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.

An increasing number of single-particle measurements show that organic and inorganic constituents of the atmospheric aerosol are internally mixed within the particles. Therefore, the phases of the aerosol will be influenced by both mixing of the organic substances with each other and mixing between organic and inorganic constituents. In this work, the mixing properties of the organic aerosol fractions have been investigated theoretically and experimentally with respect to melting and deliquescence. We show that a liquid (or an amorphous solid) is the thermodynamically stable phaseseven in the absence of water as a solventsprovided that a sufficiently high number of miscible components are present. Furthermore, we show experimentally that the deliquescence relative humidity of aqueous solutions of dicarboxylic acids decreases with an increasing number of components present in the solution. A five-component mixture consisting of malic, malonic, maleic, glutaric, and methylsuccinic acids deliquesces at a relative humidity (RH) as low as 45.4% RH, while the pure dicarboxylic acids exhibit deliquescence points between 72 and 96% RH. A further reduction of the deliquescence relative humidity is observed when an inorganic salt is added to the dicarboxylic acid fivecomponent mixture. For NaCl, deliquescence of the eutonic composition occurred at 41.3%, for ammonium sulfate at 36.4%, and for ammonium nitrate even at 27.1% RH. Interactions between the solutes lead to either higher or lower solubilities in the multicomponent mixture as compared to the respective single-component aqueous solutions. In the mixed dicarboxylic acids/inorganic salt solutions, the solubilities of ammonium nitrate and sulfate are increased by ∼40%, the one of sodium chloride is decreased by a similar amount. In summary, these mixing properties suggest that small fractions of organic species prevent tropospheric aerosols from becoming fully solid, and the organic fraction may even stay fully liquid irrespective of tropospheric humidity.

The complex interplay of processes that govern the size, composition, phase and morphology of aerosol particles in the atmosphere is challenging to understand and model. Measurements on single aerosol particles (2 to 100 μm in diameter) held in electrodynamic, optical and acoustic traps or deposited on a surface can allow the individual processes to be studied in isolation under controlled laboratory conditions. In particular, measurements can now be made of particle size with unprecedented accuracy (sub-nanometre) and over a wide range of timescales (spanning from milliseconds to many days). The physical state of a particle can be unambiguously identified and its composition and phase can be resolved with a high degree of spatial resolution. In this review, we describe the advances made in our understanding of aerosol properties and processes from measurements made of phase behaviour, hygroscopic growth, morphology, vapour pressure and the kinetics of water transport for single particles. We also show that studies of the oxidative aging of single particles, although limited in number, can allow the interplay of these properties to be investigated. We conclude by considering the contributions that single particle measurements can continue to make to our understanding of the properties and processes occurring in atmospheric aerosol.

Abstract. Tropospheric aerosols contain mixtures of inorganic salts, acids, water, and a large variety of organic compounds. Interactions between these substances in liquid mixtures lead to discrepancies from ideal thermodynamic behaviour. By means of activity coefficients, non-ideal behaviour can be taken into account. We present here a thermodynamic model named AIOMFAC (Aerosol InorganicOrganic Mixtures Functional groups Activity Coefficients) that is able to calculate activity coefficients covering inorganic, organic, and organic-inorganic interactions in aqueous solutions over a wide concentration range. This model is based on the activity coefficient model LIFAC by Yan et al. (1999) that we modified and reparametrised to better describe atmospherically relevant conditions and mixture compositions. Focusing on atmospheric applications we considered H + , Li + , Na + , K + , NH 4 as cations and anions and a wide range of alcohols/polyols composed of the functional groups CH n and OH as organic compounds. With AIOMFAC, the activities of the components within an aqueous electrolyte solution are well represented up to high ionic strength. Most notably, a semi-empirical middle-range parametrisation of direct organic-inorganic interactions in alcohol+water+salt solutions strongly improves the agreement between experimental and modelled activity coefficients. At room temperature, this novel thermodynamic model offers the possibility to compute equilibrium relative humidities, gas/particle partitioning and liquid-liquid phase separations with high accuracy. In further studies, other organic functional groups will be introduced. The model framework is not restricted to specific ions or organic compounds and is therefore also applicable for other research topics.