estimate the total heterogeneous surface area present in a droplet, whereas the suitability of an individual heterogeneous site to trigger nucleation is described by the compatibility function (or contact angle) in CNT. The intensity ratio of homogeneous to heterogeneous freezing peaks is in good agreement with the assumption that the ATD particles are randomly distributed amongst the emulsion droplets. The observed dependence of the heterogeneous freezing temperatures on ATD concentrations cannot be described by assuming a constant contact angle for all ATD particles, but requires the ice nucleation efficiency of ATD particles to be (log)normally distributed amongst the particles. Best quantitative agreement is reached when explicitly assuming that high-compatibility sites are rare and that therefore larger particles have on average more and better active sites than smaller ones. This analysis suggests that a particle has to have a diameter of at least 0.1 µm to exhibit on average one active site.

Ice nucleation is an important process in numerous environmental systems such as atmospheric aerosol droplets or biological tissues. Here we analyze two widely used approaches for describing homogeneous ice nucleation in aqueous solutions with respect to their applicability to heterogeneous ice nucleation processes: the lambda approach and the water-activity-based approach. We study experimentally the heterogeneous ice nucleation behaviour of mineral dust particles and biological ice nuclei (Snomax; Pseudomonas syringae) in aqueous solutions as a function of solute concentration for various solutes (sulfuric acid, ammonium sulfate, glucose, and poly(ethylene glycol) with two different molar masses of 400 and 6000 g mol(-1)). We show that the ice nucleation temperature and the corresponding lambda values depend on both the type of ice nucleus and the type of solute, while the water-activity-based approach depends only on the type of ice nucleus when the solution water activity is known. Finally, we employ both approaches to the study of ice nucleation in biological systems such as the supercooling point of living larvae and insects. We show that the behaviour of freeze tolerant and freeze avoiding species can be described using the two approaches and we discuss how the analysis can be used to interpret experimental results of the freezing behaviour of living species.

Heterogeneous ice nucleation experiments have been performed with four different ice nuclei (IN), namely nonadecanol, silica, silver iodide and Arizona test dust. All IN are either immersed in the droplets or located at the droplets surface. The IN were exposed to various aqueous solutions, which consist of (NH4)2SO4, H2SO4, MgCl2, NaCl, LiCl, Ca(NO3)2, K2CO3, CH3COONa, ethylene glycol, glycerol, malonic acid, PEG300 or a NaCl/malonic acid mixture. Freezing was studied using a differential scanning calorimeter and a cold finger cell. The results show that the heterogeneous ice freezing temperatures decrease with increasing solute concentration; however, the magnitude of this effect is solute dependent. In contrast, when the results are analyzed in terms of the solution water activity a very consistent behavior emerges: heterogeneous ice nucleation temperatures for all four IN converge each onto a single line, irrespective of the nature of the solute. We find that a constant offset with respect to the ice melting point curve, Deltaaw,het, can describe the observed freezing temperatures for each IN. Such a behavior is well-known for homogeneous ice nucleation from supercooled liquid droplets and has led to the development of water-activity-based ice nucleation theory. The large variety of investigated solutes together with different general types of ice nuclei studied (monolayers, ionic crystals, covalently bound network-forming compounds, and a mixture of chemically different crystallites) underlines the general applicability of water-activity-based ice nucleation theory also for heterogeneous ice nucleation in the immersion mode. Finally, the ice nucleation efficiencies of the various IN, as well as the atmospheric implication of the developed parametrization are discussed.

Abstract. Emulsion and bulk freezing experiments were performed to investigate immersion ice nucleation on clay minerals in pure water, using various kaolinites, montmorillonites, illites as well as natural dust from the Hoggar Mountains in the Saharan region. Differential scanning calorimeter measurements were performed on three different kaolinites (KGa-1b, KGa-2 and K-SA), two illites (Illite NX and Illite SE) and four natural and acid-treated montmorillonites (SWy-2, STx-1b, KSF and K-10). The emulsion experiments provide information on the average freezing behaviour characterized by the average nucleation sites. These experiments revealed one to sometimes two distinct heterogeneous freezing peaks, which suggest the presence of a low number of qualitatively distinct average nucleation site classes. We refer to the peak at the lowest temperature as "standard peak" and to the one occurring in only some clay mineral types at higher temperatures as "special peak". Conversely, freezing in bulk samples is not initiated by the average nucleation sites, but by a very low number of "best sites". The kaolinites and montmorillonites showed quite narrow standard peaks with onset temperatures 238 K < T std on < 242 K and best sites with averaged median freezing temperature T best med = 257 K, but only some featuring a special peak (i.e. KSF, K-10, K-SA and SWy-2) with freezing onsets in the range 240-248 K. The illites showed broad standard peaks with freezing onsets at 244 K < T std on < 246 K and best sites with averaged median freezing temperature T best med = 262 K. The large difference between freezing temperatures of standard and best sites shows that characterizing ice nucleation efficiencies of dust particles on the basis of freezing onset temperatures from bulk experiments, as has been done in some atmospheric studies, is not appropriate. Our investigations demonstrate that immersion freezing temperatures of clay minerals strongly depend on the amount of clay mineral present per droplet and on the exact type (location of collection and pre-treatment) of the clay mineral. We suggest that apparently contradictory results obtained by different groups with different setups are indeed in good agreement when only clay minerals of the same type and amount per droplet are compared. The natural sample from the Hoggar Mountains, a region whose dusts have been shown to be composed mainly of illite, showed very similar freezing characteristics (standard and best) to the illites. Relating the concentration of best IN to the dust concentration in the atmosphere suggested that the best IN in the Hoggar sample would be common enough downwind of their source region to account for ambient IN number densities in the temperature range of 250-260 K at least during dust events.

Abstract. Heterogeneous ice freezing points of aqueous solutions containing various immersed solid dicarboxylic acids (oxalic, adipic, succinic, phthalic and fumaric) have been measured with a differential scanning calorimeter. The results show that only the dihydrate of oxalic acid (OAD) acts as a heterogeneous ice nucleus, with an increase in freezing temperature between 2 and 5 K depending on solution composition. In several field campaigns, oxalic acid enriched particles have been detected in the upper troposphere with single particle aerosol mass spectrometry. Simulations with a microphysical box model indicate that the presence of OAD may reduce the ice particle number density in cirrus clouds by up to ∼50% when compared to exclusively homogeneous cirrus formation without OAD. Using the ECHAM4 climate model we estimate the global net radiative effect caused by this heterogeneous freezing to result in a cooling as high as −0.3 Wm −2 .