AS3.15

Ice Nucleation (Cirrus clouds) and aviation contrail formation in the wake of jet engine exhaust in the free troposphere and lower stratosphere as well as accelerated atmospheric cloud formation in the presence of atmospheric mineral dust particles are only partially understood phenomena that await a more profound fundamental knowledge base. In this presentation we report measurements from a flowing gas experiment in which probe molecules such as H₂O, HCl and NO₂ interact with a solid substrate such as Processed Amorphous Carbon (PAC) or mineral dust materials such as the clay mineral Bentonite or Arizona Test Dust (ATD. In these experiments that are performed under molecular flow conditions inside a Knudsen flow reactor both uptake and desorption experiments have been conducted that resulted in the measurement of the rate constants k_a and k_d for the reversible adsorption/desorption kinetics of the probe gas M in the presence of the solid substrate according to the Langmuir-type equilibrium M + SS = M_ads wherein SS and M_ads are the surface site density and the density of adsorbed probe gas molecules. Typical results for M = H₂O adsorption on PAC are saturation at 0.3% of a monolayer, a surface residence time of the adsorbate M_ads of 2500 s at ambient temperature and a rate constant k_a that is accelerated by a factor of 75-125 when measured at desorption compared to adsorption. Initial adsorption of H₂O on PAC is slow (“dry” case) with an uptake probability on the order of 10^-4 to 10^-3 per collision. In contrast, desorption from a H₂O-saturated PAC surface is from large molecular clusters or nanodroplets adhering to the PAC surface (“wet” case) and is larger by the acceleration factor given above. The adsorption process is therefore autocatalytic in adsorbed H₂O abundance which means that the more water that has been adsorbed the larger the adsorption rate constant k_a is because the H₂O molecules preferentially "choose" already adsorbed H₂O for adsorption owing to a higher uptake probability. Bentonite clay and ATD are H₂O or D₂O saturated at a coverage of 10.6 and 11.7% of a formal molecular monolayer, respectively, with an associated surface residence time t_s (= 1/k_d) of 170 s for both substrates at ambient temperature. The corresponding acceleration factors for k_a in going from the dry to the wet case are 34 and 80, respectively. We are aware that the transition from dry to wet or inversely is smooth, whereas in this work we have characterized merely the extremes, namely dry and wet. Future work will interpolate k_a in parametrized form in order to encourage the use of numerical models describing the uptake and desorption of H₂O and other small polar molecules by suitable atmospheric nuclei.

How to cite: Rossi, M. J., Iannarelli, R., and Ludwig, C.: Autocatalytic Uptake of Small Polar Molecules (H2O, HCl, NO2) on Processed Amorphous Carbon (Soot) and atmospheric Mineral Dust Materials, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1894, https://doi.org/10.5194/egusphere-egu22-1894, 2022.

Heterogeneous nucleation of cloud droplets or ice occurs on insoluble aerosol when the supersaturation of water vapor exceeds 100%. It is one of the least well understood processes in the climate system [Seinfeld et. al., PNAS, 2016]. The propensity of the different types of particles to nucleate cloud or ice droplets is affected by molecular scale chemical and topological properties of their surface [Kanji et. al., Meteor. Mon., 2017]. Heterogeneous nucleation is commonly represented in climate models by the classical nucleation theory (CNT) [Fletcher, J. Chem. Phys., 1958], whose only tunable input parameter is the contact angle, which does not allow for the inclusion such details. In this work we use the example of soot to demonstrate that a single contact angle, used as a thermodynamic parameter, is an ambiguous descriptor of the hydrophilicity - and therefore of the heterogeneous nucleation efficiency - of a surface. We also show that the adsorption nucleation theory (ANT) [Sorjaama, Atmos. Chem. Phys. 2007], in which the contact angle serves as geometric parameter that links the droplet shape to the amount water adsorbed at the surface, can account for molecular scale surface properties.

We perform molecular dynamics simulations of water nanodroplets on model graphene and soot surfaces whose hydrophilicity is tuned by A) uniformly scaling the interaction energy between the surface and droplet and B) by adding hydroxyl groups in different concentration and topology. We estimate the mean equilibrium contact angle of the droplets, and we present spatial distributions of local contact angles as novel and unusual approach to describe the real shape of nanodroplets, which strongly deviate from the idealized assumption of a spherical cap and fluctuate in time.

The average contact angle is a good descriptor of the hydrophilicity only in the case of type A systems, for which, in accordance with previous simulation results, we observe a linear relationship between the contact angle the surface hydrophilicity expressed as pairwise Ɛ_LJ parameter between the water and the surface. For the chemically and topologically heterogeneous type B systems we could not identify any significant correlation. Since the same mean contact angle can correspond to very different surfaces, CNT is not expected to differentiate between their heterogeneous nucleation activity.  The contact angle distributions on the other hand provide a unique description of the droplet shape for each of the systems. The distributions are bimodal for type A and trimodal for type B systems, with the marked differences in the weight and position of the hydrophobic peak. These distributions are however strictly geometrical properties of the droplet, and hence can only be used in the framework provided by ANT.

How to cite: Lbadaoui-Darvas, M., Nenes, A., Takahama, S., and Laaksonen, A.: Which properties of adsorbed droplets can describe heterogeneous nucleation on carbonaceous surfaces? Insights from molecular simulations and theoretical models., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11282, https://doi.org/10.5194/egusphere-egu22-11282, 2022.

The widespread plant pathogenic bacteria Pseudomonas Syringae are one the most efficient ice nucleating organisms found in the atmospheric clouds. Various strains of P. Syringae have been identified not only in agricultural regions and on the plant leaves, but also in the samples of cloud water and in fresh snow and rain collected far away from the ecosystems of origin.

Not only P. Syringae survive the transport over several hundred kilometers, they are also able to multiply in the cloud droplets. At low temperature, bacteria initiate the freezing of the supercooled water droplet owing to the ice nucleation active (INA) protein molecules anchored on the outer shell of the cell membrane. As liquid water converts to ice, ice crystals grow fast via water diffusion and droplet coalescence, finally returning to the ground as rain or snow.

How do microorganisms survive the freezing of the cloud droplets? Under what conditions are their chances of survival highest, and which factors play the most important role? We address these questions by freezing microscopic water droplets containing P. Syringae levitated in an electrodynamic trap under realistic cloud conditions and observing the freezing events with a high-speed video camera. The droplets are then extracted from the trap and transferred to a Petri dish containing nutritious media, where the number of surviving bacteria is determined by colony counting. We find that the P. Syringae bacteria have a good chance of survival especially if the freezing of the drops takes a lot of time and the bacteria are able to adapt to the new conditions. At low ambient temperature, the bacteria counteract rapid freezing by initiating ice nucleation at low supercooling, highlighting the role of the INA proteins in the survival mechanism.  By modelling the water flow through the cell wall during freezing numerically, we demonstrate that the permeability of the bacteria cell membrane plays a decisive role in the fight for survival in a freezing environment. Thus, we suggest an explanation of the bacteria survival mechanism based on the thermodynamic model.

How to cite: Kiselev, A. A., Wieber, C., Rabe, K., Zoheir, A., and Leisner, T.: Survival of ice nucleating bacteria in freezing cloud droplets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10075, https://doi.org/10.5194/egusphere-egu22-10075, 2022.

Heavy aerosol loading threatens human health across the globe and is typically related to photochemical processing associated with emission of organic, inorganic and trace metal compounds. Aerosol particles dominated by organic solutes may attain a high or ultra-high viscosity (> 10¹² Pa s) becoming solid-like in cold and dry air, limiting diffusion of organic and reactive molecules through the particle volume, thus slowing chemistry. In contrast, illumination and thus photochemistry to produce radicals may occur through the bulk of light absorbing particles irrespective of diffusion limitations, but its efficiency is not well constrained. We investigated iron oxidation state changes in particles containing various concentrations of citric acid, iron(III)-citrate, copper(II)-citrate and copper(II) salts using environmental X-ray spectromicroscopy with control of relative humidity, RH, and temperature, T. Chemical images of single aerosol particles with resolution currently as low as 35 × 35 nm² were acquired in a humidified microreactor revealing spatial gradients in the concentration of iron(II), iron(III), copper(I) and copper(II) compounds. We have also quantified the CO₂ formation from coated wall flowtube experiments due to decarboxylation subsequent to ligand to metal charge transfer and the condensed and gas phase products using proton-transfer reaction mass spectrometry to characterize the complex chemical reaction scheme. We observed that iron was largely reduced in particles despite being in an oxygen atmosphere immediately after exposure to atmospherically relevant UV light exposure i.e. using 375 nm LED and a measured intensity of 3 W m^‑2 for 15 min. This implies that oxygen uptake, diffusion, reactive oxygen species generation and metal reoxidation reactions were slow compared to photochemical reduction. When relative humidity, RH < 50%, there was significant oxidation only near the surface of particles extending over scales of tens of nanometers. At higher RH, particles became more homogeneously oxidized. We concluded that O₂ reaction and diffusion is limited and results in organic radical persistence in particles. In the presence of copper, iron was immediately oxidized after UV exposure, which is in sharp contrast to particles without copper. If oxygen is limited, and therefore cannot quickly reoxidize iron, then copper oxidation reactions or cross iron-copper redox reactions must generate more radicals than expected. We aim to improve the kinetic treatment of radical production from copper and iron, which can affect redox cycling in organic aerosol. Such information is necessary for the accurate prediction of aerosol phase radical generation, chemical loss of oxygenated organic aerosol dominated by carboxyl functionalities and identifying diffusion limitations leading to the preservation of reactive oxygen species and free radicals.

How to cite: Kilchhofer, K., Alpert, P., Schneider, F., Dou, J., Luo, B., Peter, T., Krieger, U., Corral Arroyo, P., Schaefer, T., Herrmann, H., Xto, J., Huthwelker, T., Borca, C., Henzler, K., Raabe, J., Watts, B., and Ammann, M.: Imaging and modelling trace metal photochemistry in highly viscous organic aerosol particles, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11162, https://doi.org/10.5194/egusphere-egu22-11162, 2022.

The organic fraction of atmospheric particulate matter contains tens of thousands of complex compounds that have one or more functional groups. Quantifying the physical and chemical properties of each of these compounds experimentally is challenging and time consuming. The glass transition temperature, Tg, is one of these properties since it can help determine the phase state of aerosols in different parts of the atmosphere. This phase state influences gas-particle partitioning of semi-volatile compounds, the timescales of diffusion inside the particle, water uptake, as well as the rates and kinetics of heterogeneous reactions and oxidation that take place on aerosols. Experimental Tg determination can be demanding, because of the challenges presented by the synthesis and purification of the corresponding organic compounds.
Molecular Dynamics (MD) simulations have the advantage of detailed prediction of the desired properties on the molecular level with relatively low cost compared to actual experiments. In our work, we implement MD simulations to determine Tg of various organic compounds. Although Tg of organic compounds has been examined experimentally, the discrepancies in the bibliography are vast not only between experiments but also between experimental and predicted values, derived from theoretical or semi-empirical proposed equations. In the current work we focus on organic compounds of atmospheric interest, and we investigate in detail the contributions of the various functional groups to Tg. The investigated organic compounds vary in the carbon chain length as well as in the number and type of functional groups (i.e., hydroxyl and carboxylic groups). The Tg is determined by applying different cooling rates over a wide temperature span for several independent initial configurations of the examined organic molecules in bulk phase and by analyzing properties such as the density and the energy of non-bonded interactions. The results of the molecular simulations are compared with available experimental data in the bibliography, and theoretical or empirical Tg predictions.

How to cite: Siachouli, P., Karadima, K. S., Mavrantzas, V. G., and Pandis, S. N.: On the glass transition temperature of organic compounds via molecular dynamics simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6382, https://doi.org/10.5194/egusphere-egu22-6382, 2022.

Using Delaunay tessellation followed by Monte Carlo integration, we geometrically analyze atomistically-detailed model structures of aerosol nanoparticles to connect their free volume to their phase state. Nanoparticles investigated consist of water, organic molecules (such as cis-pinonic acid) and inorganic species (such as sulfate and ammonium ions). Our emphasis is on the effects of relative humidity and organic content on nanoparticle free volume, and its spatial distribution within the nanoparticle. Our analysis provides information for the distribution of empty pores in the nanoparticle, the available free volume that a guest molecule (e.g., water) can reside, and the connectivity of such pockets of accessible volume. Indirectly, our geometric analysis provides exact measures of the shape, surface area and volume of the nanoparticle.

It is found that with increasing organic concentration, the total unoccupied as well as the total accessible volume to a hypothetical penetrant in the nanoparticle increase. It is also found that the unoccupied and accessible volumes in the organic islands of the nanoparticle or at its surface are always larger compared to those in its aqueous or inorganic domains. Pores accessible to a water molecule are mainly found in the intermediate and outer areas of the nanoparticle which are dominated by organic molecules.

The largest pores accessible to a water molecule were discovered in the nanoparticle with the highest organic mass fraction and the lowest relative humidity (RH). With increasing RH, the presence of additional water molecules disturbs these cavities since organic mass is pushed to the outer regions of the nanoparticle. Simultaneously, at these highest-RH nanoparticles, the pure inorganic volume vanishes and the same happens with its organic-inorganic interfacial domains, implying a complete separation of organic molecules from inorganic ions (with the latter showing a strong preference to accumulate in the internal areas of the nanoparticle). Under the same conditions, the cis-pinonic acid was found to form a single island inside the nanoparticle characterized by a density almost identical to that of bulk cis-pinonic acid, indicative of a liquid-like phase. In contrast, the inorganic mass prefers to form a single large island whose density is very similar to that of ammonium sulfate; this indicates a solid-like phase at the core of the NP. This finding agrees with another finding that domains dominated by inorganic ions are rather dense having no cavities wherein any realistic penetrant with a radius greater than 1 Å could be accommodated. Water, on the other hand, prefers to reside in several islands, each of which of the same volume (practically) when RH is kept at low levels. In contrast, at higher levels of RH, water prefers to form a big island with numerous smaller water droplets around it.

How to cite: Mermigkis, P. G., Karadima, K. S., Mavrantzas, V. G., and Pandis, S. N.: On the phase state of aerosol nanoparticles from a detailed geometric analysis of their free volume accessible to small penetrants, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13250, https://doi.org/10.5194/egusphere-egu22-13250, 2022.

We investigated sodium/halides (F, Cl, Br and I) aqueous droplets in vacuo at the microscopic level from molecular dynamics simulations at the 100 ns scale performed using a sophisticated polarizable all atom force field whose parameters are assigned only from high end quantum ab initio computations [1]. Long range electrostatic and polarizable forces are computed according to a Fast Multipole Method scheme devoted to polarizable force fields based on the induced dipole moment approach [2]. For Cl, Br and I we simulated 10k water droplets corresponding to 0.2, 0.6, 2.0 and 4.6 M salt concentrations and on a range of temperatures included between 260 and 320K. For F, we simulated 10k droplets corresponding to 0.2, 0.6, 0.8 and 1M salt concentrations. We also simulated a reduced set of salty droplets at 300K (in particular corresponding to NaCl salts) at the 100k (up to 100 ns) and at the 1M water molecules scale (up to 30 ns) [3]. We present here a detailed analysis of the structural properties of these droplets regarding ion spatial distributions, ion aggregates (size, composition, morphology, lifetime and distribution), water ordering (relative to pure water droplets) and droplet surface potentials. In the particular case of NaCl droplets, we also discuss droplet curvature effects on the latter properties from data corresponding to 10k, 100k and 1M systems.

[1] Trumm et al, Modeling the Hydration of Mono-Atomic Anions From the Gas Phase to the Bulk Phase: The Case of the Halide Ions F-, Cl-, and Br-, J. Chem. Phys., 136 (2012) 044509.; Réal F et al,  Revisiting a Many-Body Model for Water Based on a Single Polarizable Site. From Gas Phase Clusters to Liquid and Air/Liquid Water Systems, J. Chem. Phys. 139 (2013) 114502.; Réal F et al, Structural, Dynamical, and Transport Properties of the Hydrated Halides: How Do At- and I- Bulk Properties Compare with those of the other Halides, from F- to I-, J. Chem. Phys., 144 (2016) 124513

How to cite: Masella, M., Vallet, V., Réal, F., and Houriez, C.: Sodium/halides aqueous droplets in vacuo at the sub micron scale : size, temperature and concentration effects on their structural properties from simulations at the microscopic level., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-32, https://doi.org/10.5194/egusphere-egu22-32, 2022.