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Freedman, Miriam Arak

Publications and source records attributed to Freedman, Miriam Arak.

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Influence of Ions on the Size Dependent Morphology of Aerosol Particles

The study of aerosol particles composed of mixtures of organic and inorganic compounds provides insights for understanding chemistry in the atmosphere as well as information about phase transitions of systems under confinement. In the submicron size regime, we have previously found that liquid-liquid phase separation can be inhibited at sufficiently small particle diameters leading to phase separated particles at larger sizes and homogeneous particles below a threshold diameter. In this paper, we have investigated the influence of cations and anions in the inorganic compound (NH 4 + , Na + , SO 4 2- , HSO 4 - , and Cl-) on the phase separation of submicron aerosol particles. Each of five salts were studied with two different organic compounds. Surprisingly, a strong dependence on the identity of the cation is evident in the size dependence of the particle morphology and no dependence on the anion was found. Sodium containing samples exhibit phase separation in particles below 20 nm in diameter whereas ammonium containing samples cease to undergo phase separation in particles between 45 and 65 nm in diameter. The separation relative humidity for supermicron droplets also depends on the identity of the cation in the inorganic component if the anion is the same across the salts compared. In addition, the correlation between the separation relative humidity and size dependence was investigated and displays a trend, but is not a dominant feature of the data. We explain the results in terms of hard and soft ions, where the harder cation (Na + ) leads to phase separation down to smaller sizes while the softer cation (NH 4 + ) prevents phase separation and causes particles up to a larger diameter to remain homogenous. This study has implications for atmospheric chemistry in regions dominated by sea spray aerosol, which contains sodium as the primary cation, when compared to continental aerosol, where ammonium is an abundant cation. Additionally, these findings can be used to understand the influence of cations on the phase transitions of confined materials.

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DOE OSTI2021

Use of Transmission Electron Microscopy for Analysis of Aerosol Particles and Strategies for Imaging Fragile Particles

For over 25 years, transmission electron microscopy (TEM) has provided a method for the study of aerosol particles with sizes from below the optical diffraction limit to several microns, resolving the particles as well as smaller features. The wide use of this technique to study aerosol particles has contributed important insights about environmental aerosol particle samples and model atmospheric systems. TEM produces an image that is a 2D projection of aerosol particles that have been impacted onto grids and, through associated techniques and spectroscopies, can contribute additional information such as the determination of elemental composition, crystal structure, and 3D particle structures. Soot, mineral dust, and organic/inorganic particles have all been analyzed using TEM and spectroscopic techniques. TEM, however, has limitations that are important to understand when interpreting data including the ability of the electron beam to damage and thereby change the structure and shape of particles, especially in the case of particles composed of organic compounds and salts. In this paper, we concentrate on the breadth of studies that have used TEM as the primary analysis technique. Another focus is on common issues with TEM and cryogenic-TEM. Insights for new users on best practices for fragile particles, that is, particles that are easily susceptible to damage from the electron beam, with this technique are discussed. Tips for readers on interpreting and evaluating the quality and accuracy of TEM data in the literature are also provided and explained.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Dynamics of Liquid–Liquid Phase Separation in Submicrometer Aerosol

Nanoscale materials, when compared to their bulk components, possess unique properties. In particular, shifts in phase transitions can occur for submicron particles. For instance, small particles do not undergo the process of liquid-liquid phase separation (LLPS). LLPS has applications in emulsions such as Janus particles, controllable morphology to create drug-rich phases during drug delivery, and is often observed in atmospheric aqueous aerosol particles. In atmospheric particles, LLPS is tracked as a function of particle water activity, which is equivalent to the relative humidity (RH) at equilibrium. We probed three organic/inorganic aerosol systems in the range of RH over which phase separation occurs (SRH). Our findings indicate that SRH for submicron aerosol particles is lower than for micron-sized droplets. These findings show that it may be necessary to update the representation of phase transitions in aerosol particles in climate models. The vast majority of organic/inorganic aerosol particles have submicron diameters, and a decrease in SRH for submicron particles indicates that the current estimation of phase separated aerosols may be overestimated. Furthermore, understanding the properties of LLPS at the nanoscale can provide key parameters to describe these systems and may lead to better control of phase separation in submicron particles.

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DOE OSTI2020

Liquid–Liquid Phase Separation in Supermicrometer and Submicrometer Aerosol Particles

The interactions of aerosol particles with light and clouds are among the most uncertain aspects of anthropogenic climate forcings. The effects of aerosol particles on climate depend on their optical properties, heterogeneous chemistry, water uptake behavior, and ice nucleation activity. These properties in turn depend on aerosol physics and chemistry including composition, size, shape, internal structure (morphology), and phase state. The greatest numbers of particles are found at small, submicrometer sizes, and the properties of aerosol particles can differ on the nanoscale compared with measurements of bulk materials. As a result, our focus has been on characterizing the phase transitions of aerosol particles in both supermicrometer and submicrometer particles. The phase transition of particular interest for us has been liquid–liquid phase separation (LLPS), which occurs when components of a solution phase separate due to a difference in solubilities. For example, organic compounds can have limited solubility in salt solutions especially as the water content decreases, increasing the concentration of the salt solution, and causing phase separation between organic-rich and inorganic-rich phases. To characterize the systems of interest, we primarily use optical microscopy for supermicrometer particles and cryogenic-transmission microscopy for submicrometer particles. This Account details our main results to date for the phase transitions of supermicrometer particles and the morphology of submicrometer aerosol. Herein, we have found that the relative humidity (RH) at which LLPS occurs (separation RH; SRH) is highly sensitive to the composition of the particles. For supermicrometer particles, SRH decreases as the pH is lowered to atmospherically relevant values. SRH also decreases when non-phase-separating organic compounds are added to the particles. For submicrometer particles, a size dependence of morphology is observed in systems that undergo LLPS in supermicrometer particles. In the limit of slow drying rates, particles <30 nm are homogeneous and larger particles are phase-separated. This size dependence of aerosol morphology arises because small particles cannot overcome the activation barrier needed to form a new phase when phase separation occurs by a nucleation and growth mechanism. Additionally, the inhibition of LLPS in small particles is observed for mixtures of ammonium sulfate with single organic compounds as well as complex organics like α-pinene secondary organic matter. The morphology of particles affects activation diameters for the formation of cloud condensation nuclei. These results more generally have implications for aerosol properties that affect the climate system. In addition, LLPS is also widely studied in materials and biological chemistry, and our results could potentially translate to implications for these fields.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Flash Freeze Flow Tube to Vitrify Aerosol Particles at Fixed Relative Humidity Values

Development of methods to measure the phase transitions and physical properties of submicron atmospheric aerosol particles is needed to better model these systems. Here, we introduce a method to flash freeze submicron particles to measure phase transitions as a function of relative humidity (RH). Particles are equilibrated at a fixed RH, vitrified in a temperature-controlled flow tube, and imaged with cryogenic transmission electron microscopy (cryo-TEM). We demonstrate the use of the technique for measuring the efflorescence relative humidity (ERH) of potassium sulfate and potassium chloride aerosol as well as the separation RH for a multicomponent organic/inorganic system that undergoes liquid-liquid phase separation (LLPS). The location of phase transitions can shift between the micrometer and nanometer size regimes and particles in a given population may have a range of RH over which a phase transition occurs. This technique addresses these requirements by allowing for characterization of the phase transitions for individual particles in a population on the submicron scale.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Effects of Sucrose on Phase Transitions of Organic/Inorganic Aerosols

Aerosol morphology influences the interactions of particles with light, water uptake, and heterogeneous chemistry. Through these mechanisms, morphology may be important to the role of aerosol particles on the climate system. A factor that impacts morphology is the diverse and complex mixture of organic compounds that make up the aerosol particles in the atmosphere. In this paper, we investigate the effect of sucrose on liquid-liquid phase separation. As the amount of sucrose is increased, the oxygen to carbon ratio (O:C) of the particles increases. We demonstrate that the morphology of the particle changes as O:C is increased with phase separation causing the particle to take on a textured appearance due to large or small inclusions, as opposed to a core-shell particle. As a result of the increased O:C ratio, the separation relative humidity (SRH) of the particles decreases and, in all cases, the SRH can be completely arrested with the addition of a sufficiently large concentration of sucrose. The quantity of sucrose required to arrest phase separation of the particles shows a direct correlation with the SRH of the system without sucrose. The identity of the salt further impacts the amount of sucrose required to arrest phase separation and is controlled more strongly by the identity of the anion. When the mixture contains an organic/inorganic combination that is strongly phase separating, phase separation is observed at an O:C of 0.92 which is a higher O:C value than previously shown (O:C > 0.8). Sucrose is compared with several other high O:C compounds to confirm the influence of the average O:C ratio. In all, these effects could drastically influence aerosol processes in the climate system.

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