Liquid-Liquid Phase Separation in Supermicrometer and Submicrometer Aerosol Particles

Kinetic and Thermodynamic Interplay of Polymer-Mediated Liquid-Liquid Phase Separation for Poorly Water-Soluble Drugs

Understanding the interplay between kinetics and thermodynamics of polymer-mediated liquid-liquid phase separation is crucial for designing and implementing an amorphous solid dispersion formulation strategy for poorly water-soluble drugs. This work investigates the phase behaviors of a poorly water-soluble model drug, celecoxib (CXB), in a supersaturated aqueous solution with and without polymeric additives (PVP, PVPVA, HPMCAS, and HPMCP). Drug-polymer-water ternary phase diagrams were also constructed to estimate the thermodynamic behaviors of the mixtures at room temperature. The liquid-liquid phase separation onset point for CXB was detected using an inline UV/vis spectrometer equipped with a fiber optic probe. Varying CXB concentrations were achieved using an accurate syringe pump throughout this study. The appearance of the transient nanodroplets was verified by cryo-EM and total internal reflection fluoresence microscopic techniques. The impacts of various factors, such as polymer composition, drug stock solution pumping rates, and the types of drug-polymer interactions, are tested against the onset points of the CXB liquid-liquid phase separation (LLPS). It was found that the types of drug-polymer interactions, i.e., hydrogen bonding and hydrophobic interactions, are vital to the position and shapes of LLPS in the supersaturation drug solution. A relation between the behaviors of LLPS and its location in the CXB-polymer-water ternary phase diagram was drawn from the findings.

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.

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.

Role of WSOCs and pH on Ammonium Nitrate Aerosol Efflorescence: Insights into Secondary Aerosol Formation

Efflorescence of ammonium nitrate (AN) aerosols significantly impacts atmospheric secondary aerosol formation, climate, and human health. We investigated the effect of representative water-soluble organic compounds (WSOCs) (sucralose (SUC), glycerol (GLY), and citric acid (CA) on AN:WSOC aerosol efflorescence using vacuum Fourier transform infrared spectroscopy. Combining efflorescence relative humidity (ERH) measurements, heterogeneous nucleation rates, and model predictions, we found that aerosol viscosity, correlating with molecular diffusion, effectively predicted ERH variations among the AN:WSOC aerosols. WSOCs with higher viscosity (SUC and CA) hindered efflorescence, while GLY with a lower viscosity showed a minor effect. At a low AN:CA molar ratio (10:1), CA promoted ERH, likely due to CA crystallization. Increasing the droplet pH inhibited AN:CA aerosol efflorescence. In contrast, for AN:SUC and AN:GLY aerosols, efflorescence is pH-insensitive. With the addition of trivial sulfate, AN:SUC droplets exhibited two-stage efflorescence, coinciding with ammonium sulfate and AN efflorescence. Given the atmospheric abundance, the morphology, phase, and mixing state of nitrate aerosols are significant for atmospheric chemistry and physics. Our results suggest that AN:WSOCs aerosols can exist in the amorphous phase in the atmosphere, with efflorescence behavior depending on the aerosol composition, viscosity, pH, and the cation and anion interactions in a complex manner.

Liquid-Liquid Phase Separation Can Drive Aerosol Droplet Growth in Supersaturated Regimes

It is well known that atmospheric aerosol size and composition impact air quality, climate, and health. The aerosol composition is typically a mixture and consists of a wide range of organic and inorganic particles that interact with each other. Furthermore, water vapor is ubiquitous in the atmosphere, in indoor air, and within the human body's respiratory system, and the presence of water can alter the aerosol morphology and propensity to form droplets. Specifically, aerosol mixtures can undergo liquid-liquid phase separation (LLPS) in the presence of water vapor. However, the experimental conditions for which LLPS impacts water uptake and the subsequent prediction of aerosol mixtures are poorly understood. To improve our understanding of aerosol mixtures and droplets, this study explores two ternary systems that undergo LLPS, namely, the 2MGA system (sucrose + ammonium sulfate + 2-methylglutaric acid) and the PEG1000 system (sucrose + ammonium sulfate + polyethylene glycol 1000). In this study, the ratio of species and the O:C ratios are systematically changed, and the hygroscopic properties of the resultant aerosol were investigated. Here, we show that the droplet activation above 100% RH of the 2MGA system was influenced by LLPS, while the droplet activation of the PEG1000 system was observed to be linearly additive regardless of chemical composition, O:C ratio, and LLPS. A theoretical model that accounts for LLPS with O:C ratios was developed and predicts the water uptake of internally mixed systems of different compositions and phase states. Hence, this study provides a computationally efficient algorithm to account for the LLPS and solubility parameterized by the O:C ratio for droplet activation at supersaturated relative humidity conditions and may thus be extended to mixed inorganic-organic aerosol populations with unspeciated organic composition found in the ambient environment.

Impact of pH on Gas-Particle Partitioning of Semi-Volatile Organics in Multicomponent Aerosol

The partitioning of semivolatile organic compounds (SVOCs) between the condensed and gas phases can have significant implications for the properties of aerosol particles. In addition to affecting size and composition, this partitioning can alter radiative properties and impact cloud activation processes. We present measurements and model predictions on how activity and pH influence the evaporation of SVOCs from particles to the gas phase, specifically investigating aqueous inorganic particles containing dicarboxylic acids (DCAs). The aerosols are studied at the single-particle level by using optical trapping and cavity-enhanced Raman spectroscopy. Optical resonances in the spectra enable precise size tracking, while vibrational bands allow real-time monitoring of pH. Results are compared to a Maxwell-type model that accounts for volatile and nonvolatile solutes in aqueous droplets that are held at a constant relative humidity. The aerosol inorganic-organic mixture functional group activity coefficients thermodynamic model and Debye-Hückel theory are both used to calculate the activities of the species present in the droplet. For DCAs, we find that the evaporation rate is highly sensitive to the particle pH. For acidity changes of approximately 1.5 pH units, we observe a shift from a volatile system to one that is completely nonvolatile. We also observe that the pH itself is not constant during evaporation; it increases as DCAs evaporate, slowing the rate of evaporation until it eventually ceases. Whether a DCA evaporates or remains a stable component of the droplet is determined by the difference between the lowest pKa of the DCA and the pH of the droplet.

Phase Behavior and Viscosity in Biomass Burning Organic Aerosol and Climatic Impacts

Smoke particles generated by burning biomass consist mainly of organic aerosol termed biomass burning organic aerosol (BBOA). BBOA influences the climate by scattering and absorbing solar radiation or acting as nuclei for cloud formation. The viscosity and the phase behavior (i.e., the number and type of phases present in a particle) are properties of BBOA that are expected to impact several climate-relevant processes but remain highly uncertain. We studied the phase behavior of BBOA using fluorescence microscopy and showed that BBOA particles comprise two organic phases (a hydrophobic and a hydrophilic phase) across a wide range of atmospheric relative humidity (RH). We determined the viscosity of the two phases at room temperature using a photobleaching method and showed that the two phases possess different RH-dependent viscosities. The viscosity of the hydrophobic phase is largely independent of the RH from 0 to 95%. We use the Vogel-Fulcher-Tamman equation to extrapolate our results to colder and warmer temperatures, and based on the extrapolation, the hydrophobic phase is predicted to be glassy (viscosity >1012 Pa s) for temperatures less than 230 K and RHs below 95%, with possible implications for heterogeneous reaction kinetics and cloud formation in the atmosphere. Using a kinetic multilayer model (KM-GAP), we investigated the effect of two phases on the atmospheric lifetime of brown carbon within BBOA, which is a climate-warming agent. We showed that the presence of two phases can increase the lifetime of brown carbon in the planetary boundary layer and polar regions compared to previous modeling studies. Hence, the presence of two phases can lead to an increase in the predicted warming effect of BBOA on the climate.

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.

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.

Significance of liquid-liquid phase separation (LLPS)-related genes in breast cancer: a multi-omics analysis

Currently, the role of liquid-liquid phase separation (LLPS) in cancer has been preliminarily explained. However, the significance of LLPS in breast cancer is unclear. In this study, single cell sequencing datasets GSE188600 and GSE198745 for breast cancer were downloaded from the GEO database. Transcriptome sequencing data for breast cancer were downloaded from UCSC database. We divided breast cancer cells into high-LLPS group and low-LLPS group by down dimension clustering analysis of single-cell sequencing data set, and obtained differentially expressed genes between the two groups. Subsequently, weighted co-expression network analysis (WGCNA) was performed on transcriptome sequencing data, and the module genes most associated with LLPS were obtained. COX regression and Lasso regression were performed and the prognostic model was constructed. Subsequently, survival analysis, principal component analysis, clinical correlation analysis, and nomogram construction were used to evaluate the significance of the prognostic model. Finally, cell experiments were used to verify the function of the model's key gene, PGAM1. We constructed a LLPS-related prognosis model consisting of nine genes: POLR3GL, PLAT, NDRG1, HMGB3, HSPH1, PSMD7, PDCD2, NONO and PGAM1. By calculating LLPS-related risk scores, breast cancer patients could be divided into high-risk and low-risk groups, with the high-risk group having a significantly worse prognosis. Cell experiments showed that the activity, proliferation, invasion and healing ability of breast cancer cell lines were significantly decreased after knockdown of the key gene PGAM1 in the model. Our study provides a new idea for prognostic stratification of breast cancer and provides a novel marker: PGAM1.

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.

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.

Exploring the Nanostructures Accessible to an Organic Surfactant Atmospheric Aerosol Proxy

The composition of atmospheric aerosols varies with time, season, location, and environment. This affects key aerosol properties such as hygroscopicity and reactivity, influencing the aerosol’s impact on the climate and air quality. The organic fraction of atmospheric aerosol emissions often contains surfactant material, such as fatty acids. These molecules are known to form three-dimensional nanostructures in contact with water. Different nanostructures have marked differences in viscosity and diffusivity that are properties whose understanding is essential when considering an aerosol’s atmospheric impact. We have explored a range of nanostructures accessible to the organic surfactant oleic acid (commonly found in cooking emissions), simulating variation that is likely to happen in the atmosphere. This was achieved by changing the amount of water, aqueous phase salinity and by addition of other commonly coemitted compounds: sugars and stearic acid (the saturated analogue of oleic acid). The nanostructure was observed by both synchrotron and laboratory small/wide angle X-ray scattering (SAXS/WAXS) and found to be sensitive to the proxy composition. Additionally, the spacing between repeat units in these nanostructures was water content dependent (i.e., an increase from 41 to 54 Å in inverse hexagonal phase d-spacing when increasing the water content from 30 to 50 wt %), suggesting incorporation of water within the nanostructure. A significant decrease in mixture viscosity was also observed with increasing water content from ∼10⁴ to ∼10² Pa s when increasing the water content from 30 to 60 wt %. Time-resolved SAXS experiments on levitated droplets of this proxy confirm the phase changes observed in bulk phase mixtures and demonstrate that coexistent nanostructures can form in droplets. Aerosol compositional and subsequent nanostructural changes could affect aerosol processes, leading to an impact on the climate and urban air pollution.

Toward a molecular understanding of the surface composition of atmospherically relevant organic particles

Many mass spectrometry methods using various ionization sources provide bulk composition of airborne particles, but little is known about the surface species that play a major role in determining their physicochemical properties that impact air quality, climate, and health. The present work shows that the composition of surface layers of atmospherically relevant submicron organic particles can be probed without the use of an external ionization source. Solid dicarboxylic acid particles are used as models, with glutaric acid being the most efficient at generating ions. Coating with small diacids or products from α-pinene ozonolysis demonstrates that ions are ejected from the surface, providing surface molecular characterization of organic particles on the fly. This unique approach provides a path forward for elucidating the role of the surface in determining chemical and physical properties of particles, including heterogeneous reactions, particle growth, water uptake, and interactions with biological systems.

Decomposition of multifunctionalized α-alkoxyalkyl-hydroperoxides derived from the reactions of Criegee intermediates with diols in liquid phases

The oxidation of volatile organic compounds in the atmosphere produces organic hydroperoxides (ROOHs) that typically possess not only -OOH but also other functionalities such as -OH and -C(O). Because of their high hydrophilicity and low volatility, such multifunctionalized ROOHs are expected to be taken up in atmospheric condensed phases such as aerosols and fog/cloud droplets. However, the characteristics of ROOHs that control their fates and lifetimes in liquid phases are poorly understood. Here, we report a study of the liquid-phase decomposition kinetics of multifunctionalized α-alkoxyalkyl-hydroperoxides (α-AHs) that possessed an ether, a carbonyl, a hydroperoxide, and two hydroxy groups. These ROOHs were synthesized by ozonolysis of α-terpineol in water in the presence of 1,3-propanediol, 1,4-butanediol, or 1,5-pentanediol. Their decomposition products were detected as chloride anion adducts by electrospray mass spectrometry as a function of reaction time. Experiments using H₂¹⁸O and D₂O revealed that hemiacetal species were α-AH decomposition products that further transformed into other products. The result that the rate coefficients (k) of the decomposition of C₁₅ α-AHs increased exponentially from pH 5.0 to 3.9 was consistent with an H⁺-catalyzed decomposition mechanism. The temperature dependence of k and an Arrhenius plot yielded activation energies (E_a) of 15.7 ± 0.8, 15.0 ± 2.4, and 15.9 ± 0.3 kcal mol^-1 for the decomposition of α-AHs derived from the reaction of α-terpineol CIs with 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol, respectively. The determined E_a values were compared with those of related ROOHs. We found that alkyl chain length is not a critical factor for the decomposition mechanism, whereas the presence of additional -OH groups would modulate the reaction barriers to decomposition via the formation of hydrogen-bonding with surrounding water molecules. The derived E_a values for the decomposition of the multifunctionalized, terpenoid-derived α-AHs will facilitate atmospheric modeling by serving as representative values for ROOHs in atmospheric condensed phases.

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.

Solid organic-coated ammonium sulfate particles at high relative humidity in the summertime Arctic atmosphere

Physical and chemical properties of individual atmospheric particles determine their climate impacts. Hygroscopic inorganic salt particles mixed with trace amounts of organic material are predicted to be liquid under typical tropospheric conditions in the summertime Arctic. Yet, we unexpectedly observed a significant concentration of solid particles composed of ammonium sulfate with an organic coating under conditions of high relative humidity and low temperature. These particle properties are consistent with marine biogenic-derived new particle formation and growth, with particle collision hypothesized to result in the solid phase. This particle source is predicted to have increasing relevance in the context of declining Arctic sea ice and increasing open water, with impacts on clouds, and therefore climate.

The ability of atmospheric aerosols to impact climate through water uptake and cloud formation is fundamentally determined by the size, composition, and phase (liquid, semisolid, or solid) of individual particles. Particle phase is dependent on atmospheric conditions (relative humidity and temperature) and chemical composition and, importantly, solid particles can inhibit the uptake of water and other trace gases, even under humid conditions. Particles composed primarily of ammonium sulfate are presumed to be liquid at the relative humidities (67 to 98%) and temperatures (−2 to 4 °C) of the summertime Arctic. Under these atmospheric conditions, we report the observation of solid organic-coated ammonium sulfate particles representing 30% of particles, by number, in a key size range (<0.2 µm) for cloud activation within marine air masses from the Arctic Ocean at Utqiaġvik, AK. The composition and size of the observed particles are consistent with recent Arctic modeling and observational results showing new particle formation and growth from dimethylsulfide oxidation to form sulfuric acid, reaction with ammonia, and condensation of marine biogenic sulfate and highly oxygenated organic molecules. Aqueous sulfate particles typically undergo efflorescence and solidify at relative humidities of less than 34%. Therefore, the observed solid phase is hypothesized to occur from contact efflorescence during collision of a newly formed Aitken mode sulfate particle with an organic-coated ammonium sulfate particle. With declining sea ice in the warming Arctic, this particle source is expected to increase with increasing open water and marine biogenic emissions.

Liquid-Liquid Phase Separation in Nucleation Process of Biomineralization

Biomineralization is a typical interdisciplinary subject attracting biologists, chemists, and geologists to figure out its potential mechanism. A mounting number of studies have revealed that the classical nucleation theory is not suitable for all nucleation process of biominerals, and phase-separated structures such as polymer-induced liquid precursors (PILPs) play essential roles in the non-classical nucleation processes. These structures are able to play diverse roles biologically or pathologically, and could also give inspiring clues to bionic applications. However, a lot of confusion and dispute occurred due to the intricacy and interdisciplinary nature of liquid precursors. Researchers in different fields may have different opinions because the terminology and current state of understanding is not common knowledge. As a result, our team reviewed the most recent articles focusing on the nucleation processes of various biominerals to clarify the state-of-the-art understanding of some essential concepts and guide the newcomers to enter this intricate but charming field.

Characterization of Atmospheric Processes of Brevetoxins in Sea Spray Aerosols from Red Tide Events

Atmospheric processes can affect the longevity of harmful toxins in sea spray aerosols (SSA). This study characterized the degradation of brevetoxin (BTx) in SSA under different environmental conditions. The samples of seawater collected during a Karenia brevis bloom in Manasota, Florida, were nebulized into a large outdoor photochemical chamber to mimic the atmospheric oxidation of aerosolized toxins and then aged in the presence or absence of sunlight and/or O₃. Aerosol samples were collected during the aging process using a Particle-Into-Liquid Sampler. Their BTx concentrations were measured using an enzyme-linked immuno-sorbent assay (ELISA) and high-performance liquid chromatography/tandem mass spectroscopy. The BTx ozonolysis rate constant measured by ELISA was 5.74 ± 0.21 × 10³ M^-1 s^-1. The corresponding lifetime for decay of 87.5% BTx in the presence of 20 ppb of O₃ was 7.08 ± 0.26 h, suggesting that aerosolized BTx can still travel long distances at night before SSA deposition. BTx concentrations in SSA decreased more rapidly in the presence of sunlight than in its absence due to oxidation with photochemically produced OH radicals.

Organic Coating Reduces Hygroscopic Growth of Phase-Separated Aerosol Particles

A large fraction of secondary aerosol particles are liquid-liquid phase-separated with an organic shell and an inorganic core. This has the potential to regulate the hygroscopicity of such particles, with significant implications for their optical properties, reactivity, and lifetime. However, it is unclear how this phase separation affects the hygroscopic growth of the particles. Here, we showed a large variation in hygroscopic growth (e.g., 1.14-1.32 under a relative humidity (RH) of 90%) of particles from the forest and urban atmosphere, which had different average core-shell ratios. For this reason, a controlled laboratory experiment further quantifies the impact of the organic shell on particle growth with different RH values. Laboratory experiments demonstrated that (NH₄)₂SO₄ particles with thicker secondary organic shells have a lower growth factor at an RH below 94%. Organic shells started to deliquesce first (RH > 50%) and the phase changes of sulfate cores from solid to liquid took place at an RH higher than 80% as deliquescence relative humidity of pure (NH₄)₂SO₄. Our study provides the first direct evidence on an individual particle basis that hygroscopic growth behavior of phase-separated particles is dependent on the thickness of organic shells, highlighting the importance of organic coating in water uptake and possible heterogeneous reactions of the phase-separated particles.

Probing Liquid-Liquid Phase Separation in Secondary Organic Aerosol Mimicking Solutions Using Articulated Straws

The presence or absence of liquid-liquid phase separation (LLPS) in aerosol particles containing oxidized organic species and inorganic salts affects particle morphology and influences uptake into, diffusion through, and reactivity within those particles. We report here an accessible method, similar to ice core analyses, using solutions that are relevant for both aerosol chemical systems and aqueous two-phase extraction systems and contain ammonium sulfate and one of eight alcohols (methanol, ethanol, 1-propanol, 2-propanol, 2-butaonol, 3-methyl-2-butanol, 1,2-propanediol, or 1,3-propanediol) frozen in articulated (bendable) straws to probe LLPS. For alcohols with negative octanol-water partitioning coefficient (K _OW) values and O/C ratios ≥0.5, no LLPS occurs, while for alcohols with positive K _OW values and O/C ratios ≤0.33, phase separation always occurs, both findings consistent with observations using different experimental techniques. When a third species, glyoxal, is added, the glyoxal stays in the aqueous phase, regardless of whether LLPS occurs. When phase separation occurs, the glyoxal forms a strong intermolecular interaction with the sulfate ion, red-shifting the ν₃(SO₄ ^2-) peak by 15 cm^-1. These results provide evidence of chemical interactions within phase-separated systems that have implications for understanding chemical reactivity within those, and related, systems.

Prediction of Phase State of Secondary Organic Aerosol Internally Mixed with Aqueous Inorganic Salts

In the presence of inorganic salts, secondary organic aerosol (SOA) undergoes liquid-liquid phase separation (LLPS), liquid-solid phase separation, or a homogeneous phase in ambient air. In this study, a regression model was derived to predict aerosol phase separation relative humidity (SRH) for various organic and inorganic mixes. The model implemented organic physicochemical parameters (i.e., oxygen to carbon ratio, molecular weight, and hydrogen-bonding ability) and the parameters related to inorganic compositions (i.e., ammonium, sulfate, nitrate, and water). The aerosol phase data were observed using an optical microscope and also collected from the literature. The crystallization of aerosols at the effloresce RH (ERH) was semiempirically predicted with a neural network model. Overall, the greater SRH appeared for the organic compounds with the lower oxygen to carbon ratios or the greater molecular weight and the higher aerosol acidity or the larger fraction of inorganic nitrate led to the lower SRH. The resulting model has been demonstrated for three different chamber-generated SOA (originated from β-pinene, toluene, and 1,3,5-trimethylbenzene), which were internally mixed with the inorganic aqueous system of ammonium-sulfate-water. For all three SOA systems, both observations and model predictions showed LLPS at RH <80%. In the urban atmosphere, LLPS is likely a frequent occurrence for the typical anthropogenic SOA, which originates from aromatic and alkane hydrocarbon.

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.

Airborne virus transmission via respiratory droplets: Effects of droplet evaporation and sedimentation

Airborne transmission is considered as an important route for the spread of infectious diseases, such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and is primarily determined by the droplet sedimentation time, that is, the time droplets spend in air before reaching the ground. Evaporation increases the sedimentation time by reducing the droplet mass. In fact, small droplets can, depending on their solute content, almost completely evaporate during their descent to the ground and remain airborne as so-called droplet nuclei for a long time. Considering that viruses possibly remain infectious in aerosols for hours, droplet nuclei formation can substantially increase the infectious viral air load. Accordingly, the physical-chemical factors that control droplet evaporation and sedimentation times and play important roles in determining the infection risk from airborne respiratory droplets are reviewed in this article.

The persistence of a proxy for cooking emissions in megacities: a kinetic study of the ozonolysis of self-assembled films by simultaneous small and wide angle X-ray scattering (SAXS/WAXS) and Raman microscopy

Cooking emissions account for a significant proportion of the organic aerosols emitted into the urban environment and high pollution events have been linked to an increased organic content on urban particulate matter surfaces. We present a kinetic study on surface coatings of self-assembled (semi-solid) oleic acid-sodium oleate cooking aerosol proxies undergoing ozonolysis. We found clear film thickness-dependent kinetic behaviour and measured the effect of the organic phase on the kinetics for this system. In addition to the thickness-dependent kinetics, we show that significant fractions of unreacted proxy remain after extensive ozone exposure and that this effect scales approximately linearly with film thickness, suggesting that a late-stage inert reaction product may form and inhibit reaction progress - effectively building up an inert crust. We determine this by using a range of simultaneous analytical techniques; most notably Small-Angle X-ray Scattering (SAXS) has been used for the first time to measure the reaction kinetics of films of a wide range of thicknesses from ca. 0.59 to 73 μm with films <10 μm thick being of potential atmospheric relevance. These observations have implications for the evolution of particulate matter in the urban environment, potentially extending the atmospheric lifetimes of harmful aerosol components and affecting the local urban air quality and climate.