Chemical Thermodynamics of Water Soluble Organic Compounds Found in Aqueous Atmospheric Aerosols: Modeling and Microfluidic Measurements

University of Minnesota Ph.D. dissertation. June 2018. Major: Mechanical Engineering. Advisor: Cari Dutcher. 1 computer file (PDF); xxiv, 305 pages.Atmospheric aerosols, suspensions of tiny particulates in atmosphere, are known to have a major impact on Earth’s climate. Due to the highly chemically and physically complex nature of aerosol particles, large uncertainty in climate modeling arises when attempting to predict the aerosol effect. This dissertation comprises of (1) development of thermodynamic statistical mechanics models to predict solute and water content in aqueous aerosols, and (2) development of an experimental microfluidics approach to measure water loss and study liquid-liquid phase separation. The research effort will significantly advance understanding of aerosol particle thermodynamics by assessing the water content of multiphase particles containing soluble organic compounds, and reduce uncertainty in climate modeling associated with aerosol properties and dynamics. The specific objectives attained in this dissertation research are as follows. I. Aqueous Solution Thermodynamic Model Development: Thermodynamic analytic predictive models using statistical mechanics were developed for multicomponent systems across the entire range of equilibrium relative humidity (RH - 0 to 100%). The models predicted solute activity for a wide range of compounds consisting of partially dissociating organic and inorganic acids, fully dissociating symmetric and asymmetric electrolytes, and neutral organic compounds to capture their chemical behavior. II. Model Applications: (1) pH of aerosols was evaluated in a collaborative work, which is of significant interest due to its effect on the environment. (2) Hygroscopicity was estimated in a collaborative work, which has effects on the optical properties of aerosol particles. III. Experimental Microfluidics: The thermodynamic model was parameterized and validated with measurements of water uptake of multicomponent aerosol particles. The influence of relative humidity on phase behavior to assess the effects on water loss properties was studied for improved understanding of liquid-liquid morphologies. Hydrodynamic trapping of atmospheric aerosol chemical mimics in microfluidic channels was used to perform the experiments, that also represented supersaturated solutions. The efforts in this dissertation together will enhance understanding of atmospheric aerosol phase, solid/liquid/gas partitioning, and liquid-liquid morphologies found in the troposphere. Additionally, the measurements and modeling performed here are useful to any application that requires thermodynamic predictions of water content in complex fluids, like emulsions

Influence of organic compound functionality on aerosol hygroscopicity:dicarboxylic acids, alkyl-substituents, sugars and amino acids

Hygroscopicity data for 36 organic compounds, including amino acids, organic acids, alcohols and sugars, are determined using a comparative kinetics electrodynamic balance (CK-EDB). The CK-EDB applies an electric field to trap-charged aqueous droplets in a chamber with controlled temperature and relative humidity (RH). The dual micro dispenser set-up allows for sequential trapping of probe and sample droplets for accurate determination of droplet water activities from 0.45 to > 0.99. Here, we validate and benchmark the CK-EDB for the homologous series of straight-chain dicarboxylic acids (oxalic–pimelic) with measurements in better agreement with Universal Quasichemical Functional Group Activity Coefficients (UNIFAC) predictions than the original data used to parametrise UNIFAC. Furthermore, a series of increasingly complex organic compounds, with subtle changes to molecular structure and branching, are used to rigorously assess the accuracy of predictions by UNIFAC, which does not explicitly account for molecular structure. We show that the changes in hygroscopicity that result from increased branching and chain length are poorly represented by UNIFAC, with UNIFAC under-predicting hygroscopicity. Similarly, amino acid hygroscopicity is under-predicted by UNIFAC predictions, a consequence of the original data used in the parametrisation of the molecular subgroups. New hygroscopicity data are also reported for a selection of alcohols and sugars and they show variable levels of agreement with predictions

Influence of Particle Viscosity on Mass Transfer and Heterogeneous Ozonolysis Kinetics in Aqueous-Sucrose-Maleic Acid Aerosol

The ozonolysis kinetics of viscous aerosol particles containing maleic acid are studied. Kinetic fits are constrained by measured particle viscosities.</p

Quantitative Spectroscopy of BA-type Supergiants

Luminous BA-SGs allow topics ranging from NLTE physics and the evolution of massive stars to the chemical evolution of galaxies and cosmology to be addressed. A hybrid NLTE technique for the quantitative spectroscopy of BA-SGs is discussed. Thorough tests and first applications of the spectrum synthesis method are presented for four bright Galactic objects. Stellar parameters are derived from spectroscopic indicators. The internal accuracy of the method allows the 1sigma-uncertainties to be reduced to <1-2% in Teff and to 0.05-0.10dex in log g. Elemental abundances are determined for over 20 chemical species, with many of the astrophysically most interesting in NLTE. The NLTE computations reduce random errors and remove systematic trends in the analysis. Inappropriate LTE analyses tend to systematically underestimate iron group abundances and overestimate the light and alpha-process element abundances by up to factors of 2-3 on the mean. Contrary to common assumptions, significant NLTE abundance corrections of ~0.3dex can be found even for the weakest lines. NLTE abundance uncertainties amount to typically 0.05-0.10dex (random) and \~0.10dex (systematic 1sigma-errors). Near-solar abundances are derived for the heavier elements, and patterns indicative of mixing with nuclear-processed matter for the light elements. These imply a blue-loop scenario for Eta Leo, while the other three objects appear to have evolved directly from the main sequence. In the most ambitious computations several ten-thousand spectral lines are accounted for, permitting the accurate reproduction of the entire observed spectra from the visual to NIR. This prerequisite for the quantitative interpretation of medium-resolution spectra opens up BA-SGs as versatile tools for extragalactic stellar astronomy beyond the Local Group. (abridged)Comment: 36 pages, 18 figures, accepted for publication in A&

Phase Behavior of Ammonium Sulfate with Organic Acid Solutions in Aqueous Aerosol Mimics Using Microfluidic Traps

Water-soluble organic acids such as dicarboxylic acids are known to form a significant fraction of organic aerosol mass, yet the chemical composition and interactions between components in an organic acid–inorganic salt mixed particle remain unclear. In this study, phase behavior of different mixing ratios of the salt and organic acids, here 3-methyl glutaric acid and 3-methyl adipic acid, are investigated with respect to their water activity. A microfluidic pervaporation approach is used to study different phase transitions of internally mixed aqueous droplets. Single droplets of varied compositions are trapped and stored in microfluidic wells until dehydration, where both the water content and the solution volume of the droplet decrease slowly with time. The volume is calculated by imaging techniques and correlated with the initial known concentration of the solution to determine concentrations at each time interval. The phase transitions of the droplets with changing concentrations are also observed under an inverted microscope. This study will help determine the concentration at which a mixture droplet, mimicking organic and inorganic atmospheric aerosols, changes phase

Isotherm-Based Thermodynamic Model for Solute Activities of Asymmetric Electrolyte Aqueous Solutions

Adsorption isotherm-based statistical thermodynamic models can be used to determine solute concentration and solute and solvent activities in aqueous solutions. Recently, the number of adjustable parameters in the isotherm model of Dutcher et al. <i>J. Phys. Chem. A/C</i> 2011, 2012, 2013 were reduced for neutral solutes as well as symmetric 1:1 electrolytes by using a Coulombic model to describe the solute–solvent energy interactions (Ohm et al. <i>J. Phys. Chem. A</i> 2015, Nandy et al. <i>J. Phys. Chem. A</i> 2016). Here, the Coulombic treatment for symmetric electrolytes is extended to establish improved isotherm model equations for asymmetric 1–2 and 1–3 electrolyte systems. The Coulombic model developed here results in prediction of activities and other thermodynamic properties in multicomponent systems containing ions of arbitrary charge. The model is found to accurately calculate the osmotic coefficient over the entire solute concentration range with two model parameters, related to intermolecular solute–solute and solute–solvent spacing. The inorganic salts and acids treated here are generally considered to be fully dissociated. However, there are certain weak acids that do not dissociate completely, such as the bisulfate ion. In this work, partial dissociation of the bisulfate ion from sulfuric acid is treated as a mixture, with an additional model parameter that accounts for the dissociation ratio of the dissociated ions to nondissociated ions

Heterogeneous Ice Nucleation in Model Crystalline Porous Organic Polymers: Influence of Pore Size on Immersion Freezing

Heterogeneous ice nucleation activity is affected by aerosol particle composition, crystallinity, pore size, and surface area. However, these surface properties are not well understood, regarding how they act to promote ice nucleation and growth to form ice clouds. Therefore, synthesized materials for which surface properties can be tuned were examined in immersion freezing mode in this study. To establish the relationship between particle surface properties and efficiency of ice nucleation, materials, here, covalent organic frameworks (COFs), with different pore diameters and degrees of crystallinity (ordering), were characterized. Results showed that out of all the highly crystalline COFs, the sample with a pore diameter between 2 and 3 nm exhibited the most efficient ice nucleation activity. We posit that the highly crystalline structures with ordered pores have an optimal pore diameter where the ice nucleation activity is maximized and that the not highly crystalline structures with nonordered pores have more sites for ice nucleation. The results were compared and discussed in the context of other synthesized porous particle systems. Such studies give insight into how material features impact ice nucleation activity

Isotherm-Based Thermodynamic Models for Solute Activities of Organic Acids with Consideration of Partial Dissociation

Organic acids make up a significant fraction of the organic mass in atmospheric aerosol particles. The calculation of gas–liquid–solid equilibrium partitioning of the organic acid is therefore critical for accurate determination of atmospheric aerosol physicochemical properties and processes such as new particle formation and activation to cloud condensation nuclei. Previously, an adsorption isotherm-based statistical thermodynamic model was developed for capturing solute concentration–activity relationships for multicomponent aqueous solutions over the entire concentration range (Dutcher et al. <i>J. Phys. Chem. C/A</i> <b>2011</b>, <b>2012</b>, <b>2013</b>), with model parameters for energies of adsorption successfully related to dipole–dipole electrostatic forces in solute–solvent and solvent–solvent interactions for both electrolytes and organics (Ohm et al. <i>J. Phys. Chem. A</i> <b>2015</b>). However, careful attention is needed for weakly dissociating semivolatile organic acids. Dicarboxylic acids, such as malonic acid and glutaric acid are treated here as a mixture of nondissociated organic solute (HA) and dissociated solute (H⁺ + A^–). It was found that the apparent dissociation was greater than that predicted by known dissociation constants alone, emphasizing the effect of dissociation on osmotic and activity coefficient predictions. To avoid additional parametrization from the mixture approach, an expression was used to relate the Debye–Hückel hard-core collision diameter to the adjustable solute–solvent intermolecular distance. An improved reference state treatment for electrolyte–organic aqueous mixtures, such as that observed here with partial dissociation, has also been proposed. This work results in predictive correlations for estimation of organic acid and water activities for which there is little or no activity data