Size dependence of phase transitions in aerosol nanoparticles

Effect of Aerosol Size on Glass Transition Temperature

The amorphous phase state of suspended nanoparticles affects their atmospheric lifetimes and environmental impact. Influence of relative humidity and chemical composition on the glass-to-liquid transition is well-known. However, the influence of the particle size on the phase transition remains uncertain. Here we show experimental data that probe the amorphous phase transition of suspended sucrose particles as a function of particle size. The depression in glass-transition temperature follows the Gibbs-Thomson or Keesom-Laplace predicted proportionality of ΔTg ∝ D-1 for particles 100-700 nm in diameter, but the proportionality changes to ΔTg ∝ D-1/2 for smaller sizes. Literature data for glass-transition temperature depression in thin films and nanoconfined compounds show similar and strong deviations from the expected D-1 behavior. While the observed proportionalities remain incompletely understood, the results here provide evidence that the deviation from ΔTg ∝ D-1 is not attributable to substrate effects.

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.

Rapid hydrolysis of NO2 at High Ionic Strengths of Deliquesced Aerosol Particles

Nitrogen dioxide (NO2) hydrolysis in deliquesced aerosol particles forms nitrous acid and nitrate and thus impacts air quality, climate, and the nitrogen cycle. Traditionally, it is considered to proceed far too slowly in the atmosphere. However, the significance of this process is highly uncertain because kinetic studies have only been made in dilute aqueous solutions but not under high ionic strength conditions of the aerosol particles. Here, we use laboratory experiments, air quality models, and field measurements to examine the effect of the ionic strength on the reaction kinetics of NO2 hydrolysis. We find that high ionic strengths (I) enhance the reaction rate constants (kI) by more than an order of magnitude compared to that at infinite dilution (kI=0), yielding log10(kI/kI=0) = 0.04I or rate enhancement factor = 100.04I. A state-of-the-art air quality model shows that the enhanced NO2 hydrolysis reduces the negative bias in the simulated concentrations of nitrous acid by 28% on average when compared to field observations over the North China Plain. Rapid NO2 hydrolysis also enhances the levels of nitrous acid in other polluted regions such as North India and further promotes atmospheric oxidation capacity. This study highlights the need to evaluate various reaction kinetics of atmospheric aerosols with high ionic strengths.

Improving Aerosol Characterization Using an Optical Particle Counter Coupled with a Quartz Crystal Microbalance with an Integrated Microheater

Aerosols, as well as suspended particulate matter, impact atmospheric pollution, the climate, and human health, directly or indirectly. Particle size, chemical composition, and other aerosol characteristics are determinant factors for atmospheric pollution dynamics and more. In the last decade, low-cost devices have been widely used in instrumentation to measure aerosols. However, they present some issues, such as the problem of discriminating whether the aerosol is composed of liquid particles or solid. This issue could lead to errors in the estimation of mass concentration in monitoring environments where there is fog. In this study, we investigate the use of an optical particle counter (OPC) coupled to a quartz crystal microbalance with an integrated microheater (H-QCM) to enhance measurement performances. The H-QCM was used not only to measure the collected mass on its surface but also, by using the integrated microheater, it was able to heat the collected mass by performing heating cycles. In particular, we tested the developed system with aerosolized saline solutions of sodium chloride (NaCl), with three decreasing concentrations of salt and three electronic cigarette solutions (e-liquid), with different concentrations of propylene glycol and glycerin mixtures. The results showed that the OPC coherently counted the salt dilution effects, and the H-QCM output confirmed the presence of liquid and solid particles in the aerosols. In the case of e-liquid aerosols, the OPC counted the particles, and the HQCM output highlighted that in the aerosol, there were no solid particles but a liquid phase only. These findings contribute to the refinement of aerosol measurement methodologies by low-cost sensors, fostering a more comprehensive understanding.

Second inflection point of supercooled water surface tension induced by hydrogen bonds: A molecular-dynamics study

Surface tension of supercooled water is a fundamental property in various scientific processes. In this study, we perform molecular dynamics simulations with the TIP4P-2005 model to investigate the surface tension of supercooled water down to 220 K. Our results show a second inflection point (SIP) in the surface tension at temperature TSIP ≈ 267.5 ± 2.3 K. Using an extended IAPWS-E functional fit for the water surface tension, we calculate the surface excess internal-energy and entropy terms of the excess Helmholtz free energy. Similar to prior studies [Wang et al., Phys. Chem. Chem. Phys. 21, 3360 (2019); Gorfer et al., J. Chem. Phys. 158, 054503 (2023)], our results show that the surface tension is governed by two driving forces: a surface excess entropy change above the SIP and a surface excess internal-energy change below it. We study hydrogen-bonding near the SIP because it is the main cause of water's anomalous properties. With decreasing temperature, our results show that the entropy contribution to the surface tension reaches a maximum slightly below the SIP and then decreases. This is because the number of hydrogen bonds increases more slowly below the SIP. Moreover, the strengths and lifetimes of the hydrogen bonds also rise dramatically below the SIP, causing the internal-energy term to dominate the excess surface free energy. Thus, the SIP in the surface tension of supercooled TIP4P-2005 water is associated with an increase in the strengths and lifetimes of hydrogen bonds, along with a decrease in the formation rate (#/K) of new hydrogen bonds.

Hygroscopy as an Indicator of Specific Surface Area in Polymer Materials

Specific surface area (SSA) is an integral characteristic of the interfacial surface in poly-disperse systems, widely used for the assessment of technological properties in polymer materials and composites. Hygroscopic water content (Wh) is an obligate indicator of dispersed materials prior to any analysis of their chemical composition. This study links both indicators for the purpose of the express assessment of SSA using widely available Wh data, on the example of natural (starch, cellulose) and synthetic (acrylic hydrogels) polymer materials. The standard BET analysis of SSA using water vapor desorption was chosen as a reference method. In contrast to the known empirical correlations, this study is based on the fundamental thermodynamic theory of the disjoining water pressure for the connection of the analyzed quantities. The statistical processing of the results for the new methodology and the standard BET method showed their good compliance in a wide range of SSA from 200 to 900 m2/g. The most important methodological conclusion is the possibility of an accurate physically based calculation of hydrophilic SSA in polymer materials using their Wh data at a known relative humidity in the laboratory.

Convergence of dissolving and melting at the nanoscale

Phase transitions of water and its mixtures are of fundamental importance in physical chemistry, the pharmaceutical industry, materials sciences, and atmospheric sciences. However, current understanding remains elusive to explain relevant observations, especially at the nanoscale. Here, by using molecular dynamics simulations, we investigate the dissolution of sodium chloride (NaCl) nanocrystals with volume-equivalent diameters from 0.51 to 1.75 nm. Our results show that the dissolution of NaCl in aqueous nanodroplets show a strong size dependence, and its solubility can be predicted by the Ostwald-Freundlich equation and Gibbs-Duhem equation after considering a size-dependent solid-liquid surface tension. We find that the structure of dissolved ions in the saturated aqueous nanodropplet resembles the structure of a molten NaCl nanoparticle. With decreasing nanodroplet size, this similarity grows and the average potential energy of NaCl in solution, the molten phase and the crystal phase converges.

Assessment of Secondary Sulfate Aqueous-Phase Formation Pathways in the Tropical Island City of Haikou: A Chemical Kinetic Perspective

Sulfate (SO42-) is an essential chemical species in atmospheric aerosols and plays an influential role in their physical-chemical characteristics. The mechanisms of secondary SO42- aerosol have been intensively studied in air-polluted cities. However, few studies have focused on cities with good air quality. One-year PM2.5 samples were collected in the tropical island city of Haikou, and water-soluble inorganic ions, as well as water-soluble Fe and Mn, were analyzed. The results showed that non-sea-salt SO42- (nss-SO42-) was the dominant species of water-soluble inorganic ions, accounting for 40-57% of the total water-soluble inorganic ions in PM2.5 in Haikou. The S(IV)+H2O2 pathway was the main formation pathway for secondary SO42- in wintertime in Haikou, contributing to 57% of secondary SO42- formation. By contrast, 54% of secondary SO42- was produced by the S(IV)+Fe×Mn pathway in summer. In spring and autumn, the S(IV)+H2O2, S(IV)+Fe×Mn, and S(IV)+NO2 pathways contributed equally to secondary SO42- formation. The ionic strength was the controlling parameter for the S(IV)+NO2 pathway, while pH was identified as a key factor that mediates the S(IV)+H2O2 and S(IV)+Fe×Mn pathways to produce secondary SO42-. This study contributes to our understanding of secondary SO42- production under low PM2.5 concentrations but high SO42- percentages.

Crystal-Phase Engineering in Heterogeneous Catalysis

The performance of a chemical reaction is critically dependent on the electronic and/or geometric structures of a material in heterogeneous catalysis. Over the past century, the Sabatier principle has already provided a conceptual framework for optimal catalyst design by adjusting the electronic structure of the catalytic material via a change in composition. Beyond composition, it is essential to recognize that the geometric atomic structures of a catalyst, encompassing terraces, edges, steps, kinks, and corners, have a substantial impact on the activity and selectivity of a chemical reaction. Crystal-phase engineering has the capacity to bring about substantial alterations in the electronic and geometric configurations of a catalyst, enabling control over coordination numbers, morphological features, and the arrangement of surface atoms. Modulating the crystallographic phase is therefore an important strategy for improving the stability, activity, and selectivity of catalytic materials. Nonetheless, a complete understanding of how the performance depends on the crystal phase of a catalyst remains elusive, primarily due to the absence of a molecular-level view of active sites across various crystal phases. In this review, we primarily focus on assessing the dependence of catalytic performance on crystal phases to elucidate the challenges and complexities inherent in heterogeneous catalysis, ultimately aiming for improved catalyst design.

Assessment of submicron aerosol liquid water content and mass-based growth factors in South Asian outflow over the Indian Ocean

Aerosol-bound water, a ubiquitous and abundant component of atmospheric aerosols, has an impact on regional climate, visibility, human health, the hydrological cycle, and atmospheric chemistry. Yet, the intricate relationship between aerosol liquid water (ALWC) and chemical composition and relative humidity (RH) was not well understood. The present study explores ALWC derived from the ISORROPIA II model using real-time, high-resolution data of non-refractory submicron chemical species and meteorological parameters (temperature and RH) collected over the Indian Ocean as part of the ICARB (Integrated Campaign for Aerosols, Gases, and Radiation Budget)-2018 experiment. Results show that ALWC values over the South Eastern Arabian Sea (SEAS) were found to be higher by 4-6 times than those observed over the Equatorial Indian Ocean (EIO) due to a large decrease in aerosol loading from SEAS to EIO. ALWC peaked in the early morning hours (4:00-7:00), with greater values during the nighttime and lower values during the daytime across SEAS, which is comparable with RH variation. While the ratio of organics-to-SO42- mass fraction linearly decreased with increasing mass-based growth factors (MGFs) over EIO, such a scenario was not observed over SEAS. The latitudinal gradient of mass fraction of ALWC had shown a decrease towards EIO, consistent with organic fraction. The extinction coefficient of the dry mass of submicron particles is noticeably increased by 40 % by ALWC over SEAS and EIO. Moreover, ALWC could enhance the aerosol negative forcing by an average of 66 % (64 %) over SEAS (EIO) at the top of the atmosphere during the cruise period. These inferences imply that ALWC is the key factor in assessing the role of aerosols on atmospheric radiative forcing. Overall, the present study highlights the serious need to consider the ALWC in climate forcing simulations, particularly in moist tropical environments where their effect can be significant.

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.

Spontaneous Takeoff of Single Sulfur Nanoparticles during Sublimation Studied by Dark-Field Microscopy

The Leidenfrost effect describes a fascinating phenomenon in which a liquid droplet, when deposited onto a very hot substrate, will levitate on its own vapor layer and undergo frictionless movements. Driven by the significant implications for heat transfer engineering and drag reduction, intensive efforts have been made to understand, manipulate, and utilize the Leidenfrost effect on macrosized objects with a typical size of millimeters. The Leidenfrost effect of nanosized objects, however, remains unexplored. Herein, we report on an unprecedented Leidenfrost effect of single nanosized sulfur particles at room temperature. It was discovered when advanced dark-field optical microscopy was employed to monitor the dynamic sublimation process of single sulfur nanoparticles sitting on a flat substrate. Despite the phenomenological similarity, including the vapor-cushion-induced levitation and the extended lifetime, the Leidenfrost effect at the nanoscale exhibited two extraordinary features that were obviously distinct from its macroscopic counterpart. First, there was a critical size below which single sulfur nanoparticles began to levitate. Second, levitation occurred in the absence of the temperature difference between the nanoparticle and the substrate, which was barely possible for macroscopic objects and underscored the value of bridging the gap connecting the Leidenfrost effect and nanoscience. The sublimation-triggered spontaneous takeoff of single sulfur nanoparticles shed new light on its further applications, such as nanoflight.

Unexpected hygroscopic behaviors of individual sub-50 nm NaNO3 nanoparticles observed by in situ atomic force microscopy

Hygroscopicity is one of the most important physicochemical properties of salt nanoparticles, greatly influencing the environment, climate and human health. However, the hygroscopic properties of salt nanoparticles are poorly understood owing to the great challenges of the preparation, preservation and in situ characterization. Here we show the unexpected shape- and size-dependent hygroscopic behaviors of NaNO₃ nanoparticles prepared from molten salts using in situ environment-controlled atomic force microscopy. During the humidifying process, the angular and round sub-50 nm NaNO₃ particles display anisotropic and isotropic water adsorption behaviors, respectively. The sub-10 nm NaNO₃ nanoparticles abnormally shrink and disappear. The growth factors of the NaNO₃ nanoparticles are highly sensitive to their sizes and shapes, and quite different from those of NaNO₃ microparticles. These findings show that the hygroscopic behaviors of salt nanoparticles may not be comprehensively described by the traditional growth factors, and open up a new pathway to study the hygroscopic behaviors of salt nanoparticles.

Aerosol pH and Ion Activities of HSO4- and SO42- in Supersaturated Single Droplets

Accurate determination of acidity (pH) and ion activities in aqueous droplets is a major experimental and theoretical challenge for understanding and simulating atmospheric multiphase chemistry. Here, we develop a ratiometric Raman spectroscopy method to measure the equilibrium concentration of sulfate (SO₄^2-) and bisulfate (HSO₄^-) in single microdroplets levitated by aerosol optical tweezers. This approach enables determination of ion activities and pH in aqueous sodium bisulfate droplets under highly supersaturated conditions. The experimental results were compared against aerosol thermodynamic model calculations in terms of simulating aerosol ion concentrations, ion activity coefficients, and pH. We found that the Extended Aerosol Inorganics Model (E-AIM) can well reproduce the experimental results. The alternative model ISORROPIA, however, exhibits substantial deviations in SO₄^2- and HSO₄^- concentrations and up to a full unit of aerosol pH under acidic conditions, mainly due to discrepancies in simulating ion activity coefficients of SO₄^2--HSO₄^- equilibrium. Globally, this may cause an average deviation of ISORROPIA from E-AIM by 25 and 65% in predicting SO₄^2- and HSO₄^- concentrations, respectively. Our results show that it is important to determine aerosol pH and ion activities in the investigation of sulfate formation and related aqueous phase chemistry.

A database for deliquescence and efflorescence relative humidities of compounds with atmospheric relevance

Deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH), the two parameters that regulate phase state and hygroscopicity of substances, play important roles in atmospheric science and many other fields. A large number of experimental studies have measured the DRH and ERH values of compounds with atmospheric relevance, but these values have not yet been summarized in a comprehensive manner. In this work, we develop for the first-of-its-kind a comprehensive database which compiles the DRH and ERH values of 110 compounds (68 inorganics and 42 organics) measured in previous studies, provide the preferred DRH and ERH values at 298 K for these compounds, and discuss the effects of a few key factors (e.g., temperature and particle size) on the measured DRH and ERH values. In addition, we outline future work that will broaden the scope of this database and enhance its accessibility.

A core-shell box model for simulating viscosity dependent secondary organic aerosol (CSVA) and its application

Secondary organic aerosol (SOA) plays a key role in air pollution and global climate change. However, the understanding and modelling of SOA properties and evolution are still limited. In this paper, we developed a novel kinetic Core-Shell box model for Viscosity dependent SOA simulation (CSVA), which includes explicit gas-phase reactions (MCM), homogeneous nucleation by H₂SO₄-NH₃-H₂O, viscosity dependent mass transfer between gas and particle phases (organic and aqueous phases) and particle-phase reactions. The gas-particle mass transfer is represented by chainlike reactions analogizing to electrical resistance. The CSVA model is verified and applied to chamber experiments of toluene oxidation systems. The monomers and dimers of SOA are determined by coupling the high-resolution Orbitrap mass spectra and MCM mechanism. The majority of dimers are confirmed to be peroxyhemiacetals formed by reactions of hydroperoxides with aldehydes in the particle phase. The results show that CSVA can well capture the following processes: (1) relative humidity (RH) dependent nucleation of the H₂SO₄-NH₃-H₂O system, (2) particle size-dependent hygroscopic growth of inorganics (e.g., NaCl and (NH₄)₂SO₄) and organics (levoglucosan and SOA), (3) NO_x dependent SOA formation, (4) viscosity-induced evolution of particle size distribution, and (5) effect of RH on SOA formation. In particular, our model reproduces the phenomenon that the evolution of SOA particle size distribution from a one-peak mode into a two-peak mode is due to viscosity.

Phase Behavior of Hydrocarbon-like Primary Organic Aerosol and Secondary Organic Aerosol Proxies Based on Their Elemental Oxygen-to-Carbon Ratio

A large fraction of atmospheric aerosols can be characterized as primary organic aerosol (POA) and secondary organic aerosol (SOA). Knowledge of the phase behavior, that is, the number and type of phases within internal POA + SOA mixtures, is crucial to predict their effect on climate and air quality. For example, if POA and SOA form a single phase, POA will enhance the formation of SOA by providing organic mass to absorb SOA precursors. Using microscopy, we studied the phase behavior of mixtures of SOA proxies and hydrocarbon-like POA proxies at relative humidity (RH) values of 90%, 45%, and below 5%. Internal mixtures of POA and SOA almost always formed two phases if the elemental oxygen-to-carbon ratio (O/C) of the POA was less than 0.11, which encompasses a large fraction of atmospheric hydrocarbon-like POA from fossil fuel combustion. SOA proxies mixed with POA proxies having 0.11 ≤ O/C ≤ 0.29 mostly resulted in particles with one liquid phase. However, two liquid phases were also observed, depending on the type of SOA and POA surrogates, and an increase in phase-separated particles was observed when increasing the RH in this O/C range. The results have implications for predicting atmospheric SOA formation and policy strategies to reduce SOA in urban environments.

High Pressure Inside Nanometer-Sized Particles Influences the Rate and Products of Chemical Reactions

The composition of organic aerosol has a pivotal influence on aerosol properties such as toxicity and cloud droplet formation capability, which could affect both climate and air quality. However, a comprehensive and fundamental understanding of the chemical and physical processes that occur in nanometer-sized atmospheric particles remains a challenge that severely limits the quantification and predictive capabilities of aerosol formation pathways. Here, we investigated the effects of a fundamental and hitherto unconsidered physical property of nanoparticles-the Laplace pressure. By studying the reaction of glyoxal with ammonium sulfate, both ubiquitous and important atmospheric constituents, we show that high pressure can significantly affect the chemical processes that occur in atmospheric ultrafine particles (i.e., particles < 100 nm). Using high-resolution mass spectrometry and UV-vis spectroscopy, we demonstrated that the formation of reaction products is strongly (i.e., up to a factor of 2) slowed down under high pressures typical of atmospheric nanoparticles. A size-dependent relative rate constant is determined and numerical simulations illustrate the reduction in the production of the main glyoxal reaction products. These results established that the high pressure inside nanometer-sized aerosols must be considered as a key property that significantly impacts chemical processes that govern atmospheric aerosol growth and evolution.

Representation of Multiphase OH Oxidation of Amorphous Organic Aerosol for Tropospheric Conditions

Organic aerosol (OA) is ubiquitous in the atmosphere and, during transport, can experience chemical transformation with consequences for air quality and climate. Prediction of the chemical evolution of OA depends on its reactivity with atmospheric oxidants such as the OH radical. OA particles undergo amorphous phase transitions from liquid to solid (glassy) states in response to temperature changes, which, in turn, will impact its reactivity toward OH oxidation. To improve the predictability of OA reactivity toward OH oxidation, the reactive uptake coefficients (γ) of OH radicals reacting with triacontane and squalane serving as amorphous OA surrogates were measured at temperatures from 213-293 K. γ increases strongest with temperature when the organic species is in the liquid phase, compared to when being in the semisolid or solid phase. The resistor model is applied, accounting for the amorphous phase state changes using the organic species' glass transition temperature and fragility, to evaluate the physicochemical parameters of the temperature dependent OH uptake process. This allows for the derivation of a semiempirical formula, applicable to models, to predict the degree of oxidation and chemical lifetime of the condensed-phase organic species for typical tropospheric temperature and humidity when OA particle viscosity is known.

Phase Diagram of Binary Alloy Nanoparticles under High Pressure

CALPHAD (CALculation of PHAse Diagram) is a useful tool to construct phase diagrams of various materials under different thermodynamic conditions. Researchers have extended the use of the CALPHAD method to nanophase diagrams and pressure phase diagrams. In this study, the phase diagram of an arbitrary A–B nanoparticle system under pressure was investigated. The effects of the interaction parameter and excess volume were investigated with increasing pressure. The eutectic temperature was found to decrease in most cases, except when the interaction parameter in the liquid was zero and that in the solid was positive, while the excess volume parameter of the liquid was positive. Under these conditions, the eutectic temperature increased with increasing pressure.

The impact of atmospheric oxidation on hygroscopicity and cloud droplet activation of inorganic sea spray aerosol

Sea spray aerosol (SSA) contributes significantly to natural aerosol particle concentrations globally, in marine areas even dominantly. The potential changes of the omnipresent inorganic fraction of SSA due to atmospheric ageing is largely unexplored. In the atmosphere, SSA may exist as aqueous phase solution droplets or as dried solid or amorphous particles. We demonstrate that ageing of liquid NaCl and artificial sea salt aerosol by exposure to ozone and UV light leads to a substantial decrease in hygroscopicity and cloud activation potential of the dried particles of the same size. The results point towards surface reactions on the liquid aerosols that are more crucial for small particles and the formation of salt structures with water bound within the dried aerosols, termed hydrates. Our findings suggest an increased formation of hydrate forming salts during ageing and the presence of hydrates in dried SSA. Field observations indicate a reduced hygroscopic growth factor of sub-micrometre SSA in the marine atmosphere compared to fresh laboratory generated NaCl or sea salt of the same dry size, which is typically attributed to organic matter or sulphates. Aged inorganic sea salt offers an additional explanation for such a measured reduced hygroscopic growth factor and cloud activation potential.

Microscopic Evidence for Phase Separation of Organic Species and Inorganic Salts in Fine Ambient Aerosol Particles

Phase separation is an important microscopic phenomenon in aerosol particles and reflects the surface properties of particles and the aging degree of organic components. However, few data are available to directly reveal phase separation in ambient aerosol particles, although there are abundant data from laboratory experiments. In this study, different state-of-the-art microscopic technologies were used to study the phase separation of organic matter (OM) and inorganic salts in individual particles collected from different atmospheric environments, with one type of surrogate particles prepared in the laboratory. We found that most of the collected particles with an equivalent sphere diameter of >100 nm have a secondary inorganic aerosol core with OM coating in the continental atmosphere. In addition, secondary inorganic aerosol and OM phase separation are more frequent in rural particles than suburban particles, suggesting that particle aging enhances the phase separation. Our results show that the phase separation is a frequent phenomenon that forms organic coatings on inorganic particles of individual particles (>100 nm), and their number abundances depend on the particle size and OM aging degree. The resulting morphology shows that OM is an important particle surface in the atmosphere, which influences gas partitioning, optical and hygroscopic properties, and cloud condensation nuclei formation activities.

Particle Size Distribution Dynamics Can Help Constrain the Phase State of Secondary Organic Aerosol

Particle phase state is a property of atmospheric aerosols that has important implications for the formation, evolution, and gas/particle partitioning of secondary organic aerosol (SOA). In this work, we use a size-resolved chemistry and microphysics model (Statistical Oxidation Model coupled to the TwO Moment Aerosol Sectional (SOM-TOMAS)), updated to include an explicit treatment of particle phase state, to constrain the bulk diffusion coefficient (D_b) of SOA produced from α-pinene ozonolysis. By leveraging data from laboratory experiments performed in the absence of a seed and under dry conditions, we find that the D_b for SOA can be constrained ((1-7) × 10^-15 cm² s^-1 in these experiments) by simultaneously reproducing the time-varying SOA mass concentrations and the evolution of the particle size distribution. Another version of our model that used the predicted SOA composition to calculate the glass-transition temperature, viscosity, and, ultimately, D_b (∼10^-15 cm² s^-1) of the SOA was able to reproduce the mass and size distribution measurements when we included oligomer formation (oligomers accounted for about a fifth of the SOA mass). Our work highlights the potential of a size-resolved SOA model to constrain the particle phase state of SOA using historical measurements of the evolution of the particle size distribution.

New Multiphase Chemical Processes Influencing Atmospheric Aerosols, Air Quality, and Climate in the Anthropocene

Atmospheric aerosols and fine particulate matter (PM_2.5) are strongly affecting human health and climate in the Anthropocene, that is, in the current era of globally pervasive and rapidly increasing human influence on planet Earth. Poor air quality associated with high aerosol concentrations is among the leading health risks worldwide, causing millions of attributable excess deaths and years of life lost every year. Besides their health impact, aerosols are also influencing climate through interactions with clouds and solar radiation with an estimated negative total effective radiative forcing that may compensate about half of the positive radiative forcing of carbon dioxide but exhibits a much larger uncertainty. Heterogeneous and multiphase chemical reactions on the surface and in the bulk of solid, semisolid, and liquid aerosol particles have been recognized to influence aerosol formation and transformation and thus their environmental effects. However, atmospheric multiphase chemistry is not well understood because of its intrinsic complexity of dealing with the matter in multiple phases and the difficulties of distinguishing its effect from that of gas phase reactions.

Recently, research on atmospheric multiphase chemistry received a boost from the growing interest in understanding severe haze formation of very high PM_2.5 concentrations in polluted megacities and densely populated regions. State-of-the-art models suggest that the gas phase reactions, however, are not capturing the high concentrations and rapid increase of PM_2.5 observed during haze events, suggesting a gap in our understanding of the chemical mechanisms of aerosol formation. These haze events are characterized by high concentrations of aerosol particles and high humidity, especially favoring multiphase chemistry. In this Account, we review recent advances that we have made, as well as current challenges and future perspectives for research on multiphase chemical processes involved in atmospheric aerosol formation and transformation. We focus on the following questions: what are the key reaction pathways leading to aerosol formation under polluted conditions, what is the relative importance of multiphase chemistry versus gas-phase chemistry, and what are the implications for the development of efficient and reliable air quality control strategies? In particular, we discuss advances and challenges related to different chemical regimes of sulfate, nitrate, and secondary organic aerosols (SOAs) under haze conditions, and we synthesize new insights into the influence of aerosol water content, aerosol pH, phase state, and nanoparticle size effects. Overall, there is increasing evidence that multiphase chemistry plays an important role in aerosol formation during haze events. In contrast to the gas phase photochemical reactions, which are self-buffered against heavy pollution, multiphase reactions have a positive feedback mechanism, where higher particle matter levels accelerate multiphase production, which further increases the aerosol concentration resulting in a series of record-breaking pollution events. We discuss perspectives to fill the gap of the current understanding of atmospheric multiphase reactions that involve multiple physical and chemical processes from bulk to nanoscale and from regional to global scales. A synthetic approach combining laboratory experiments, field measurements, instrument development, and model simulations is suggested as a roadmap to advance future research.

Predicting the influence of particle size on the glass transition temperature and viscosity of secondary organic material

Atmospheric aerosols can assume liquid, amorphous semi-solid or glassy, and crystalline phase states. Particle phase state plays a critical role in understanding and predicting aerosol impacts on human health, visibility, cloud formation, and climate. Melting point depression increases with decreasing particle diameter and is predicted by the Gibbs-Thompson relationship. This work reviews existing data on the melting point depression to constrain a simple parameterization of the process. The parameter [Formula: see text] describes the degree to which particle size lowers the melting point and is found to vary between 300 and 1800 K nm for a wide range of particle compositions. The parameterization is used together with existing frameworks for modeling the temperature and RH dependence of viscosity to predict the influence of particle size on the glass transition temperature and viscosity of secondary organic aerosol formed from the oxidation of [Formula: see text]-pinene. Literature data are broadly consistent with the predictions. The model predicts a sharp decrease in viscosity for particles less than 100 nm in diameter. It is computationally efficient and suitable for inclusion in models to evaluate the potential influence of the phase change on atmospheric processes. New experimental data of the size-dependence of particle viscosity for atmospheric aerosol mimics are needed to thoroughly validate the predictions.

Hygroscopicity of amino acids and their effect on the water uptake of ammonium sulfate in the mixed aerosol particles

Amino acids are important water-soluble nitrogen-containing compounds in atmospheric aerosols. They can be involved in cloud formation due to their hygroscopicity and have significant influences on the hygroscopicity of inorganic compounds, which have not yet been well characterized. In this work, the hygroscopic properties of three amino acids, including aspartic acid, glutamine, and serine, as well as their mixtures with ammonium sulfate (AS) were investigated using a hygroscopicity tandem differential mobility analyzer (HTDMA) system. The gradual water uptake of aspartic acid, glutamine and serine particles indicates that they exist as liquid phase at low RH. When mixing either aspartic acid or glutamine with AS by mass ratio of 1:3, we observed a clear phase transition but with a lower deliquescence relative humidity (DRH) with respect to that of pure AS. This suggests the crystallization of AS in the presence of each of these two amino acids. However, as the mass fractions of these two amino acids increased in the mixed particles, the deliquescence transition process was not obvious. In contrast, the crystallization of AS was efficiently hampered even at low content (i.e., 25% by mass) of serine in the mixed particles. The Zdanovskii-Stokes-Robinson (ZSR) method in general underestimated the hygroscopic growth of any mixtures at RH below 79% (prior to AS deliquescence), suggesting both amino acid and the partially dissolved AS contributed the overall hygroscopicity at RH in this range. Relatively good agreements were reached between the measurements and model predictions using the Extended Aerosol Inorganic Model (E-AIM) assuming solid state AS in the mixed particles for 1:3 aspartic acid-AS and glutamine-AS systems. However, the model failed to simulate the water uptake behaviors of any other systems. It demonstrates that the interactions between components within the aerosols have a significant effect on the phase state of the mixed particles.

Trimetallic Nanoparticles: Greener Synthesis and Their Applications

Nanoparticles (NPs) and multifunctional nano-sized materials have significant applications in diverse fields, namely catalysis, sensors, optics, solar energy conversion, cancer therapy/diagnosis, and bioimaging. Trimetallic NPs have found unique catalytic, active food packaging, biomedical, antimicrobial, and sensing applications; they preserve an ever-superior level of catalytic activities and selectivity compared to monometallic and bimetallic nanomaterials. Due to these important applications, a variety of preparation routes, including hydrothermal, microemulsion, selective catalytic reduction, co-precipitation, and microwave-assisted methodologies have been reported for the syntheses of these nanomaterials. As the fabrication of nanomaterials using physicochemical methods often have hazardous and toxic impacts on the environment, there is a vital need to design innovative and well-organized eco-friendly, sustainable, and greener synthetic protocols for their assembly, by applying safer, renewable, and inexpensive materials. In this review, noteworthy recent advancements relating to the applications of trimetallic NPs and nanocomposites comprising these NPs are underscored as well as their eco-friendly and sustainable synthetic preparative options.

Reconciling atmospheric water uptake by hydrate forming salts

Magnesium and calcium chloride salts contribute to the global atmospheric aerosol burden via emission of sea spray and mineral dust. Their influence on aerosol hygroscopicity and cloud forming potential is important but uncertain with ambiguities between results reported in the literature. To address this, we have conducted measurements of the hygroscopic growth and critical supersaturation of dried, size selected nano-particles made from aqueous solution droplets of MgCl₂ and CaCl₂, respectively, and compare experimentally derived values with results from state-of-the-art thermodynamic modelling. It is characteristic of both MgCl₂ and CaCl₂ salts that they bind water in the form of hydrates under a range of ambient conditions. We discuss how hydrate formation affects the particles' water uptake and provide an expression for hydrate correction factors needed in calculations of hygroscopic growth factors, critical super-saturations, and derived κ values of particles containing hydrate forming salts. We demonstrate the importance of accounting for hydrate forming salts when predicting hygroscopic properties of sea spray aerosol.

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.

Hygroscopic properties of NaCl nanoparticles on the surface: a scanning force microscopy study

We investigated the hygroscopic growth of sodium chloride (NaCl) nanoparticles with curvature related diameters ranging from 10 nm to 200 nm, at different relative humidities using scanning force microscopy.

Interfaces of Gas-Aerosol Particles: Relative Humidity and Salt Concentration Effects

The growth of aerosol particles is intimately related to chemical reactions in the gas phase and particle phase and at gas-aerosol particle interfaces. While chemical reactions in gas and particle phases are well documented, there is very little information regarding interface-related reactions. The interface of gas-aerosol particles not only facilitates a physical channel for organic species to enter and exit but also provides a necessary lane for culturing chemical reactions. The physical and chemical properties of gas-particle interfaces have not been studied extensively, nor have the reactions occurring at the interfaces been well researched. This is mainly due to the fact that there is a lack of suitable in situ interface-sensitive analytical techniques for direct measurements of interfacial properties. The motivation behind this research is to understand how interfaces play a role in the growth of aerosol particles. We have developed in situ interface-specific second harmonic scattering to examine interfacial behaviors of molecules of aerosol particles under different relative humidity (RH) and salt concentrations. Both the relative humidity and salt concentration can change the particle size and the phase of the aerosol. RH not only varies the concentration of solutes inside aerosol particles but also changes interfacial hydration in local regions. Organic molecules were found to exhibit distinct behaviors at the interfaces and bulk on NaCl particles under different RH levels. Our quantitative analyses showed that the interfacial adsorption free energies remain unchanged while interfacial areas increase as the relative humidity increases. Furthermore, the surface tension of NaCl particles decreases as the RH increases. Our experimental findings from the novel nonlinear optical scattering technique stress the importance of interfacial water behaviors on aerosol particles in the atmosphere.

Novel ionization processes for use in mass spectrometry: 'Squeezing' nonvolatile analyte ions from crystals and droplets

Together with my group and collaborators, I have been fortunate to have had a key role in the discovery of new ionization processes that we developed into new flexible, sensitive, rapid, reliable, and robust ionization technologies and methods for use in mass spectrometry (MS). Our current research is focused on how best to understand, improve, and use these novel ionization processes which convert volatile and nonvolatile compounds from solids or liquids into gas-phase ions for analysis by MS using e.g. mass-selected fragmentation and ion mobility spectrometry to provide reproducible, accurate, and improved mass and drift time resolution. In my view, the apex was the discovery of vacuum matrix-assisted ionization (vMAI) in 2012 on an intermediate pressure matrix-assisted laser desorption/ionization (MALDI) source without the use of a laser, high voltages, or any other added energy. Only exposure of the matrix:analyte to the sub-atmospheric pressure of the mass spectrometer was necessary to initiate ionization. These findings were initially rejected by three different scientific journals, with comments related to 'how can this work?', 'where do the charges come from?', and 'it is not analytically useful'. Meanwhile, we and others have demonstrated analytical utility without a complete understanding of the mechanism. In reality, MALDI and electrospray ionization are widely used in science and their mechanisms are still controversially discussed despite use and optimization of now 30 years. This Perspective covers the applications and mechanistic aspects of the novel ionization processes for use in MS that guided us in instrument developments, and provides our perspective on how they relate to traditional ionization processes.

Multiphase reactivity of polycyclic aromatic hydrocarbons is driven by phase separation and diffusion limitations

Polycyclic aromatic hydrocarbons (PAHs) are among the most prominent toxic compounds in the air. Heterogeneous reactions involving O₃ can change the toxicity of PAHs, but the reaction mechanism and kinetics remain to be elucidated. Based on new experiments combined with state-of-the-art kinetic and thermodynamic models, we show that phase separation plays a critical role in the ozonolysis of PAHs mixed with secondary organic aerosols and organic oils. Ozonolysis products of PAHs phase separate to form viscous surface crusts, which protect underlying PAHs from ozonolysis to prolong their chemical lifetime. These results have significant implications for outdoor and indoor air quality by affecting PAH long-range transport and fate in indoor environments.

Benzo[a]pyrene (BaP), a key polycyclic aromatic hydrocarbon (PAH) often associated with soot particles coated by organic compounds, is a known carcinogen and mutagen. When mixed with organics, the kinetics and mechanisms of chemical transformations of BaP by ozone in indoor and outdoor environments are still not fully elucidated. Using direct analysis in real-time mass spectrometry (DART-MS), kinetics studies of the ozonolysis of BaP in thin films exhibited fast initial loss of BaP followed by a slower decay at long exposure times. Kinetic multilayer modeling demonstrates that the slow decay of BaP over long times can be simulated if there is slow diffusion of BaP from the film interior to the surface, resolving long-standing unresolved observations of incomplete PAH decay upon prolonged ozone exposure. Phase separation drives the slow diffusion time scales in multicomponent systems. Specifically, thermodynamic modeling predicts that BaP phase separates from secondary organic aerosol material so that the BaP-rich layer at the surface shields the inner BaP from ozone. Also, BaP is miscible with organic oils such as squalane, linoleic acid, and cooking oil, but its oxidation products are virtually immiscible, resulting in the formation of a viscous surface crust that hinders diffusion of BaP from the film interior to the surface. These findings imply that phase separation and slow diffusion significantly prolong the chemical lifetime of PAHs, affecting long-range transport of PAHs in the atmosphere and their fates in indoor environments.

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.

Second inflection point of water surface tension in the deeply supercooled regime revealed by entropy anomaly and surface structure using molecular dynamics simulations

The surface tension of supercooled water is of fundamental importance in physical chemistry and materials and atmospheric sciences. Controversy, however, exists over its temperature dependence in the supercooled regime, especially on the existence of the "second inflection point (SIP)". Here, we use molecular dynamics simulations of the SPC/E water model to study the surface tension of water (σw) as a function of temperature down to 198.15 K, and find a minimum point of surface excess entropy per unit area around ∼240-250 K. Additional simulations with the TIP4P/2005 water model also show consistent results. Hence, we predict an SIP of σw roughly in this region, at the boundary where the "no man's land" happens. The increase of surface entropy with decreasing temperature in the region below the inflection point is clearly an anomalous behavior, unknown for simple liquids. Furthermore, we find that σw has a near-linear correlation with the interfacial width, which can be well explained by the capillary wave theory. Deep in the supercooled regime, a compact water layer at the interface is detected in our simulations, which may be a key component that contributes to the deviation of surface tension from the International Association for the Properties of Water and Steam relationship. Our findings may advance the understanding of the origin of the anomalous properties of liquid water in the supercooled regime.

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.

Strengths and Weaknesses of Molecular Simulations of Electrosprayed Droplets

The origin and the magnitude of the charge in a macroion are critical questions in mass spectrometry analysis coupled to electrospray and other ionization techniques that transfer analytes from the bulk solution into the gaseous phase via droplets. In many circumstances, it is the later stages of the existence of a macroion in the containing solvent drop before the detection that determines the final charge state. Experimental characterization of small (with linear dimensions of several nanometers) and short-lived droplets is quite challenging. Molecular simulations in principle may provide insight exactly in this challenging for experiments regime. We discuss the strengths and weaknesses of the molecular modeling of electrosprayed droplets using molecular dynamics. We illustrate the limitations of the molecular modeling in the analysis of large macroions and specifically proteins away from their native states. Graphical Abstract ᅟ.

Nanomaterials in Neural-Stem-Cell-Mediated Regenerative Medicine: Imaging and Treatment of Neurological Diseases

Patients are increasingly being diagnosed with neuropathic diseases, but are rarely cured because of the loss of neurons in damaged tissues. This situation creates an urgent clinical need to develop alternative treatment strategies for effective repair and regeneration of injured or diseased tissues. Neural stem cells (NSCs), highly pluripotent cells with the ability of self-renewal and potential for multidirectional differentiation, provide a promising solution to meet this demand. However, some serious challenges remaining to be addressed are the regulation of implanted NSCs, tracking their fate, monitoring their interaction with and responsiveness to the tissue environment, and evaluating their treatment efficacy. Nanomaterials have been envisioned as innovative components to further empower the field of NSC-based regenerative medicine, because their unique physicochemical characteristics provide unparalleled solutions to the imaging and treatment of diseases. By building on the advantages of nanomaterials, tremendous efforts have been devoted to facilitate research into the clinical translation of NSC-based therapy. Here, recent work on emerging nanomaterials is highlighted and their performance in the imaging and treatment of neurological diseases is evaluated, comparing the strengths and weaknesses of various imaging modalities currently used. The underlying mechanisms of therapeutic efficacy are discussed, and future research directions are suggested.

Surface Composition of Free Mixed NaCl/Na2SO4 Nanoscale Aerosols Probed by X-ray Photoelectron Spectroscopy

The local chemical surface composition of unsupported mixed solid NaCl/Na₂SO₄ aerosols ( d ∼ 70 nm) is studied by X-ray photoelectron spectroscopy. The solid aerosols are generated by drying aqueous droplets containing mixtures of the two salts in different mole fractions. The mole fraction of these salts is found to deviate at the solid aerosol surface significantly from the initial droplet composition. The minority species in the droplets are found to be enhanced at the surface of the solid mixed aerosols. This surface enhancement is rationalized in terms of the nucleation/crystallization process, where the salts evidently do not cocrystallize, rather than each salt forms pure crystal moieties. Characteristic variations of the surface ion concentration as a function of the mole fraction of the salts in the initial droplet are observed in the nanometer size regime. This is unlike core-shell architectures previously found in mixed micron salt aerosols, indicating that aerosol models derived from micron-sized aerosols are evidently not fully reliable to describe the surface composition of nanosized aerosols. Furthermore, surface enhancement of the minority component in mixed NaCl/Na₂SO₄ aerosols is also different from previous results on surface segregation of mixed NaCl/NaBr aerosols, where one of the anionic species is surface segregated for all mole fractions, which was explained in terms of the ability of the involved salts to cocrystallize and forming solid solutions. The present results rather indicate that mixed NaCl/Na₂SO₄ aerosols do not cocrystallize. Electron microscopy of deposited mixed salt aerosols reveals mostly a cubic structure of pure NaCl aerosols, whereas mixed salt aerosols are found to show a grainy structure composed of multiple small crystals which supports the present findings obtained from photoelectron spectroscopy.

The viscosity of atmospherically relevant organic particles

The importance of organic aerosol particles in the environment has been long established, influencing cloud formation and lifetime, absorbing and scattering sunlight, affecting atmospheric composition and impacting on human health. Conventionally, ambient organic particles were considered to exist as liquids. Recent observations in field measurements and studies in the laboratory suggest that they may instead exist as highly viscous semi-solids or amorphous glassy solids under certain conditions, with important implications for atmospheric chemistry, climate and air quality. This review explores our understanding of aerosol particle phase, particularly as identified by measurements of the viscosity of organic particles, and the atmospheric implications of phase state.

Temperature effect on phase state and reactivity controls atmospheric multiphase chemistry and transport of PAHs

Polycyclic aromatic hydrocarbons like benzo(a)pyrene (BaP) in atmospheric particulate matter pose a threat to human health because of their high carcinogenicity. In the atmosphere, BaP is mainly degraded through a multiphase reaction with ozone, but the fate and atmospheric transport of BaP are poorly characterized. Earlier modeling studies used reaction rate coefficients determined in laboratory experiments at room temperature, which may overestimate/underestimate degradation rates when applied under atmospheric conditions. Moreover, the effects of diffusion on the particle bulk are not well constrained, leading to large discrepancies between model results and observations. We show how regional and global distributions and transport of BaP can be explained by a new kinetic scheme that provides a realistic description of the temperature and humidity dependence of phase state, diffusivity, and reactivity of BaP-containing particles. Low temperature and humidity can substantially increase the lifetime of BaP and enhance its atmospheric dispersion through both the planetary boundary layer and the free troposphere. The new scheme greatly improves the performance of multiscale models, leading to better agreement with observed BaP concentrations in both source regions and remote regions (Arctic), which cannot be achieved by less-elaborate degradation schemes (deviations by multiple orders of magnitude). Our results highlight the importance of considering temperature and humidity effects on both the phase state of aerosol particles and the chemical reactivity of particulate air pollutants.

Phase transition dynamics of single optically trapped aqueous potassium carbonate particles

This study reveals that complex multiple processes occur during efflorescence and deliquescence in unsupported, submicron sized particles.

Effect of Drying Rate on Aerosol Particle Morphology

The morphology of aerosol particles impacts their role in the climate system. In the submicron size regime, the morphology of particles that undergo liquid-liquid phase separation is dependent on their size, where for some systems small particles are homogeneous and large particles are phase-separated. We use cryogenic transmission electron microscopy to probe the morphology of model organic aerosol systems. We observe that the transition region (where both homogeneous and phase-separated morphologies are seen) spans 121 nm at the fastest drying rates with a midpoint diameter > 170 nm. By slowing the drying rate over several orders of magnitude, the transition region shifts to smaller diameters (midpoint < 40 nm) and the width narrows to 4 nm. Our results suggest that the size-dependent morphology originates from an underlying finite size effect, rather than solely kinetics, due to the presence of a size dependence even at the slowest drying rates.

Revising the hygroscopicity of inorganic sea salt particles

Sea spray is one of the largest natural aerosol sources and plays an important role in the Earth’s radiative budget. These particles are inherently hygroscopic, that is, they take-up moisture from the air, which affects the extent to which they interact with solar radiation. We demonstrate that the hygroscopic growth of inorganic sea salt is 8–15% lower than pure sodium chloride, most likely due to the presence of hydrates. We observe an increase in hygroscopic growth with decreasing particle size (for particle diameters <150 nm) that is independent of the particle generation method. We vary the hygroscopic growth of the inorganic sea salt within a general circulation model and show that a reduced hygroscopicity leads to a reduction in aerosol-radiation interactions, manifested by a latitudinal-dependent reduction of the aerosol optical depth by up to 15%, while cloud-related parameters are unaffected. We propose that a value of κ_s=1.1 (at RH=90%) is used to represent the hygroscopicity of inorganic sea salt particles in numerical models.

Sea spray, one of the largest natural aerosol sources, plays an important role in the Earth’s radiative budget. Here the authors show that the ability of sea salt particles to take up water is smaller than for pure salt, with implications for the parameterization of the direct aerosol radiative effect.

Microphysical explanation of the RH-dependent water affinity of biogenic organic aerosol and its importance for climate

A large fraction of atmospheric organic aerosol (OA) originates from natural emissions that are oxidized in the atmosphere to form secondary organic aerosol (SOA). Isoprene (IP) and monoterpenes (MT) are the most important precursors of SOA originating from forests. The climate impacts from OA are currently estimated through parameterizations of water uptake that drastically simplify the complexity of OA. We combine laboratory experiments, thermodynamic modeling, field observations, and climate modeling to (1) explain the molecular mechanisms behind RH-dependent SOA water-uptake with solubility and phase separation; (2) show that laboratory data on IP- and MT-SOA hygroscopicity are representative of ambient data with corresponding OA source profiles; and (3) demonstrate the sensitivity of the modeled aerosol climate effect to assumed OA water affinity. We conclude that the commonly used single-parameter hygroscopicity framework can introduce significant error when quantifying the climate effects of organic aerosol. The results highlight the need for better constraints on the overall global OA mass loadings and its molecular composition, including currently underexplored anthropogenic and marine OA sources.

Global distribution of particle phase state in atmospheric secondary organic aerosols

Secondary organic aerosols (SOA) are a large source of uncertainty in our current understanding of climate change and air pollution. The phase state of SOA is important for quantifying their effects on climate and air quality, but its global distribution is poorly characterized. We developed a method to estimate glass transition temperatures based on the molar mass and molecular O:C ratio of SOA components, and we used the global chemistry climate model EMAC with the organic aerosol module ORACLE to predict the phase state of atmospheric SOA. For the planetary boundary layer, global simulations indicate that SOA are mostly liquid in tropical and polar air with high relative humidity, semi-solid in the mid-latitudes and solid over dry lands. We find that in the middle and upper troposphere SOA should be mostly in a glassy solid phase state. Thus, slow diffusion of water, oxidants and organic molecules could kinetically limit gas–particle interactions of SOA in the free and upper troposphere, promote ice nucleation and facilitate long-range transport of reactive and toxic organic pollutants embedded in SOA.

Secondary organic aerosols (SOA) are important for climate and aerosol quality, but the phase state is unclear. Here, the authors show that SOA is liquid in tropical and polar air, semi-solid in the mid-latitudes, solid over dry lands and in a glassy solid phase state in the middle and upper troposphere.

Characterization of the temperature and humidity-dependent phase diagram of amorphous nanoscale organic aerosols

Atmospheric aerosols can exist in amorphous semi-solid or glassy phase states. These states are determined by the temperature (T) and relative humidity (RH). New measurements of viscosity for amorphous semi-solid nanometer size sucrose particles as a function of T and RH are reported. Viscosity is measured by inducing coagulation between two particles and probing the thermodynamic states that induce the particle to relax into a sphere. It is shown that the glass transition temperature can be obtained by extrapolation to 10¹² Pa s from the measured temperature-dependent viscosity in the 10⁶ to 10⁷ Pa s range. The experimental methodology was refined to allow isothermal probing of RH dependence and to increase the range of temperatures over which the dry temperature dependence can be studied. Several experiments where one monomer was sodium dodecyl sulfate (SDS), which remains solid at high RH, are also reported. These sucrose-SDS dimers were observed to relax into a sphere at T and RH similar to those observed in sucrose-sucrose dimers, suggesting that amorphous sucrose will flow over an insoluble particle at a viscosity similar to that characteristic of coalescence between two sucrose particles. Possible physical and analytical implications of this observation are considered. The data reported here suggest that semi-solid viscosity between 10⁴ and 10¹² Pa s can be modelled over a wide range of T and RH using an adapted Vogel-Fulcher-Tammann equation and the Gordon-Taylor mixing rule. Sensitivity of modelled viscosity to variations in dry glass transition temperature, Gordon-Taylor constant, and aerosol hygroscopicity are explored, along with implications for atmospheric processes such as ice nucleation of glassy organic aerosols in the upper free troposphere. The reported measurement and modelling framework provides a template for characterizing the phase diagram of other amorphous aerosol systems, including secondary organic aerosols.

Using electric current to surpass the microstructure breakup limit

The elongated droplets and grains can break up into smaller ones. This process is driven by the interfacial free energy minimization, which gives rise to a breakup limit. We demonstrated in this work that the breakup limit can be overpassed drastically by using electric current to interfere. Electric current free energy is dependent on the microstructure configuration. The breakup causes the electric current free energy to reduce in some cases. This compensates the increment of interfacial free energy during breaking up and enables the processing to achieve finer microstructure. With engineering practical electric current parameters, our calculation revealed a significant increment of the obtainable number of particles, showing electric current a powerful microstructure refinement technology. The calculation is validated by our experiments on the breakup of Fe₃C-plates in Fe matrix. Furthermore, there is a parameter range that electric current can drive spherical particles to split into smaller ones.