The unique properties of water in the supercooled (metastable) state are not fully understood. In particular, the effects of solutes and mechanical pressure on the kinetics of the liquid-to-solid phase transition of supercooled water and aqueous solutions to ice have remained unresolved. Here we show from experimental data that the homogeneous nucleation of ice from supercooled aqueous solutions is independent of the nature of the solute, but depends only on the water activity of the solution--that is, the ratio between the water vapour pressures of the solution and of pure water under the same conditions. In addition, we show that the presence of solutes and the application of pressure have a very similar effect on ice nucleation. We present a thermodynamic theory for homogeneous ice nucleation, which expresses the nucleation rate coefficient as a function of water activity and pressure. Recent observations from clouds containing ice are in good agreement with our theory and our results should help to overcome one of the main weaknesses of numerical models of the atmosphere, the formulation of cloud processes.

Abstract. Interactions with water are crucial for the properties, transformation and climate effects of atmospheric aerosols. Here we present a conceptual framework for the interaction of amorphous aerosol particles with water vapor, outlining characteristic features and differences in comparison to crystalline particles. We used a hygroscopicity tandem differential mobility analyzer (H-TDMA) to characterize the hydration and dehydration of crystalline ammonium sulfate, amorphous oxalic acid and amorphous levoglucosan particles (diameter ∼100 nm, relative humidity 5-95% at 298 K). The experimental data and accompanying Köhler model calculations provide new insights into particle microstructure, surface adsorption, bulk absorption, phase transitions and hygroscopic growth. The results of these and related investigations lead to the following conclusions:(1) Many organic substances, including carboxylic acids, carbohydrates and proteins, tend to form amorphous rather than crystalline phases upon drying of aqueous solution droplets. Depending on viscosity and microstructure, the amorphous phases can be classified as glasses, rubbers, gels or viscous liquids.(2) Amorphous organic substances tend to absorb water vapor and undergo gradual deliquescence and hygroscopic growth at lower relative humidity than their crystalline counterparts.(3) In the course of hydration and dehydration, certain organic substances can form rubber-or gel-like structures Correspondence to: U. Pöschl (u.poschl@mpic.de) (supramolecular networks) and undergo transitions between swollen and collapsed network structures.(4) Organic gels or (semi-)solid amorphous shells (glassy, rubbery, ultra-viscous) with low molecular diffusivity can kinetically limit the uptake and release of water and may influence the hygroscopic growth and activation of aerosol particles as cloud condensation nuclei (CCN) and ice nuclei (IN). Moreover, (semi-)solid amorphous phases may influence the uptake of gaseous photo-oxidants and the chemical transformation and aging of atmospheric aerosols.(5) The shape and porosity of amorphous and crystalline particles formed upon dehydration of aqueous solution droplets depend on chemical composition and drying conditions. The apparent volume void fractions of particles with highly porous structures can range up to ∼50% or more (xerogels, aerogels).(6) For efficient description of water uptake and phase transitions of aerosol particles, we propose not to limit the terms deliquescence and efflorescence to equilibrium phase transitions of crystalline substances. Instead we propose generalized definitions according to which amorphous and crystalline components can undergo gradual or prompt, partial or full deliquescence or efflorescence.We suggest that (semi-)solid amorphous phases may be important not only in the upper atmosphere as suggested in recent studies of glass formation at low temperatures. Depending on relative humidity, (semi-)solid phases and moisture-induced glass transitions may also play a role in gas-particle intera...

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.

Abstract. We present laboratory experiments employing differential scanning calorimetry as well as flow cell microscopy to study the microphysics of aqueous NaC1 and sea-salt solutions and droplets at temperatures below 273 K. The freezing and melting points of ice and other precipitates were determined in NaC1 and sea-salt bulk samples as well as in emulsion samples. Using flow cell microscopy, we have determined the deliquescence and efflorescence relative humidities of NaC1 and sea-salt droplets at temperatures between 249 and 273 K, extending the existing room temperature data to polar conditions. Our measurements suggest that sea-salt aerosols will most likely be liquid most of the time under polar marine boundary conditions. In addition, we show that sea-salt aerosols or seawater spray deposited on the polar ice pack will remain partly liquid down to 230 K, with concentrations of C1-and Br-increasing by more than an order of magnitude upon cooling when compared to normal seawater concentrations. This is likely to enhance the rate at which heterogeneous bromine activation reactions occur in the sea-salt deposits. Such reaction rate enhancements with decreasing temperatures are currently not implemented in chemical models, and might help explain the fast bromine activation and subsequent ozone destruction observed during ozone depletion events in the polar marine boundary layer in spring. , 1994]. Information on the microphysics of sea-salt aerosols and patches of sea salt deposited on the ice pack under these extrcme polar conditions is still lacking. In this paper we present results on the microphysics of NaC1 and sea-salt solution droplets at low temperatures. We have preformed calorimetric freezing experiments with emulsified droplets as well as flow cell microscopy of deposited droplets to investigate the behavior of sea-salt aerosols under polar conditions. In addition, we have conducted bulk freezing experiments to mimic the behavior of patches of sea-salt aerosols and seawater deposited on the polar ice pack. Experimental Procedure CalorimetryWe have employed differential scanning calorimetry (DSC) to study phase transitions occurring in bulk and emulsion samples of aqueous NaC1 and sea-salt solutions. The experimental setup used in this paper has been described in detail previously [Chang et al., 1999;Koop et al., 1999]. Solutions of NaC1 were prepared by adding deionized water to NaC1 crystals (Mallinckrodt, >99%). The NaC1 crystals were dried in an oven at a temperature of about 380 K for at least 2 hours prior to preparing the solutions. Sea-salt solutions were prepared by diluting an aqueous stock solution of a synthetic sea-salt mixture (Instant Ocean ©, Aquarium Systems). Since sea salt is a very hygroscopic salt mixture, the composition of the stock 26,393

A new optical microscope technique has been developed to investigate phase transitions in micrometer-sized droplets. This technique has been used to study the nucleation of ice from aqueous H2SO4 aerosols 0−35 wt % in composition in the temperature range from 273 to 170 K. The aerosols were produced with a nebulizer and were deposited on a quartz plate, which was coated with a hydrophobic silane monolayer to minimize the effects of heterogeneous nucleation. More than 1200 aerosol particles were monitored individually with the optical microscope, and their freezing temperatures and melting points were recorded. The observed freezing temperatures are lower than the ones from comparable aerosol studies reported in the literature, the differences in the freezing temperature being up to 30 K, especially for the more concentrated aerosols. No freezing was observed above 170 K for compositions greater than 27 wt %. A thermodynamic model has been used to apply the new freezing temperature data to the formation of clouds in the upper troposphere and lower stratosphere. The results indicate that the homogeneous nucleation of ice particles in cirrus clouds requires saturation ratios with respect to ice ranging from about 1.5 at 230 K to 1.6 at 205 K. In addition, the formation of type II polar stratospheric clouds under volcanically perturbed conditions where H2SO4 is the main aqueous aerosol component at low temperatures is predicted to occur about 3 K below the ice frost point.

Calorimetric freezing experiments with aqueous sulfuric and nitric acid solutions are presented and applied to the formation of polar stratospheric clouds (PSCs). We show that the nucleation of hydrates from these solutions is a stochastic process and that nucleation rates and their uncertainties can be determined using Poisson statistics. Under thermodynamic equilibrium conditions above the ice frost point, the homogeneous nucleation rates of stratospheric aerosols are exceedingly low, ruling out homogeneous freezing as a pathway for PSC formation. Several stratospherically important substrates were tested concerning their ability to induce heterogeneous nucleation. None of the experiments indicated a relevant enhancement of the freezing probability of liquid aerosols. Moreover, the experiments reveal that the freezing process of the solutions under stratospheric conditions is limited by the nucleation rates of the hydrates, rather than their crystal growth rates, thus ruling out the possibility of a glassy state of stratospheric aerosol droplets. Also, we argue why a glacial state of the aerosols seems to be unlikely. The only processes leading to freezing of the hydrates appear to be the heterogeneous nucleation on water ice crystals forming below the frost point and the homogeneous freezing of almost binary HNO3/H2O droplets with H2SO4 concentrations below approximately 0.01 wt %.

This review provides an introduction to ice nucleation processes in supercooled water and aqueous solutions. Concepts for experimental techniques suitable to study homogeneous ice nucleation are addressed, in particular differential scanning calorimetry of inverse emulsions. Ice nucleation data from aqueous solutions have been analyzed using two approaches, and the interrelations between those are examined. It is argued that the ice nucleation process is driven entirely by thermodynamic quantities and how this can be understood in the context of three proposed theories for supercooled liquid water. Ice nucleation data for pure water droplets surrounded by a gas have been compiled and evaluated; within experimental uncertainty neither a volume dependent nucleation process nor a surface dependent nucleation process is convincingly supported by the analysis. Finally, open questions in the area of supercooled aqueous solutions and ice nucleation are discussed.

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 .