Experimental phase diagram and its temporal evolution for submicron 2-methylglutaric acid and ammonium sulfate aerosol particles.

Liquid-liquid phase separation (LLPS) in aerosol particles is important for the climate system due to its potential to impact heterogeneous chemistry, cloud condensation nuclei, and new particle growth. Our group and others have shown a lower separation relative humidity for submicron particles, but whether the suppression is due to thermodynamics or kinetics is unclear. Herein, we characterize the experimental LLPS phase diagram of submicron 2-methylglutaric acid and ammonium sulfate aerosol particles and compare it to that of supermicron-sized particles. Surprisingly, as the equilibration time of submicron-sized aerosol particles was increased from 20 min to 60 min, the experimental phase diagram converges with the results for supermicron-sized particles. Our findings indicate that nucleation kinetics are responsible for the observed lower separation relative humidities in submicron aerosol particles. Therefore, experiments and models that investigate atmospheric processes of organic aerosol particles may need to consider the temporal evolution of aerosol LLPS.

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.

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 submicrometer 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 submicrometer aerosol particles is lower than for micrometer-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 submicrometer diameters, and a decrease in SRH for submicrometer 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 submicrometer particles.

Inhibition of Phase Separation in Aerosolized Water-Soluble Polymer-Polymer Nanoparticles at Small Sizes and the Effects of Molecular Weight.

The effects of confinement on the phase separation behavior of polymer-polymer mixtures have been frequently studied in morphologies such as thin films and rods, but little research exists with respect to the nanoscale droplet size regime. This paper addresses the phase separation of water-soluble polymers in submicron aerosol droplets. Atomized aerosol particles were prepared from aqueous solutions and dried using diffusion dryers. For poly(ethylene) glycol/dextran and poly(vinyl alcohol)/poly(4-styrene sulfonic acid) systems, small particles remain homogeneous, while larger particles undergo phase separation within a single particle. As the molecular weight of the polymers increases while a constant ratio between monomers of polymers A and B is maintained, phase separation occurs in smaller diameter particles. These trends are modeled using a combination of equations describing the nucleation of a new phase and the Flory-Huggins theory and provide qualitative agreement. These results provide insight into the phase separation of aqueous nanoscale polymer-polymer systems. Potential exists to make new polymer materials with unique properties due to the mixing of polymer combinations that normally undergo phase separation.

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. In this paper, we present 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 (SRH) 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.

Role of pH in Aerosol Processes and Measurement Challenges.

pH is one of the most basic chemical properties of aqueous solution, but its measurement in nanoscale aerosol particles presents many challenges. The pH of aerosol particles is of growing interest in the atmospheric chemistry community because of its demonstrated effects on heterogeneous chemistry and human health, as well as potential effects on climate. The authors have shown that phase transitions of aerosol particles are sensitive to pH, focusing on systems that undergo liquid-liquid phase separation. Currently, aerosol pH is calculated indirectly from knowledge of species present in the gas and aerosol phases through the use of thermodynamic models. From these models, ambient aerosol is expected to be highly acidic (pH ∼ 0-3). Direct measurements have focused on model systems due to the difficulty of this measurement. This area is one in which physical chemists should be encouraged to contribute because of the potential consequences for aerosol processes in the environment.

Effects of High Acidity on Phase Transitions of an Organic Aerosol.

Aerosol particle morphology influences the effect of particles on climate. Recent studies have documented the high acidity found in many ambient aerosol particles. The effect of this acidity on the phase transitions of mixed organic/inorganic aerosol particles has not been addressed. To investigate this effect, six organic compounds and ammonium sulfate were investigated individually with varying amounts of sulfuric acid to determine the role of low pH in the separation, efflorescence, and deliquescence transitions. All phase transitions were affected by the addition of sulfuric acid. This effect was attributed primarily to the change in the identity of the inorganic component as the ammonium/sulfate ratio (ASR) was changed from 2 to 1.5 to 1. The separation relative humidity (SRH) decreased with increasing amounts of sulfuric acid for each system studied, with the largest effect seen in compounds that have a lower SRH when mixed with ammonium sulfate. Control studies without an inorganic salt revealed that for some systems, phase separation occurs for mixtures of sulfuric acid and an organic acid. Overall, it was found that for aerosol particles at low pH (≤0.35) composed of organic acids and inorganic salts, phase separation can be impeded in some cases.