Dynamic Non-Covalent Bonds Powering Enhanced Temporary Shape Retention Temperature and Mechanical Robustness in Shape Memory Polyurethane

Shape memory polyurethane (SMPU) with excellent mechanical properties holds significant application value in engineering. However, achieving high strength and toughness typically relies on hydrogen bonding for energy dissipation, which limits the application of such PUs due to their deformation temperature being below room temperature. Here, we introduce a rigid long-chain polyamide acid with a rich aromatic structure as a chain extender, combined with metal coordination, to develop a shape memory polyurethane with a phase transition temperature of 50 °C and outstanding mechanical performance. The presence of rigid segments of polyamic acid (PAA) and -COOH not only increases the rigidity of the polyurethane chains but also promotes the formation of hydrogen bonds and π-π conjugation, leading to physical cross-linking points and significant microphase separation, resulting in superior mechanical properties for PU-PAA. The dynamic bonding characteristics impart self-healing and solvent recyclability to PU-PAA. The coordination interactions enhance the cross-linking points, enabling PU-PAA-Eu to exhibit excellent shape fixation and recovery rates, as well as fluorescence properties. Additionally, due to the presence of -COOH, PU-PAA demonstrates remarkable adhesion to various metals. This work provides a strategy toward the development of high performance SMPU and holds promising potential for applications such as anticounterfeit coatings.

Size-Dependent Nascent Sea Spray Aerosol Bounce Fractions and Estimated Viscosity: The Role of Divalent Cation Enrichment, Surface Tension, and the Kelvin Effect

Viscosity, or the "thickness," of aerosols plays a key role in atmospheric processes like ice formation, water absorption, and heterogeneous kinetics. However, the viscosity of sea spray aerosols (SSA) has not been widely studied. This research explored the relationship between particle size and viscosity of authentic SSA particles through particle bounce, atomic force microscopy analysis, and predictive viscosity modeling from molecular composition. The study found that 40 nm SSA particles had estimated viscosities around 104 Pa·s and bounce fractions three times higher than 100 and 200 nm particles with less than 102 Pa·s at a relative humidity (RH) of 60%. Additional studies revealed the Kelvin effect and particle density, influenced by particle size, have a greater impact on size-dependent bounce fractions than changes in RH across impactor stages. While changes in the level of surfactants can impact particle bounce, the increased viscosity in smaller SSA is attributed to the formation of gel-like phase states caused by cation-organic cross-links between divalent calcium ions and organic anions enriched in the smaller particles. This work shows the smallest gel-like SSA particles observed in the field are highly viscous, which has implications for cloud formation, secondary aerosol growth, and pollutant transport in coastal environments.

Aerosolized algal bloom toxins are not inert

Harmful algal blooms (HABs) are projected to become increasingly prevalent, extending over longer periods and wider geographic regions due to the warming surface ocean water and other environmental factors, including but not limited to nutrient concentrations and runoff for marine and freshwater environments. Incidents of respiratory distress linked to the inhalation of marine aerosols containing HAB toxins have been documented, though the risk is typically associated with the original toxins. However, aerosolized toxins in micrometer and submicrometer particles are vulnerable to atmospheric processing. This processing can potentially degrade HAB toxins and produce byproducts with varying potencies compared to the parent toxins. The inhalation of aerosolized HAB toxins, especially in conjunction with co-morbid factors such as exposure to air pollutants from increased commercial activities in ports, may represent a significant exposure pathway for a considerable portion of the global population. Understanding the chemistry behind the transformation of these toxins can enhance public protection by improving the existing HAB alert systems.

Phase Transitions in Organic and Organic/Inorganic Aerosol Particles

The phase state of aerosol particles can impact numerous atmospheric processes, including new particle growth, heterogeneous chemistry, cloud condensation nucleus formation, and ice nucleation. In this article, the phase transitions of inorganic, organic, and organic/inorganic aerosol particles are discussed, with particular focus on liquid-liquid phase separation (LLPS). The physical chemistry that determines whether LLPS occurs, at what relative humidity it occurs, and the resultant particle morphology is explained using both theoretical and experimental methods. The known impacts of LLPS on aerosol processes in the atmosphere are discussed. Finally, potential evidence for LLPS from field and chamber studies is presented. By understanding the physical chemistry of the phase transitions of aerosol particles, we will acquire a better understanding of aerosol processes, which in turn impact human health and climate.

Relative Humidity-Dependent Phase Transitions in Submicron Respiratory Aerosols

Respiratory viruses, such as influenza and severe acute respiratory syndrome coronavirus 2, represent a substantial public health burden and are largely transmitted through respiratory droplets and aerosols. Environmental factors such as relative humidity (RH) and temperature impact virus transmission rates, and a precise mechanistic understanding of the connection between these environmental factors and virus transmission would improve efforts to mitigate respiratory disease transmission. Previous studies on supermicrometer particles observed RH-dependent phase transitions and linked particle phase state to virus viability. Phase transitions in atmospheric aerosols are dependent on size in the submicrometer range, and actual respiratory particles are expelled over a large size range, including submicrometer aerosols that can transmit diseases over long distances. Here, we directly investigated the phase transitions of submicrometer model respiratory aerosols. A probe molecule, Nile red, was added to particle systems including multiple mucin/salt mixtures, a growth medium, and simulated lung fluid. For each system, the polarity-dependent fluorescence emission was measured following RH conditioning. Notably, the fluorescence measurements of mucin/NaCl and Dulbecco's modified Eagle's medium particles indicated that liquid-liquid phase separation (LLPS) also occurs in submicron particles, suggesting that LLPS can also impact the viability of viruses in submicron particles and thus affect aerosol virus transmission. Furthermore, the utility of fluorescence-based measurements to study submicrometer respiratory particle physicochemical properties in situ is demonstrated.

The Significant Role of New Particle Composition and Morphology on the HNO3-Driven Growth of Particles down to Sub-10 nm

New particle formation and growth greatly influence air quality and the global climate. Recent CERN Cosmics Leaving OUtdoor Droplets (CLOUD) chamber experiments proposed that in cold urban atmospheres with highly supersaturated HNO3 and NH3, newly formed sub-10 nm nanoparticles can grow rapidly (up to 1000 nm h-1). Here, we present direct observational evidence that in winter Beijing with persistent highly supersaturated HNO3 and NH3, nitrate contributed less than ∼14% of the 8-40 nm nanoparticle composition, and overall growth rates were only ∼0.8-5 nm h-1. To explain the observed growth rates and particulate nitrate fraction, the effective mass accommodation coefficient of HNO3 (αHNO3) on the nanoparticles in urban Beijing needs to be 2-4 orders of magnitude lower than those in the CLOUD chamber. We propose that the inefficient uptake of HNO3 on nanoparticles is mainly due to the much higher particulate organic fraction and lower relative humidity in urban Beijing. To quantitatively reproduce the observed growth, we show that an inhomogeneous "inorganic core-organic shell" nanoparticle morphology might exist for nanoparticles in Beijing. This study emphasized that growth for nanoparticles down to sub-10 nm was largely influenced by their composition, which was previously ignored and should be considered in future studies on nanoparticle growth.

Probing the evaporation dynamics of semi-volatile organic compounds to reveal the thermodynamics of liquid-liquid phase separated aerosol

Liquid-liquid phase separation (LLPS) is a thermodynamically driven process that occurs in mixtures of low miscibility material. LLPS is an important process in chemical, biological, and environmental systems. In atmospheric chemistry, LLPS in aerosol containing internally-mixed organic and inorganic particles has been an area of significant interest, with particles separating to form organic-rich and aqueous phases on dehydration. This alters the optical properties of the particles, has been connected to changes in the cloud nucleation ability of the aerosol, and potentially changes the reactivity of particles towards gas-phase oxidants. Although the chemical systems that undergo LLPS have become quite well-characterized, the properties and processes of LLPS particles are quite poorly understood. In this work, we characterize LLPS in aerosol particles containing ammonium sulfate and triethylene glycol (3EG), a semi-volatile organic molecule. We explore the relative humidity (RH) conditions under which LLPS occurs and characterize the rate of evaporation of 3EG from well-mixed and LLPS particles as a function of RH. We show that the evaporation rates vary with RH due to changes in chemical activity, however no clear change in the dynamics following LLPS are observed. We interpret our observations using a thermodynamic model (AIOMFAC) coupled with an evaporation model and show that a significant increase in the activity coefficient of 3EG as the RH decreases, required for LLPS to occur, obscures a clear step-change in the evaporation rates following LLPS. By characterizing the evaporation rates, we estimate the composition of the organic-rich phase and compare our results to thermodynamic predictions. This study is the first to explore the connection between LLPS and the chemical evolution of aerosol particles via the evaporation of semi-volatile organic material. Ultimately, we reveal that the thermodynamics of non-ideal mixing are primarily responsible for the controlling both the rate of evaporation and the onset of LLPS, with LLPS itself having limited impact on the rate of evaporation in a fluid system. These results have significant implications for understanding and predicting the lifetime of aerosol particles, their effect on cloud formation, and the chemical evolution of multiphase systems by particle-gas partitioning and heterogeneous reactions.

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.

Geometric Analysis of Free and Accessible Volume in Atmospheric Nanoparticles

Computer-generated atomistic microstructures of atmospheric nanoparticles are geometrically analyzed using Delaunay tessellation followed by Monte Carlo integration to compute their free and accessible volume. The nanoparticles studied consist of cis-pinonic acid (a biogenic organic aerosol component), inorganic ions (sulfate and ammonium), and water. Results are presented for the free or unoccupied volume in different domains of the nanoparticles and its dependence on relative humidity and organic content. We also compute the accessible volume to small penetrants such as water molecules. Most of the free volume or volume accessible to a penetrant as large as a water molecule is located in the domains occupied by organics. In contrast, regions dominated by inorganics do not have any cavities with sizes larger than 1 Å. Solid inorganic domains inside the particle are practically impermeable to any small molecule, thereby offering practically infinite resistance to diffusion. A guest molecule can find diffusive channels to wander around within the nanoparticle only through the aqueous and organic-rich domains. The largest pores are observed in nanoparticles with high levels of organic mass and low relative humidity. At high relative humidity, the presence of more water molecules reduces the empty space in the inner domains of the nanoparticle, since areas rich in organic molecules (which are the only ones where appreciable pores are found) are pushed to the outer area of the particle. This, however, should not be expected to affect the diffusive process as transport through the aqueous phase inside the particle will be, by default, fast due to its fluid-like nature.

Phase State, Surface Tension, Water Activity, and Accommodation Coefficient of Water-Organic Clusters Near the Critical Size for Atmospheric New Particle Formation

Interactions between water and organic molecules in sub-4 nm clusters play a significant role in the formation and growth of secondary organic aerosol (SOA) particles. However, a complete understanding of the relevant water microphysics has not yet been achieved due to challenges in the experimental characterization of soft nuclei. Here, we use molecular dynamics simulations to study the phase-mixing states, surface tension, water activity, and water accommodation coefficient of organic-water clusters representative of freshly nucleated SOA particles. Our results reveal large deviations from the behavior expected based on continuum theories. In particular, the phase-mixing state has a strong dependence on cluster size; surface tension displays a minimum at a specific organic-water mass ratio (morg/mw ∼ 4.5 in this study) corresponding to a minimum inhibition of droplet nucleation associated with the Kelvin effect; and the water accommodation coefficient increases by a factor of 2 with nanocluster hygroscopic growth, in agreement with recent experimental studies. Overall, our results yield parametric relations for water microphysical properties in sub-4 nm clusters and provide insight into the role of water in the initial stages of SOA nucleation and growth.

Revealing the Impacts of Chemical Complexity on Submicrometer Sea Spray Aerosol Morphology

Sea spray aerosol (SSA) ejected through bursting bubbles at the ocean surface is a complex mixture of salts and organic species. Submicrometer SSA particles have long atmospheric lifetimes and play a critical role in the climate system. Composition impacts their ability to form marine clouds, yet their cloud-forming potential is difficult to study due to their small size. Here, we use large-scale molecular dynamics (MD) simulations as a "computational microscope" to provide never-before-seen views of 40 nm model aerosol particles and their molecular morphologies. We investigate how increasing chemical complexity impacts the distribution of organic material throughout individual particles for a range of organic constituents with varying chemical properties. Our simulations show that common organic marine surfactants readily partition between both the surface and interior of the aerosol, indicating that nascent SSA may be more heterogeneous than traditional morphological models suggest. We support our computational observations of SSA surface heterogeneity with Brewster angle microscopy on model interfaces. These observations indicate that increased chemical complexity in submicrometer SSA leads to a reduced surface coverage by marine organics, which may facilitate water uptake in the atmosphere. Our work thus establishes large-scale MD simulations as a novel technique for interrogating aerosols at the single-particle level.

Size Dependence of Liquid-Liquid Phase Separation by in Situ Study of Flowing Submicron Aerosol Particles

Liquid-liquid phase separation (LLPS) of atmospheric particles impacts a range of atmospheric processes. Driven by thermodynamics, LLPS occurs in mixed organic-inorganic particles when high inorganic salt concentrations exclude organic compounds, which develop into a separate phase. The effect of particle size on the thermodynamic and kinetic drivers of LLPS, however, remains incompletely understood. Here, the size dependence was studied for the separation relative humidity (SRH) of LLPS. Submicron organic-inorganic aerosol particles of ammonium sulfate mixed with 1,2,6-hexanetriol and polyethylene glycol (PEG) were studied. In a flow configuration, upstream size selection was coupled to a downstream fluorescence aerosol flow tube (F-AFT) at 293 ± 1 K. For both mixed particle types, the SRH values for submicron particle diameters of 260-410 nm agreed with previous measurements reported in the literature for supermicron particles. For smaller particles, the SRH values decreased by approximately 5% RH for diameters of 130-260 nm for PEG-sulfate particles and of 70-190 nm for hexanetriol-sulfate particles. From these observations, the nucleation rate in the hexanetriol-sulfate system was constrained, implying an activation barrier to nucleation of +1.4 to +2.0 × 10^-19 J at 70% RH and 293 K. Quantifying the activation barrier is an approach for predicting size-dependent LLPS in the atmosphere.

Optical trapping and light scattering in atmospheric aerosol science

Aerosol particles are ubiquitous in the atmosphere, and currently contribute a large uncertainty to climate models. Part of the endeavour to reduce this uncertainty takes the form of improving our understanding of aerosol at the microphysical level, thus enabling chemical and physical processes to be more accurately represented in larger scale models. In addition to modeling efforts, there is a need to develop new instruments and methodologies to interrogate the physicochemical properties of aerosol. This perspective presents the development, theory, and application of optical trapping, a powerful tool for single particle investigations of aerosol. After providing an overview of the role of aerosol in Earth's atmosphere and the microphysics of these particles, we present a brief history of optical trapping and a more detailed look at its application to aerosol particles. We also compare optical trapping to other single particle techniques. Understanding the interaction of light with single particles is essential for interpreting experimental measurements. In the final part of this perspective, we provide the relevant formalism for understanding both elastic and inelastic light scattering for single particles. The developments discussed here go beyond Mie theory and include both how particle and beam shape affect spectra. Throughout the entirety of this work, we highlight numerous references and examples, mostly from the last decade, of the application of optical trapping to systems that are relevant to the atmospheric aerosol.

Predicting the environmental fates of emerging contaminants: Synergistic effects in ozone reactions of nitrogen-containing alkenes

While nitro and amino alkenes are common in pharmaceuticals, pesticides, and munitions, their environmental fates are not well known. Ozone is a ubiquitous atmospheric oxidant for alkenes, but the synergistic effects of nitrogen-containing groups on the reactions have not been measured. The kinetics and products of ozonolysis of a series of model compounds with different combinations of these functional groups have been measured in the condensed phase using stopped-flow and mass spectrometry methods. Rate constants span about six orders of magnitude with activation energies ranging from 4.3 to 28.2 kJ mol^-1. Vinyl nitro groups substantially decrease the reactivity, while amino groups have the opposite effect. The site of the initial ozone attack is highly structure dependent, consistent with local ionization energy calculations. The reaction of the neonicotinoid pesticide nitenpyram, which forms toxic N-nitroso compounds, was consistent with model compounds, confirming the utility of model compounds for assessing environmental fates of these emerging contaminants.

Does liquid-liquid phase separation impact ice nucleation in mixed polyethylene glycol and ammonium sulfate droplets?

Particles can undergo different phase transitions in the atmosphere including deliquescence, liquid-liquid phase separation (LLPS), melting, and freezing. In this study, phase transitions of particles/droplets containing polyethylene glycol with a molar mass of 400 g mol^-1 (PEG400) and ammonium sulfate (AS), i.e., PEG400-AS particles/droplets, were investigated at different organic-to-inorganic dry mass ratios (OIRs) under typical tropospheric temperatures and water activities (a_w). The investigated droplets (60-100 μm) with or without LLPS in the closed system froze through homogeneous ice nucleation. At temperatures lower than 200 K, multiple ice nucleation events were observed within the same individual droplets at low a_w. Droplets with and without LLPS shared similar lambda values at the same OIR according to the lambda approach indicating they form ice through the same mechanism. A parameterization of lambda values was provided which can be used to predict freezing temperature of aqueous PEG400-AS droplets. We found that adding AS reduces the temperature dependence of a_w in aqueous PEG400 droplets. Assuming incorrectly that a_w is temperature-independent for a constant droplet composition leads to a deviation between the experimental determined ice nucleation rate coefficients for droplets at OIR > 1 and the predicted values by the water-activity-based ice nucleation theory. We proposed a parameterization of temperature dependence of a_w to minimize the deviations of the measured melting temperatures and nucleation rate coefficients from the corresponding predictions for aqueous PEG400-AS system.

Nanomechanics and Morphology of Simulated Respiratory Particles

The impact of respiratory particle composition on the equilibrium morphology and phase is not well understood. Furthermore, the effects of these different phases and morphologies on the viability of viruses embedded within these particles are equally unknown. Physiologically relevant respiratory fluid analogues were constructed, and their hygroscopic behavior was measured using an ensemble technique. A relationship between hygroscopicity and protein concentration was determined, providing additional validation to the high protein content of respiratory aerosol measured in prior works (>90%). It was found that the salt component of the respiratory particles could crystallize as a single crystal, multiple crystals, or would not crystallize at all. It was found that dried protein particles at indoor-relevant climatic conditions could exist separately in a glassy (∼77% of particles) or viscoelastic state (∼23% of particles). The phase state and morphology of respiratory particles may influence the viability of embedded pathogens. We recommend that pathogen research aiming to mimic the native composition of respiratory fluid should use a protein concentration of at least 90% by solute volume to improve the representativity of the pathogen's microenvironment.

A simple theory for interfacial properties of dilute solutions

Recent studies suggest that cosolute mixtures may exert significant non-additive effects upon protein stability. The corresponding liquid–vapor interfaces may provide useful insight into these non-additive effects. Accordingly, in this work, we relate the interfacial properties of dilute multicomponent solutions to the interactions between solutes. We first derive a simple model for the surface excess of solutes in terms of thermodynamic observables. We then develop a lattice-based statistical mechanical perturbation theory to derive these observables from microscopic interactions. Rather than adopting a random mixing approximation, this dilute solution theory (DST) exactly treats solute–solute interactions to lowest order in perturbation theory. Although it cannot treat concentrated solutions, Monte Carlo (MC) simulations demonstrate that DST describes the interactions in dilute solutions with much greater accuracy than regular solution theory. Importantly, DST emphasizes a fundamental distinction between the “intrinsic” and “effective” preferences of solutes for interfaces. DST predicts that three classes of solutes can be distinguished by their intrinsic preference for interfaces. While the surface preference of strong depletants is relatively insensitive to interactions, the surface preference of strong surfactants can be modulated by interactions at the interface. Moreover, DST predicts that the surface preference of weak depletants and weak surfactants can be qualitatively inverted by interactions in the bulk. We also demonstrate that DST can be extended to treat surface polarization effects and to model experimental data. MC simulations validate the accuracy of DST predictions for lattice systems that correspond to molar concentrations.

Impact of Chemical Properties of Human Respiratory Droplets and Aerosol Particles on Airborne Viruses' Viability and Indoor Transmission

The airborne transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been identified as a potential pandemic challenge, especially in poorly ventilated indoor environments, such as certain hospitals, schools, public buildings, and transports. The impacts of meteorological parameters (temperature and humidity) and physical property (droplet size) on the airborne transmission of coronavirus in indoor settings have been previously investigated. However, the impacts of chemical properties of viral droplets and aerosol particles (i.e., chemical composition and acidity (pH)) on viability and indoor transmission of coronavirus remain largely unknown. Recent studies suggest high organic content (proteins) in viral droplets and aerosol particles supports prolonged survival of the virus by forming a glassy gel-type structure that restricts the virus inactivation process under low relative humidity (RH). In addition, the virus survival was found at neutral pH, and inactivation was observed to be best at low (<5) and high pH (>10) values (enveloped bacteriophage Phi6). Due to limited available information, this article illustrates an urgent need to research the impact of chemical properties of exhaled viral particles on virus viability. This will improve our fundamental understanding of indoor viral airborne transmission mechanisms.

pH-Dependent Chemical Transformations of Humic-Like Substances and Further Cognitions Revealed by Optical Methods

Humic-like substances (HULIS) are macromolecular complex groups in water-soluble organic compounds (WSOC). pH is a crucial factor that influences the chemical transformations of HULIS in atmospheric particles, but this has been rarely investigated, especially under varying pH conditions. This study attempted to unveil the chemical transformation mechanisms of HULIS under a range of pH conditions using optical methods. The pH-dependent light absorption and fluorescence properties of HULIS were comprehensively analyzed; the acidity coefficient (pK_a) of HULIS in relation to chemical structures was determined, and the hypothetical chemical transformation mechanisms of HULIS with increasing pH were analyzed by optical characterizations. The results suggested that pH greatly impacted the light absorption and fluorescence efficiencies of HULIS in both winter and summer seasons, and pK_a was an important inflection point. The pK_a of HULIS ranged from 3.5 to 8.0 in winter and 6.4 to 10.0 in summer. The acidic/basic groups were identified as -OH or -NH₂ substituted quinolines, carboxylic aromatics, and pyridines. The pH-sensitive species accounted for about 6% and 21% of HULIS-C (carbon concentrations of HULIS) in winter and summer, respectively. The varying optical spectra with increasing pH might result from charge transfer or complex reactions with HULIS deprotonation.

Hygroscopic Growth, Phase Morphology, and Optical Properties of Model Aqueous Brown Carbon Aerosol

Brown carbon aerosol in the atmosphere contain light-absorbing chromophores that influence the optical scattering properties of the particles. These chromophores may be hydrophobic, such as PAHs, or water soluble, such as nitroaromatics, imidazoles, and other conjugated oxygen-rich molecules. Water-soluble chromophores are expected to exist in aqueous solution in the presence of sufficient water and will exhibit physical properties (e.g., size, refractive index, and phase morphology) that depend on the environmental relative humidity (RH). In this work, we characterize the RH-dependent properties of 4-nitrocatechol (4-NC) and its mixtures with ammonium sulfate, utilizing a single-particle levitation platform coupled with Mie resonance spectroscopy to probe the size, real part of the complex refractive index (RI), and phase morphology of individual micron-sized particles. We measure the hygroscopic growth properties of pure 4-NC and apply mixing rules to characterize the growth of mixtures with ammonium sulfate. We report the RI at 589 nm for these samples as a function of RH and explore the wavelength dependence of the RI at non-absorbing wavelengths. The real part of the RI at 589 nm was found to vary in the range 1.54-1.59 for pure 4-NC from 92.5 to 75% RH, with an estimated pure component RI of 1.70. The real part of the RI was also measured for mixtures of AS and 4-NC and ranged from 1.39 to 1.51 depending on the component ratio and RH. We went on to characterize phase transitions in mixed particles, identifying the onset RH of liquid-liquid phase separation (LLPS) and efflorescence transitions. Mixtures showed LLPS in the range of 85-76% RH depending on the molar ratio, while efflorescence typically fell between 22 and 42% RH. Finally, we characterized the imaginary part of the complex RI using an effective oscillator model to capture the wavelength-dependent absorption properties of the system.

Phase Behavior of Internal Mixtures of Hydrocarbon-like Primary Organic Aerosol and Secondary Aerosol Based on Their Differences in Oxygen-to-Carbon Ratios

The phase behavior, the number and type of phases, in atmospheric particles containing mixtures of hydrocarbon-like organic aerosol (HOA) and secondary organic aerosol (SOA) is important for predicting their impacts on air pollution, human health, and climate. Using a solvatochromic dye and fluorescence microscopy, we determined the phase behavior of 11 HOA proxies (O/C = 0-0.29) each mixed with 7 different SOA materials generated in environmental chambers (O/C 0.4-1.08), where O/C represents the average oxygen-to-carbon atomic ratio. Out of the 77 different HOA + SOA mixtures studied, we observed two phases in 88% of the cases. The phase behavior was independent of relative humidity over the range between 90% and <5%. A clear trend was observed between the number of phases and the difference between the average O/C ratios of the HOA and SOA components (ΔO/C). Using a threshold ΔO/C of 0.265, we were able to predict the phase behavior of 92% of the HOA + SOA mixtures studied here, with one-phase particles predicted for ΔO/C < 0.265 and two-phase particles predicted for ΔO/C ≥ 0.265. The threshold ΔO/C value provides a relatively simple and computationally inexpensive framework for predicting the number of phases in internal SOA and HOA mixtures in atmospheric models.

Heterogeneous Nucleation Drives Particle Size Segregation in Sequential Ozone and Nitrate Radical Oxidation of Catechol

Secondary organic aerosol formation via condensation of organic vapors onto existing aerosol transforms the chemical composition and size distribution of ambient aerosol, with implications for air quality and Earth's radiative balance. Gas-to-particle conversion is generally thought to occur on a continuum between equilibrium-driven partitioning of semivolatile molecules to the pre-existing mass size distribution and kinetic-driven condensation of low volatility molecules to the pre-existing surface area size distribution. However, we offer experimental evidence in contrast to this framework. When catechol is sequentially oxidized by O₃ and NO₃ in the presence of (NH₄)₂SO₄ seed particles with a single size mode, we observe a bimodal organic aerosol mass size distribution with two size modes of distinct chemical composition with nitrocatechol from NO₃ oxidation preferentially condensing onto the large end of the pre-existing size distribution (∼750 nm). A size-resolved chemistry and microphysics model reproduces the evolution of the two distinct organic aerosol size modes─heterogeneous nucleation to an independent, nitrocatechol-rich aerosol phase.

Seeded Crystal Growth of Internally Mixed Organic-Inorganic Aerosols: Impact of Organic Phase State

Atmospheric aerosols are complex with both inorganic and organic components. The soluble inorganics can transition between aqueous and crystalline phases through efflorescence and deliquescence. This study focuses on the efflorescence of (NH₄)₂SO₄/organic particles by seeded crystal growth through contact with a crystal of (NH₄)₂SO₄. Seeded crystal growth is known to effectively shut down supersaturation of aqueous aerosols. Here, we investigate whether organics can inhibit seeded crystal growth. We demonstrate that poly(ethylene glycol) 400 (PEG-400), which phase-separates from the aqueous (NH₄)₂SO₄ and forms a core-shell structure, did not inhibit seeded crystal growth of (NH₄)₂SO₄ at all relative humidity (RH) values below deliquescence RH. The PEG-400 layer was not viscous enough to prevent the diffusion of species through the coating. In contrast, we find that although raffinose, which stays homogeneously mixed with (NH₄)₂SO₄, did not inhibit seeded crystal growth at RH > 45%, it did inhibit heterogeneous efflorescence at lower humidities. Viscosity measurements using an electrodynamic balance show a significant increase in viscosity as humidity was lowered, suggesting that inhibited diffusion of water and ions prevented efflorescence. The observed efflorescence at the higher RH also demonstrates that collisions can induce efflorescence of mixed aerosols that would otherwise not homogeneously effloresce.

Strong Acids or Bases Displaced by Weak Acids or Bases in Aerosols: Reactions Driven by the Continuous Partitioning of Volatile Products into the Gas Phase

Aerosols are ubiquitous in the atmosphere and profoundly affect climate systems and human health. To gain more insights on their broad impacts, we need to comprehensively understand the fundamental properties of atmospheric aerosols. Since aerosols are multiphase, a dispersion of condensed matter (solid particles or liquid droplets, hereafter particles) in gas, partitioning of volatile matter between the condensed and the gas phases is one defining characteristic of aerosols. For example, water content partitioning under different relative humidity conditions, known as aerosol hygroscopicity, has been extensively investigated in the past decades. Meanwhile, partitioning of volatile organic or inorganic components, which is referred to as aerosol volatility, remains understudied. Commonly, a bulk solution system is treated as a single phase, with volatility mainly determined by the nature of its components, and the composition partitioning between solution and gas phase is limited. Aerosols, however, comprise an extensive gas phase, and their volatility can also be induced by component reactions. These reactions occurring within aerosols are driven by the formation of volatile products and their continuous partitioning into the gas phase. As a consequence, the overall aerosol systems exhibit prominent volatility. Noteworthily, such volatility induced by reactions is a phenomenon exclusively observed in the multiphase aerosol systems, and it is trivial in bulk solutions due to the limited extent of liquid-gas partitioning. Take the chloride depletion in sea salt particles as an example. Recent findings have revealed that chloride depletion can be caused by reactions between NaCl and weak organic acids, which release HCl into the gas phase. Such a reaction can be described as a strong acid displaced by a weak acid, which is hardly observed in bulk phase. Generally, this unique partitioning behavior of aerosol systems and its potential to alter aerosol composition, size, reactivity, and other physicochemical properties merits more attention by atmospheric community.This Account focuses on the recent advancements in the research of component reactions that induce aerosol volatility. These reactions can be categorized into four types: chloride depletion, nitrate depletion, ammonium depletion, and salt hydrolysis. The depletion of chloride or nitrate can be regarded as a displacement reaction, in which a strong acid is displaced by a weak acid. Such a reaction releases highly volatile HCl or HNO₃ into the gas phase and leads to a loss of chloride or nitrate within the particles. Likewise, ammonium depletion is a displacement reaction in which a strong base is displaced by a weak base, resulting in release of ammonia and substantial changes in aerosol hygroscopicity. In addition, aerosol volatility can also be induced by salt hydrolysis in a specific case, which is sustained by the coexistence of proton acceptor and hydroxide ion acceptor within particles. Furthermore, we quantitatively discuss these displacement reactions from both thermodynamic and kinetic perspectives, by using the extended aerosol inorganic model (E-AIM) and Maxwell steady-state diffusive mass transfer equation, respectively. Given the ubiquity of component partitioning in aerosol systems, our discussion may provide a new perspective on the underlying mechanisms of aerosol aging and relevant climate effects.

Liquid-Liquid Phase Separation of Single Optically Levitated Water-Ionic Liquid Droplets in Air

In this study, to investigate the equilibrium morphology of liquid-liquid phase-separated droplets in air, a temperature-responsive ionic liquid (IL) showing lower critical solution temperature behavior was employed. ILs have negligible vapor pressure and do not evaporate from aerosol droplets during dehumidifying processes. We demonstrated that the liquid-liquid phase separation of single optically levitated aqueous droplets containing the temperature-responsive IL can be induced by controlling the air relative humidity. The formation of liquid-liquid phase-separated droplets of partially engulfed morphology was successfully observed under an optical microscope, and their configurations were compared with those calculated by a thermodynamic model based on interfacial tensions and relative volume ratios of two immiscible phases.

Trileucine as a dispersibility enhancer of spray-dried inhalable microparticles

The formation of trileucine-containing spray-dried microparticles intended for pulmonary delivery was studied in depth. A single-particle method was employed to study the shell formation characteristics of trileucine in the presence of trehalose as a glass former, and an empirical correlation was proposed to predict the instance of shell formation. A droplet chain instrument was used to produce and collect monodisperse particles to examine morphology and calculate particle density for different levels of trileucine. It was observed that the addition of only 0.5 mg/mL (10% w/w) trileucine to a trehalose system could lower dried particle densities by approximately 1 g/cm³. In addition, a laboratory-scale spray dryer was used to produce batches of trileucine/trehalose powders in the respirable range. Raman spectroscopy demonstrated that both components were completely amorphous. Scanning electron microscopy and time-of-flight secondary ion mass spectrometry were used to study the particle morphologies and surface compositions. For all cases with trileucine, highly rugose particles with trileucine coverages of more than 60% by mass were observed with trileucine feed fractions of as little as 2% w/w. Moreover, it was seen that at lower trileucine content, smaller and larger particles of a polydisperse powder had slightly different surface compositions. The surface activity of trileucine was also modeled via a modified form of the diffusion equation inside an evaporating droplet that took into account initial surface adsorption and eventual surface desorption due to droplet shrinkage. Finally, using the Flory-Huggins theory, it was estimated that at room temperature, liquid-liquid phase separation would start when the trileucine reached an aqueous concentration of about 18 mg/mL. Besides the surface activity of trileucine, this low concentration was assumed to explain the substantial effect of trileucine on the morphology of spray-dried particles due to early phase separation. The methodology proposed in this study can be used in the rational design of trileucine-containing microparticles.

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.

A review of efflorescence kinetics studies on atmospherically relevant particles

The efflorescence transitions of aerosol particles have been intensively investigated due to their critical impacts on global climate and atmospheric chemistry. In the present study, we present a critical review of efflorescence kinetics focusing on three key issues: the efflorescence relative humidity (ERH) and the influence factors for aerosol ERH (e.g. particle sizes, and temperature); efflorescence processes of mixed aerosols, concerning the effect of coexisting inorganic and organic components on the efflorescence of inorganic salts; homogeneous and heterogeneous nucleation rates of pure and mixed aerosols. Among the previous studies, there are significant discrepancies for measured aerosol ERH under even the same conditions. Moreover, the interactions between organic and inorganic components remain largely unclear, causing efflorescence transition behaviours and chemical composition evolutions of certain mixed systems to be debatable. Thus, it is important to better understand efflorescence to gain insights into the physicochemical properties and characterize observed efflorescence characteristics of atmospheric particles, as well as guide further studies on aerosol hygroscopicity and reactivity.

Temperature dependence of the gas-particle partitioning of selected VOCs

The gas-particle partitioning coefficients for volatile organic compounds (VOCs) are difficult to acquire because discriminating the small mass fraction of the VOCs in the aerosol particle relative to that in the gas phase is challenging. In this paper, we report the temperature dependence of the gas-particle partitioning coefficient (Kp) for n-butanol (n-BuOH) and trichloroethylene (TCE). Using the bench-scale system that we developed, we measured the Kp of surrogate VOCs, n-BuOH, and TCE onto inorganic (ammonium sulfate, Am Sulf) and organic (succinic acid, SA) aerosol particles at a fixed relative humidity (RH) of 35%. At this RH level and temperature range of 278.15-308.15 K, the ln Kp for TCE and n-BuOH partitioning on SA aerosol particles were -27.0 ± 0.70 to -27.9 ± 0.01 and -13.9 ± 0.03 to -17.4 ± 0.17. In contrast, the ln Kp for TCE and n-BuOH partitioning on Am Sulf aerosol particles ranged from -26.4 ± 0.70 to -27.4 ± 0.71 and -14.1 ± 0.03 to -17.1 ± 0.17, respectively. Results showed that TCE fitted well with the classic van't Hoff relationship. The enthalpy of desorption (ΔHdes) for TCE was constant over the temperature range of 278.15 K to 308.15 K, behaving similarly to 1,2-dichlorobenzene. At a similar temperature range, n-BuOH partitioning into both aerosol particles exhibited nonlinear temperature dependence. The minimum ratio of ΔHdes (Am Sulf:SA) for n-BuOH partitioning on each aerosol type was at ∼278.15 K. The magnitude of the entropy ΔSdes for all VOCs was <1 kJ mol-1.

The role of respiratory droplet physicochemistry in limiting and promoting the airborne transmission of human coronaviruses: A critical review

Whether virulent human pathogenic coronaviruses (SARS-CoV, MERS-CoV, SARS-CoV-2) are effectively transmitted by aerosols remains contentious. Transmission modes of the novel coronavirus have become a hot topic of research with the importance of airborne transmission controversial due to the many factors that can influence virus transmission. Airborne transmission is an accepted potential route for the spread of some viral infections (measles, chickenpox); however, aerosol features and infectious inoculum vary from one respiratory virus to another. Infectious virus-laden aerosols can be produced by natural human respiratory activities, and their features are vital determinants for virus carriage and transmission. Physicochemical characteristics of infectious respiratory aerosols can influence the efficiency of virus transmission by droplets. This critical review identifies studies reporting instances of infected patients producing airborne human pathogenic coronaviruses, and evidence for the role of physical/chemical characteristics of human-generated droplets in altering embedded viruses' viability. We also review studies evaluating these viruses in the air, field studies and available evidence about seasonality patterns. Ultimately the literature suggests that a proportion of virulent human coronaviruses can plausibly be transmitted via the air, even though this might vary in different conditions. Evidence exists for respirable-sized airborne droplet nuclei containing viral RNA, although this does not necessarily imply that the virus is transmittable, capable of replicating in a recipient host, or that inoculum is sufficient to initiate infection. However, evidence suggests that coronaviruses can survive in simulated droplet nuclei for a significant time (>24 h). Nevertheless, laboratory nebulized virus-laden aerosols might not accurately model the complexity of human carrier aerosols in studying airborne viral transport. In summary, there is disagreement on whether wild coronaviruses can be transmitted via an airborne path and display seasonal patterns. Further studies are therefore required to provide supporting evidence for the role of airborne transmission and assumed mechanisms underlying seasonality.

Coexistence of three liquid phases in individual atmospheric aerosol particles

Aerosol particles are ubiquitous in the atmosphere and play an important role in air quality and the climate system. These particles can contain mixtures of primary organic aerosol, secondary organic aerosol, and secondary inorganic aerosol. We show that such internally mixed particles can contain three liquid phases. We also demonstrate that the presence of three liquid phases impacts the time needed for the particles to reach equilibrium with the surrounding gas phase and likely impacts the ability of the particles to activate into cloud droplets. A framework is presented for predicting conditions needed for the formation of three liquid phases in the atmosphere. These results will lead to improved representations of aerosols in models for air quality and climate predictions.

Individual atmospheric particles can contain mixtures of primary organic aerosol (POA), secondary organic aerosol (SOA), and secondary inorganic aerosol (SIA). To predict the role of such complex multicomponent particles in air quality and climate, information on the number and types of phases present in the particles is needed. However, the phase behavior of such particles has not been studied in the laboratory, and as a result, remains poorly constrained. Here, we show that POA+SOA+SIA particles can contain three distinct liquid phases: a low-polarity organic-rich phase, a higher-polarity organic-rich phase, and an aqueous inorganic-rich phase. Based on our results, when the elemental oxygen-to-carbon (O:C) ratio of the SOA is less than 0.8, three liquid phases can coexist within the same particle over a wide relative humidity range. In contrast, when the O:C ratio of the SOA is greater than 0.8, three phases will not form. We also demonstrate, using thermodynamic and kinetic modeling, that the presence of three liquid phases in such particles impacts their equilibration timescale with the surrounding gas phase. Three phases will likely also impact their ability to act as nuclei for liquid cloud droplets, the reactivity of these particles, and the mechanism of SOA formation and growth in the atmosphere. These observations provide fundamental information necessary for improved predictions of air quality and aerosol indirect effects on climate.

Droplet Interfacial Tensions and Phase Transitions Measured in Microfluidic Channels

Measurements of droplet phase and interfacial tension (IFT) are important in the fields of atmospheric aerosols and emulsion science. Bulk macroscale property measurements with similar constituents cannot capture the effect of microscopic length scales and highly curved surfaces on the transport characteristics and heterogeneous chemistry typical in these applications. Instead, microscale droplet measurements ensure properties are measured at the relevant length scale. With recent advances in microfluidics, customized multiphase fluid flows can be created in channels for the manipulation and observation of microscale droplets in an enclosed setting without the need for large and expensive control systems. In this review, we discuss the applications of different physical principles at the microscale and corresponding microfluidic approaches for the measurement of droplet phase state, viscosity, and IFT.

Dark Iron-Catalyzed Reactions in Acidic and Viscous Aerosol Systems Efficiently Form Secondary Brown Carbon

Iron-driven secondary brown carbon formation reactions from water-soluble organics in cloud droplets and aerosols create insoluble and soluble products of emerging atmospheric importance. This work shows, for the first time, results on dark iron-catalyzed polymerization of catechol forming insoluble black polycatechol particles and colored water-soluble oligomers under conditions characteristic of viscous multicomponent aerosol systems with relatively high ionic strength (I = 1-12 m) and acidic pH (∼2). These systems contain ammonium sulfate (AS)/nitrate (AN) and C3-C5 dicarboxylic acids, namely, malonic, malic, succinic, and glutaric acids. Using dynamic light scattering (DLS) and ultra high pressure liquid chromatography-mass spectrometry (UHPLC-MS), we show results on the rate of particle growth/agglomeration and identity of soluble oligomeric reaction products. We found that increasing I above 1 m and adding diacids with oxygen-to-carbon molar ratio (O:C > 1) significantly reduced the rate of polycatechol formation/aggregation by a factor of 1.3 ± 0.4 in AS solution in the first 60 min of reaction time. Using AN, rates were too slow to be quantified using DLS, but particles formed after 24 h reaction time. These results were explained by the relative concentration and affinity of ligands to Fe(III). We also report detectable amounts of soluble and colored oligomers in reactions with a slow rate of polycatechol formation, including organonitrogen compounds. These results highlight that brown carbon formation from iron chemistry is efficient under a wide range of aerosol physical states and chemical composition.

Aerosol Optical Tweezers Elucidate the Chemistry, Acidity, Phase Separations, and Morphology of Atmospheric Microdroplets

ConspectusAerosol particles represent unique chemical environments because of their high surface area-to-volume ratio that promotes the effects of interfacial chemistry in confined environments. Properties such as viscosity, diffusivity, water content, pH, and morphology-following liquid-liquid phase separation-can strongly alter how a particle interacts with condensable vapors and reactive trace gases, thus modifying its continual evolution and environmental effects. Our understanding of this chemical evolution of atmospheric particulate matter and its environmental impacts is largely limited by our ability to directly observe how these critical particle properties respond to the addition or reactive uptake of new chemical components. Aerosol optical tweezers (AOT) stably trap particles in focused laser beams, providing positional control and the retrieval of many of these critical properties required to understand and predict the chemistry of aerosolized microdroplets. The analytical power of the AOT stems from the retrieval of the cavity-enhanced Raman spectrum induced by the trapping laser. Analysis of the whispering gallery modes (WGMs) that resonate as a standing wave around the droplet's interface, provide high accuracy measurements of the droplet's size, refractive index (and thus a measurement of composition), and can distinguish between core-shell, partially engulfed, and homogeneous morphologies. We have advanced the ability to determine the properties of the core and shell phases in biphasic droplets, including obtaining high-accuracy pH measurements. These capabilities were applied to perform AOT physical chemistry experiments on authentic secondary organic aerosol (SOA) produced directly in the AOT chamber by ozonolysis of terpene vapors. The propensity of the SOA to phase separate as a shell from a wide range of nonpolar to polar core phases was observed, along with the discovery of a stable emulsified state of SOA particles in an aqueous salt droplet. Micron-thick SOA shells did not impede the gain or loss of water or squalane from the core to the surrounding air, indicating no significant diffusional limitations to condensational growth or partitioning even under dry conditions. These experiments formed the foundation of a new framework that predicts how the phase-separated morphology of complex aerosols containing organic carbon evolves during continual atmospheric oxidation processes. Increases in oxidation state will quickly drive conversion from a partially engulfed to core-shell morphology that has dramatically different chemical reactivity since the core phase is completely concealed by the shell. The recent advances in the experimental capabilities of the AOT technique such as presented here enable novel experimental methodologies that provide insights into the chemistry and multidimensional properties of aerosol microdroplets, and how these coevolve and respond to continual chemical reactions.

Evolution of Hydrogen-Bond Interactions within Single Levitated Metastable Aerosols Studied by In Situ Raman Spectroscopy

Atmospheric aerosols can exist as supersaturated (metastable) liquid or glassy states, with physical and chemical properties that are distinct from the solid or liquid phases. These unique properties of aerosols have substantial implications on climate and health effects. Direct investigations on metastable aerosols remain a challenge because any interfacial contact can cause heterogeneous nucleation. In this study, in situ Raman spectroscopic and Mie scattering imaging analysis is applied to metastable aerosols in the absence of physical contact using an environment-controlled electrodynamic balance (EDB). This has allowed a detailed study of the O-H stretching regions of the Raman spectrum, revealing evidence for the rearrangement of hydrogen-bonding structures of levitated aqueous citric acid (CA) and aqueous sucrose droplets at metastable liquid states. We found that carboxyl groups in a CA droplet yield distinctive dynamics of strong and weak hydrogen bonds, whereas hydroxyl groups in a sucrose droplet show correlated strong and weak interactions. Such effects are particularly important in a supersaturated solution. These results indicate that metastable liquid aerosols from different sources may exhibit distinct physical and chemical behavior.

Aerosol Acidity: Novel Measurements and Implications for Atmospheric Chemistry

The pH of a solution is one of its most fundamental chemical properties, impacting reaction pathways and kinetics across every area of chemistry. The atmosphere is no different, with the pH of the condensed phase driving key chemical reactions that ultimately impact global climate in numerous ways. The condensed phase in the atmosphere is comprised of suspended liquid or solid particles, known as the atmospheric aerosol, which are differentiated from cloud droplets by their much smaller size (primarily <10 μm). The pH of the atmospheric aerosol can enhance certain chemical reactions leading to the formation of additional condensed phase mass from lower volatility species (secondary aerosol), alter the optical and water uptake properties of particles, and solubilize metals that can act as key nutrients in nutrient-limited ecosystems or cause oxidative stress after inhalation. However, despite the importance of aerosol acidity for climate and health, our fundamental understanding of pH has been limited due to aerosol size (by number >99% of particles are <1 μm) and complexity. Within a single atmospheric particle, there can be hundreds to thousands of distinct chemical species, varying water content, high ionic strengths, and different phases (liquid, semisolid, and solid). Making aerosol analysis even more challenging, atmospheric particles are constantly evolving through heterogeneous reactions with gases and multiphase chemistry within the condensed phase. Based on these challenges, traditional pH measurements are not feasible, and, for years, indirect and proxy methods were the most common way to estimate aerosol pH, with mixed results. However, aerosol pH needs to be incorporated into climate models to accurately determine which chemical reactions are dominant in the atmosphere. Consequently, experimental measurements that probe pH in atmospherically relevant particles are sorely needed to advance our understanding of aerosol acidity.This Account describes recent advances in measurements of aerosol particle acidity, specifically three distinct methods we developed for experimentally determining particle pH. Our acid-conjugate base method uses Raman microspectroscopy to probe an acid (e.g., HSO₄^-) and its conjugate base (e.g., SO₄^2-) in individual micrometer-sized particles. Our second approach is a field-deployable colorimetric method based on pH indicators (e.g., thymol blue) and cell phone imaging to provide a simple, low-cost approach to ensemble average (or bulk) pH for particles in distinct size ranges down to a few hundred nanometers in diameter. In our third method, we monitor acid-catalyzed polymer degradation of a thin film (∼23 nm) of poly(ε-caprolactone) (PCL) on silicon by individual particles with atomic force microscopy (AFM) after inertially impacting particles of different pH. These measurements are improving our understanding of aerosol pH from a fundamental physical chemistry perspective and have led to initial atmospheric measurements. The impact of aerosol pH on key atmospheric processes, such as secondary organic aerosol (SOA) formation, is discussed. Some unique findings, such as an unexpected size dependence to aerosol pH and kinetic limitations, illustrate that particles are not always in thermodynamic equilibrium with the surrounding gas. The implications of our limited, but improving, understanding of the fundamental chemical concept of pH in the atmospheric aerosol are critical for connecting chemistry and climate.

Role of solvent in differential phase behavior of celecoxib during spray drying

Spray drying is an industrially viable technique that can be used for modulation of the physical form of Active Pharmaceutical Ingredients (API), which is governed by inherent crystallization tendency and processing parameters during spray drying. In the current study, we investigated the role of solvent in differential phase behavior of celecoxib, a poor crystallizer, during spray drying and unveiled the underlying mechanisms. 1% w/v solutions of celecoxib in three different compositions of methanol (M)-water (W) solvent system were spray dried using a laboratory spray dryer. The proportions were 0, 5 and 10% v/v of water in methanol (MW0, MW5, and MW10, respectively). Percentage crystallinity of the spray dried products were evaluated using modulated differential scanning calorimetry and was in the order MW10 > MW5 > MW0 (i.e. 18.52% > 8.13% > 0%). Solution-state and solid-state crystallization events responsible for the experimental observations were probed using microscopy, Raman spectroscopy, and non-isothermal crystallization studies. An intermediate amorphous phase was generated for the studied samples, which underwent crystallization under the influence of chamber temperature for MW5 and MW10. Additionally, liquid-liquid phase separation (LLPS) at very high level of supersaturation led to relatively higher crystallinity for MW10. Insights from this work provide the basis for understanding of probable phase behavior of poor crystallizers during spray drying.

Predicting secondary organic aerosol phase state and viscosity and its effect on multiphase chemistry in a regional-scale air quality model

Atmospheric aerosols are a significant public health hazard and have substantial impacts on the climate. Secondary organic aerosols (SOAs) have been shown to phase separate into a highly viscous organic outer layer surrounding an aqueous core. This phase separation can decrease the partitioning of semi-volatile and low-volatile species to the organic phase and alter the extent of acid-catalyzed reactions in the aqueous core. A new algorithm that can determine SOA phase separation based on their glass transition temperature (T _g), oxygen to carbon (O : C) ratio and organic mass to sulfate ratio, and meteorological conditions was implemented into the Community Multiscale Air Quality Modeling (CMAQ) system version 5.2.1 and was used to simulate the conditions in the continental United States for the summer of 2013. SOA formed at the ground/surface level was predicted to be phase separated with core-shell morphology, i.e., aqueous inorganic core surrounded by organic coating 65.4 % of the time during the 2013 Southern Oxidant and Aerosol Study (SOAS) on average in the isoprene-rich southeastern United States. Our estimate is in proximity to the previously reported ~ 70 % in literature. The phase states of organic coatings switched between semi-solid and liquid states, depending on the environmental conditions. The semi-solid shell occurring with lower aerosol liquid water content (western United States and at higher altitudes) has a viscosity that was predicted to be 10²-10¹² Pa s, which resulted in organic mass being decreased due to diffusion limitation. Organic aerosol was primarily liquid where aerosol liquid water was dominant (eastern United States and at the surface), with a viscosity < 10² Pa s. Phase separation while in a liquid phase state, i.e., liquid-liquid phase separation (LLPS), also reduces reactive uptake rates relative to homogeneous internally mixed liquid morphology but was lower than aerosols with a thick viscous organic shell. The sensitivity cases performed with different phase-separation parameterization and dissolution rate of isoprene epoxydiol (IEPOX) into the particle phase in CMAQ can have varying impact on fine particulate matter (PM_2.5) organic mass, in terms of bias and error compared to field data collected during the 2013 SOAS. This highlights the need to better constrain the parameters that govern phase state and morphology of SOA, as well as expand mechanistic representation of multiphase chemistry for non-IEPOX SOA formation in models aided by novel experimental insights.

Simultaneous Optical Photothermal Infrared (O-PTIR) and Raman Spectroscopy of Submicrometer Atmospheric Particles

Physicochemical analysis of individual atmospheric aerosols at the most abundant sizes in the atmosphere (<1 μm) is analytically challenging, as hundreds to thousands of species are often present in femtoliter volumes. Vibrational spectroscopies, such as infrared (IR) and Raman, have great potential for probing functional groups in single particles at ambient pressure and temperature. However, the diffraction limit of IR radiation limits traditional IR microscopy to particles > ∼10 μm, which have less relevance to aerosol health and climate impacts. Optical photothermal infrared (O-PTIR) spectroscopy is a contactless method that circumvents diffraction limitations by using changes in the scattering intensity of a continuous wave visible laser (532 nm) to detect the photothermal expansion when a vibrational mode is excited by a tunable IR laser (QCL: 800-1800 cm^-1 or OPO: 2600-3600 cm^-1). Herein, we simultaneously collect O-PTIR spectra with Raman spectra at a single point for individual particles with aerodynamic diameters <400 nm (prior to impaction and spreading) at ambient temperature and pressure, by also collecting the inelastically scattered visible photons for Raman spectra. O-PTIR and Raman spectra were collected for submicrometer particles with different substrates, particle chemical compositions, and morphologies (i.e., core-shell), as well as IR mapping with submicron spatial resolution. Initial O-PTIR analysis of ambient atmospheric particles identified both inorganic and organic modes in individual sub- and supermicrometer particles. The simultaneous IR and Raman microscopy with submicrometer spatial resolution described herein has considerable potential both in atmospheric chemistry and numerous others fields (e.g., materials and biological research).

Liquid-Liquid Phase Separation in Supermicrometer and Submicrometer Aerosol Particles

ConspectusThe 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. 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. 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.

Emerging aqueous two-phase systems: from fundamentals of interfaces to biomedical applications

This review summarizes recent advances of aqueous two-phase systems (ATPSs), particularly their interfaces, with a focus on biomedical applications.

Effects of Molecular-Level Compositional Variability in Organic Aerosol on Phase State and Thermodynamic Mixing Behavior

The molecular-level composition and structure of organic aerosol (OA) affect its chemical/physical properties, transformations, and impacts. Here, we use the molecular-level chemical composition of functionalized OA from three diverse field sites to evaluate the effect of molecular-level compositional variability on OA phase state and thermodynamic mixing favorability. For these ambient sites, modeled aerosol phase state ranges from liquid to semisolid. The observed variability in OA composition has some effect on resulting phase state, but other factors like the presence of inorganic ions, aerosol liquid water, and internal versus external mixing with water are determining factors in whether these particles exist as liquids, semisolids, or solids. Organic molecular composition plays a more important role in determining phase state for phase-separated (verus well-mixed) systems. Similarly, despite the observed OA compositional differences, the thermodynamic mixing favorability for OA samples with aerosol liquid water, isoprene oxidation products, or monoterpene oxidation products remains fairly consistent within each campaign. Mixing of filter-sampled OA and isoprene or monoterpene oxidation products is often favorable in both seasons, while mixing with water is generally unfavorable.

Size-Dependent Liquid-Liquid Phase Separation in Atmospherically Relevant Complex Systems

Physical properties of aerosol particles, such as liquid-liquid phase separation (LLPS), have the potential to impact the climate system. Model systems have been shown to have size-dependent LLPS in the submicron regime; however, these systems are an extreme simplification of ambient aerosol, which can include myriad organic compounds. We expand the studies of LLPS in particles consisting of ammonium sulfate and more complex organic mixtures from multiple organic compounds to α-pinene secondary organic matter (SOM). All systems display a size-dependent morphology, with small particles remaining homogeneous while large particles phase-separate. Surprisingly, three-phase particles were also observed in some of the systems in addition to a new phase state that we have termed channel morphology, which can arise upon efflorescence. The existence of size-dependent LLPS in complex organic mixtures and SOM provides evidence that this is a relevant phenomenon for ambient aerosol and should be considered when modeling atmospheric aerosol.

Effect of Oxalate and Sulfate on Iron-Catalyzed Secondary Brown Carbon Formation

Oxalate and sulfate are ubiquitous components of ambient aerosols with a high complexation affinity to iron. However, their effect on iron-driven secondary brown carbon formation in solution from soluble aromatic and aliphatic reagents was not studied. We report masses and hydrodynamic particle sizes of insoluble particles formed from the dark aqueous phase reaction of catechol, guaiacol, fumaric, and muconic acids with Fe(III) in the presence of oxalate or sulfate. Results show that oxalate decreases particle yield in solution from the reaction of Fe(III), with a stronger effect for guaiacol than catechol. For both compounds, the addition of sulfate results in the formation of more polydisperse (0.1-5 μm) and heavier particles than those from control experiments. Reactions with fumaric and muconic acids show that oxalate (not sulfate) and pH are determining factors in the efficiency of particle formation in solution. Polymerization reactions occur readily in the presence of sulfate in solution producing particles with iron-coordinated and/or pore-trapped sulfate anions. The addition of oxalate to the reactions of Fe(III) with all organics, except guaiacol, produced fewer and larger polymeric particles (>0.5 μm). These results imply that even in the presence of competing ligands, the formation of insoluble and colored particles from soluble organic precursors still dominates over the formation of soluble iron complexes.

Bouncier Particles at Night: Biogenic Secondary Organic Aerosol Chemistry and Sulfate Drive Diel Variations in the Aerosol Phase in a Mixed Forest

Aerosol phase state is critical for quantifying aerosol effects on climate and air quality. However, significant challenges remain in our ability to predict and quantify phase state during its evolution in the atmosphere. Herein, we demonstrate that aerosol phase (liquid, semisolid, solid) exhibits a diel cycle in a mixed forest environment, oscillating between a viscous, semisolid phase state at night and liquid phase state with phase separation during the day. The viscous nighttime particles existed despite higher relative humidity and were independently confirmed by bounce factor measurements and atomic force microscopy. High-resolution mass spectrometry shows the more viscous phase state at night is impacted by the formation of terpene-derived and higher molecular weight secondary organic aerosol (SOA) and smaller inorganic sulfate mass fractions. Larger daytime particulate sulfate mass fractions, as well as a predominance of lower molecular weight isoprene-derived SOA, lead to the liquid state of the daytime particles and phase separation after greater uptake of liquid water, despite the lower daytime relative humidity. The observed diel cycle of aerosol phase should provoke rethinking of the SOA atmospheric lifecycle, as it suggests diurnal variability in gas-particle partitioning and mixing time scales, which influence aerosol multiphase chemistry, lifetime, and climate impacts.

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.

Chemical signatures of surface microheterogeneity on liquid mixtures

Many chemical reactions in Nature, the laboratory, and chemical industry occur in solvent mixtures that bring together species of dissimilar solubilities. Solvent mixtures are visually homogeneous, but are not randomly mixed at the molecular scale. In the all-important binary water-hydrotrope mixtures, small-angle neutron and dynamic light scattering experiments reveal the existence of short-lived (<50 ps), short-ranged (∼1 nm) concentration fluctuations. The presence of hydrophobic solutes stabilizes and extends such fluctuations into persistent, mesoscopic (10-100 nm) inhomogeneities. While the existence of inhomogeneities is well established, their impacts on reactivity are not fully understood. Here, we search for chemical signatures of inhomogeneities on the surfaces of W:X mixtures (W = water; X = acetonitrile, tetrahydrofuran, or 1,4-dioxane) by studying the reactions of Criegee intermediates (CIs) generated in situ from O₃(g) addition to a hydrophobic olefin (OL) solute. Once formed, CIs isomerize to functionalized carboxylic acids (FC) or add water to produce α-hydroxy-hydroperoxides (HH), as detected by surface-specific, online pneumatic ionization mass spectrometry. Since only the formation of HH requires the presence of water, the dependence of the R = HH/FC ratio on water molar fraction x w expresses the accessibility of water to CIs on the surfaces of mixtures. The finding that R increases quasi-exponentially with x w in all solvent mixtures is consistent with CIs being preferentially produced (from their OL hydrophobic precursor) in X-rich, long-lived OL:X _m W _n interfacial clusters, rather than randomly dispersed on W:X surfaces. R vs x w dependences therefore reflect the average ⟨m, n⟩ composition of OL:X _m W _n interfacial clusters, as weighted by cluster reorganization dynamics. Water in large, rigid clusters could be less accessible to CIs than in smaller but more flexible clusters of lower water content. Since mesoscale inhomogeneities are intrinsic to most solvent mixtures, these phenomena should be quite general.

α-Pinene-Derived Organic Coatings on Acidic Sulfate Aerosol Impacts Secondary Organic Aerosol Formation from Isoprene in a Box Model

Fine particulate matter (PM_2.5) is known to have an adverse impact on public health and is an important climate forcer. Secondary organic aerosol (SOA) contributes up to 80% of PM_2.5 worldwide and multiphase reactions are an important pathway to form SOA. Aerosol-phase state is thought to influence the reactive uptake of gas-phase precursors to aerosol particles by altering diffusion rates within particles. Current air quality models do not include the impact of diffusion-limiting organic coatings on SOA formation. This work examines how α-pinene-derived organic coatings change the predicted formation of SOA from the acid-catalyzed multiphase reactions of isoprene epoxydiols (IEPOX). A box model, with inputs provided from field measurements taken at the Look Rock (LRK) site in Great Smokey Mountains National Park during the 2013 Southern Oxidant and Aerosol Study (SOAS), was modified to incorporate the latest laboratory-based kinetic data accounting for organic coating influences. Including an organic coating influence reduced the modeled reactive uptake when relative humidity was in the 55-80% range, with predicted IEPOX-derived SOA being reduced by up to 33%. Only sensitivity cases with a large increase in Henry's Law values of an order of magnitude or more or in particle reaction rates resulted in the large statistically significant differences form base model performance. These results suggest an organic coating layer could have an impact on IEPOX-derived SOA formation and warrant consideration in regional and global scale models.