24 research outputs found
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<sup>+</sup> + A<sup>–</sup>).
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