Phase Transitions of Aqueous Atmospheric Particles

Freezing-induced acidification of sea ice brine

The acidity of sea ice and snow plays a key role in the chemistry of the cryosphere; an important example lies in the photochemical catalytic release of reactive bromine in polar regions, facilitated at pHs below 6.5. We apply in-situ acid-base indicators to probe the microscopic acidity of the brine within the ice matrix in artificial sea water at a range of concentrations (0.35-70 PPT) and initial pHs (6-9). The results are supported by analogous measurements of the most abundant salts in seawater: NaCl, Na2SO4, and CaCO3. In the research herein, the acidity is expressed in terms of the Hammett acidity function, H2-. The obtained results show a pronounced acidity increase in sea water after freezing at -15 °C and during the subsequent cooling down to -50 °C. Importantly, we did not observe any significant hysteresis; the values of acidity upon warming markedly resembled those at the corresponding temperatures at cooling. The acidity increase is attributed to the minerals' crystallization, which is accompanied by a loss of the buffering capacity. Our observations show that lower salinity sea water samples (≤ 3.5 PPT) reach pH values below 6.5 at the temperature of -15 °C, whereas higher salinity ices attain such values only at -30 °C. The ensuing implications for polar chemistry and the relevance to the field measurements are discussed.

Affinity-Controlled Partitioning of Biomolecules at Aqueous Interfaces and Their Bioanalytic Applications

All-aqueous phase separation systems play essential roles in bioanalytical and biochemical applications. Compared to conventional oil and organic solvent-based systems, these systems are characterized by their rich bulk and interfacial properties, offering superior biocompatibility. In particular, phase separation in all-aqueous systems facilitates the creation of compartments with specific physicochemical properties, and therefore largely enhances the accessibility of the systems. In addition, the all-aqueous compartments have diverse affinities, with an important property known as partitioning, which can concentrate (bio)molecules toward distinct immiscible phases. This partitioning affinity imparts all-aqueous interfaces with selective permeability, enabling the controlled enrichment of target (bio)molecules. This review introduces the basic principles and applications of partitioning-induced interfacial phenomena in a typical all-aqueous system, namely aqueous two-phase systems (ATPSs); these applications include interfacial chemical reactions, bioprinting, and assembly, as well as bio-sensing and detection. The primary challenges associated with designing all-aqueous phase separation systems and several future directions are also discussed, such as the stabilization of aqueous interfaces, the handling of low-volume samples, and exploration of suitable ATPSs compositions with the efficient protocol.

A General Nucleation Model for Semiconductor Nanocrystals

We introduce a nonclassical model for nanocrystal nucleation in solution which centers on the dynamic interplay of chemical bond breakage and formation coupled with the desolvation of precursor molecules, which we term the molecular chemistry (MC) model. Departing from classical theory, our model employs the bond count as the key variable rather than particle size, thereby redefining the role of supersaturation and its role in determining the so-called critical nucleus size. We apply the model to CdSe nanocrystal formation in nonpolar solvents and showcase its efficacy in predicting solvent dynamics, precursor characteristics, crystal phase, stoichiometry, "magic number" behavior, and transition states. While the coupled-cluster method is used to determine the bond energy, we show that it is possible to derive reaction pathways by reducing the calculations to algebraic approximations for the nucleation energy. This singular set of bond energy parameters allows nanocrystal nucleation and growth to be conceptualized as a straightforward chemical reaction.

Spectral Characteristics of Unsaturated and Supersaturated Inorganic Aerosols: Insights into Deliquescence Kinetics

The deliquescence phase transition of atmospheric aerosols is crucial for radiative forcing and atmospheric chemistry. However, the deliquescence kinetics of micrometer-sized aerosols, especially the formation and evolution of surface solution films, remain poorly understood. In this study, IR spectral characteristics were employed for the first time to quantify the solute concentration evolution in surface solution films. At a constant relative humidity (RH) of ∼65%, solution films on NaCl crystals exhibited a very low solute concentration (3.06 ± 0.18 mol/L), comparable to aqueous NaCl droplets above 90% RH. These films reached saturation at ∼74% RH, i.e., the deliquescence RH of NaCl, maintaining a nearly constant saturation level during deliquescence. In contrast, amorphous NaNO3 solids showed supersaturated solution films before deliquescence. Following deliquescence, the saturation level of solution phases increased due to faster solid dissolution rates than liquid water condensation. These findings address knowledge gaps in the complex nonequilibrium dissolution processes of crystalline or amorphous atmospheric aerosols.

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.

Complex Hygroscopic Behavior of Ambient Aerosol Particles Revealed by a Piezoelectric Technique

Understanding the complex interactions between atmospheric aerosols and water vapor in subsaturated regions of the atmosphere is crucial for modeling and predicting aerosol-cloud-radiation-climate interactions. However, the microphysical mechanisms of these interactions for ambient aerosols remain poorly understood. For this study, size-resolved samples were collected from a high-altitude, relatively clean site situated in the Western Ghats of India during the monsoon season, in order to study background and preindustrial processes as a baseline for climate functioning within the context of the most polluted region of the world. Measurements of humidity-dependent mass-based growth factors, hygroscopicity, deliquescence behavior, and aerosol liquid water content (ALWC) were made by a novel approach using a quartz crystal microbalance based on a piezo-electric sensor. The climate-relevant fine-mode aerosols (≤2.5 μm) exhibited strong size-dependent variations in their interactions with water vapor and contributed a high fraction of ALWC. Deliquescence occurred for relatively large aerosols (diameter >180 nm) but was absent for smaller aerosols. The deliquescence relative humidity for ambient aerosols was significantly lower than that of pure inorganic salts, suggesting a strong influence of organic species. Our study establishes an improved approach for accurately measuring aerosol water uptake characteristics of ambient aerosols in the subsaturated regime, aiding in the assessment of radiative forcing effects and improving climate models.

Compositional evolution for mixed aerosols containing gluconic acid and typical nitrate and the effect of multiply factors on hygroscopicity

The aging process of atmospheric aerosols usually leads to a mixture of inorganic salts and organic compounds of anthropogenic origin. In organic compounds, polyhydroxy organic acids are important components, however, the study on composition and hygroscopic properties of the mixture containing inorganics and polyhydroxy organic acids is scanty. In this study, gluconic acid, the proxy of polyhydroxy organic acids, is mixed with the representative nitrate (Mg(NO3)2, Ca(NO3)2) to form aerosols. ATR-FTIR and optical microscopy are employed to study the component changes and hygroscopicity as a function of relative humidity. As relative humidity fluctuates, the FTIR-ATR spectra display that the internal mixed gluconic acid (CH2(CH)4(OH)5COOH) and nitrate can react to release acidic gases, forming relevant gluconate and further affecting the hygroscopicity. The specific presentation is particles cannot be recovered to their original size after the dehydration-hydration process and there will be some disparities in GF for mixed particles. For the gluconic acid-Ca(NO3)2/Mg(NO3)2 mixtures with molar ratios of 1:1, higher degree of reaction resulting in the production of large amounts of gluconate should be responsible to the lower hygroscopicity compared to ZSR model. For 1:2 gluconic acid-nitrate mixed systems (with higher nitrate content), the hygroscopicity of mixtures are higher than the ZSR prediction. A possible reason could be 'salt-promoting effect' on the organic fractions of the surplus inorganic salt in the mixture. These data can improve the chemical composition list evaluation, in turn hygroscopic properties and phase state of atmospheric aerosol, and then the climate effect.

Relative Humidity-Dependent Phase Transitions in Submicron Respiratory Aerosols

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

The important role of nitrate in iron and manganese dissolution and sulfate formation in fine particles at a coastal site in Northern China

Bioavailable transition trace elements, such as soluble iron (Fes) and soluble manganese (Mns) in aerosols, play a crucial role in atmospheric sulfate formation and marine ecosystems. In this study conducted during the spring of 2017 in Qingdao, a coastal city in Northern China, we applied a combined approach of multiple linear regression (MLR) incorporating the results of positive matrix factorization (PMF) to estimate the solubility of Fe and Mn from various sources. PMF analysis showed that dust was the largest contributor to total Fe (FeT) (45.5 %), followed by non-ferrous smelting (20.3 %) and secondary formation processes (17.8 %). However, secondary formation processes (33.2 %), vehicle exhaust (19.3 %) and aqueous-phase processes (19.0 %) were found to be the primary contributors to Fes. For total Mn (MnT) and Mns, dust (21.2 % ∼ 35.0 %), secondary formation processes (20.3 % ∼ 25.6 %) and industry (12.6 % ∼ 16.3 %) were identified as the dominant contributors. The solubilities of Fe and Mn varied significantly depending on their sources. Interestingly, nitrate played a more pronounced role than sulfate in facilitating the dissolution of Fe and Mn during the acid processing due to the high molar ratio of NO3-/2SO42- (1.72 ± 0.54) under the average RH of 56 % ± 15 %. This phenomenon suggested that the acid processing was primarily triggered by nitrate formation due to the low deliquescence relative humidity (DRH) of nitrate. Additionally, we discovered that the catalytic oxidation of SO2 in aerosol water was primarily driven by Fe rather than Mn, serving as a more significant pathway for sulfate formation within a pH range of 2.0 to 4.4. These findings provide valuable insights into the impact of acidification on the dissolution of Fe and Mn under conditions of moderate RH in the real ambient atmosphere with the increasing of NO3-/2SO42- molar ratio.

Stability of Feline Coronavirus in aerosols and dried in organic matrices on surfaces at various environmental conditions

Enveloped respiratory viruses, including the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), can be transmitted through aerosols and contact with contaminated surfaces. The stability of these viruses outside the host significantly impacts their transmission dynamics and the spread of diseases. In this study, we investigated the tenacity of Feline Coronavirus (FCoV) in aerosols and on surfaces under varying environmental conditions. We found that airborne FCoV showed different stability depending on relative humidity (RH), with higher stability observed at low and high RH. Medium RH conditions (50-60%) were associated with increased loss of infectivity. Furthermore, FCoV remained infectious in the airborne state over 7 h. On stainless-steel surfaces, FCoV remained infectious for several months, with stability influenced by organic material and temperature. The presence of yeast extract and a temperature of 4 °C resulted in the longest maintenance of infectivity, with a 5 log10 reduction of the initial concentration after 167 days. At 20 °C, this reduction was achieved after 19 days. These findings highlight the potential risk of aerosol and contact transmission of respiratory viruses, especially in enclosed environments, over extended periods. Studying surrogate viruses like FCoV provides important insights into the behavior of zoonotic viruses like SARS-CoV-2 in the environment.

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.

Nonequilibrium Behavior in Isoprene Secondary Organic Aerosol

Recent studies have shown that instantaneous gas-particle equilibrium partitioning assumptions fail to predict SOA formation, even at high relative humidity (∼85%), and photochemical aging seems to be one driving factor. In this study, we probe the minimum aging time scale required to observe nonequilibrium partitioning of semivolatile organic compounds (SVOCs) between the gas and aerosol phase at ∼50% RH. Seed isoprene SOA is generated by photo-oxidation in the presence of effloresced ammonium sulfate seeds at <1 ppbv NOx, aged photochemically or in the dark for 0.3-6 h, and subsequently exposed to fresh isoprene SVOCs. Our results show that the equilibrium partitioning assumption is accurate for fresh isoprene SOA but breaks down after isoprene SOA has been aged for as short as 20 min even in the dark. Modeling results show that a semisolid SOA phase state is necessary to reproduce the observed particle size distribution evolution. The observed nonequilibrium partitioning behavior and inferred semisolid phase state are corroborated by offline mass spectrometric analysis on the bulk aerosol particles showing the formation of organosulfates and oligomers. The unexpected short time scale for the phase transition within isoprene SOA has important implications for the growth of atmospheric ultrafine particles to climate-relevant sizes.

Solvation of magnesium chloride dimer in water: The case of anionic and neutral clusters

The structures of magnesium chloride dimer-water clusters, (MgCl2)2(H2O)n-/0, were investigated with size-selected anion photoelectron spectroscopy and theoretical calculations to understand the dissolution of magnesium chloride in water. The most stable structures were confirmed by comparing vertical detachment energies (VDEs) with the experimental measurements. A dramatic drop of VDE at n = 3 has been observed in the experiment, which is in accordance with the structural change of (MgCl2)2(H2O)n-. Compared to the neutral clusters, the excess electron induces two significant phenomena in (MgCl2)2(H2O)n-. First, the planar D2h geometry can be converted into a C3v structure at n = 0, making the Mg-Cl bonds easier to be broken by water molecules. More importantly, a negative charge-transfer-to-solvent process occurs after adding three water molecules (i.e., at n = 3), which leads to an obvious deviation in the evolution of the clusters. Such electron transfer behavior was noticed at n = 1 in monomer MgCl2(H2O)n-, indicating that the dimerization between two MgCl2 molecules can make the cluster more capable of binding electron. In neutral (MgCl2)2(H2O)n, this dimerization provides more sites for the added water molecules, which can stabilize the entire cluster and maintain its initial structure. Specifically, filling the coordination number to be 6 for Mg atoms can be seen as a link between structural preferences in the dissolution of the monomers, dimers, and extended bulk-state of MgCl2. This work represents an important step forward into fully understanding the solvation of MgCl2 crystals and other multivalent salt oligomers.

Capillary rise induced salt deterioration on ancient wall paintings at the Mogao Grottoes

Salt deterioration has been found to be a major threat to wall paintings at culture heritage sites in arid areas along the Silk Road. However, the routes of water migration that cause the efflorescence have not been identified, and consequently, effective preservation measures have not been developed. Our microanalysis, by interrogating 93,727 individual particles collected in a Mogao cave in Dunhuang, China, revealed that capillary rise of water in the earthen plasters drives the deterioration of wall paintings. The vertical distribution of chloride and sulfate particles in the salt efflorescence and their morphologies implied a migration of salts through capillary rise and subsequent crystal growth under environmental conditions exerts sufficient pressure to cause surface decay and loss. These results indicate that blocking the water capillary rise under the porous structures is likely the most effective route to prevent rapid deterioration of the ancient wall paintings. These salt transport and deterioration mechanisms in an arid environment, suggests that a wide range of management strategies and protective measures could be developed to effectively preserve heritage sites in arid regions, especially along the Silk Road.

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.

Direct Calculation of the Interfacial Free Energy between NaCl Crystal and Its Aqueous Solution at the Solubility Limit

Salty water is the most abundant electrolyte aqueous mixture on Earth, however, very little is known about the NaCl-saturated solution interfacial free energy (γ_{s}). Here, we provide the first direct estimation of γ_{s} for several NaCl crystallographic planes by means of the mold integration technique, a highly efficient computational method to evaluate interfacial free energies with anisotropic crystal resolution. Making use of the JC-SPC/E model, one of the most benchmarked force fields for NaCl water solutions, we measure γ_{s} of four different crystal planes, (100), (110), (111), and (112[over ¯]) with the saturated solution at normal conditions. We find high anisotropy between the different crystal orientations with values ranging from 100 to 150  mJ m^{-2}, and the average value of the distinct planes being γ[over ¯]_{s}=137(20)  mJ m^{-2}. This value for the coexistence interfacial free energy is in reasonable agreement with previous extrapolations from nucleation studies. Our Letter represents a milestone in the computational calculation of interfacial free energies between ionic crystals and aqueous solutions.

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.

Evidence of non-tailpipe emission contributions to PM2.5 and PM10 near southern California highways

Particulate Matter (PM) concentrations near highways are influenced by vehicle tailpipe and non-tailpipe emissions, other emission sources, and urban background aerosols. This study collected PM_2.5 and PM₁₀ filter samples near two southern California highways (I-5 and I-710) over two weeks in winter 2020. Samples were analyzed for chemical source markers. Mean PM_2.5 and PM₁₀ concentrations were approximately 10-15 and 30 μg/m³, respectively. Organic matter, mineral dust, and elemental carbon (EC) were the most abundant PM components. EC and polycyclic aromatic hydrocarbons at I-710 were 19-26% and 47% higher than those at the I-5 sites, respectively, likely due to a larger proportion of diesel vehicles. High correlations were found for elements with common sources, such as markers for brake wear (e.g., Fe, Ba, Cu, and Zr) and road dust (e.g., Al, Si, Ca, and Mn). Based on rubber abundances, the contributions of tire tread particles to PM_2.5 and PM₁₀ mass were approximately 8.0% at I-5 and 5.5% at I-710. Two different tire brands showed significantly different Si, Zn, carbon, and natural rubber abundances.

Comparison of Phase States of PM2.5 over Megacities, Seoul and Beijing, and Their Implications on Particle Size Distribution

Although the particle phase state is an important property, there is scant information on it, especially, for real-world aerosols. To explore the phase state of fine mode aerosols (PM_2.5) in two megacities, Seoul and Beijing, we collected PM_2.5 filter samples daily from Dec 2020 to Jan 2021. Using optical microscopy combined with the poke-and-flow technique, the phase states of the bulk of PM_2.5 as a function of relative humidity (RH) were determined and compared to the ambient RH ranges in the two cities. PM_2.5 was found to be liquid to semisolid in Seoul but mostly semisolid to solid in Beijing. The liquid state was dominant on polluted days, while a semisolid state was dominant on clean days in Seoul. These findings can be explained by the aerosol liquid water content related to the chemical compositions of the aerosols at ambient RH; the water content of PM_2.5 was much higher in Seoul than in Beijing. Furthermore, the overall phase states of PM_2.5 observed in Seoul and Beijing were interrelated with the particle size distribution. The results of this study aid in a better understanding of the fundamental physical properties of aerosols and in examining how these are linked to PM_2.5 in polluted urban atmospheres.

Initial pH Governs Secondary Organic Aerosol Phase State and Morphology after Uptake of Isoprene Epoxydiols (IEPOX)

Aerosol acidity increases secondary organic aerosol (SOA) formed from the reactive uptake of isoprene-derived epoxydiols (IEPOX) by enhancing condensed-phase reactions within sulfate-containing submicron particles, leading to low-volatility organic products. However, the link between the initial aerosol acidity and the resulting physicochemical properties of IEPOX-derived SOA remains uncertain. Herein, we show distinct differences in the morphology, phase state, and chemical composition of individual organic-inorganic mixed particles after IEPOX uptake to ammonium sulfate particles with different initial atmospherically relevant acidities (pH = 1, 3, and 5). Physicochemical properties were characterized via atomic force microscopy coupled with photothermal infrared spectroscopy (AFM-PTIR) and Raman microspectroscopy. Compared to less acidic particles (pH 3 and 5), reactive uptake of IEPOX to the most acidic particles (pH 1) resulted in 50% more organosulfate formation, clearer phase separation (core-shell), and more irregularly shaped morphologies, suggesting that the organic phase transitioned to semisolid or solid. This study highlights that initial aerosol acidity may govern the subsequent aerosol physicochemical properties, such as viscosity and morphology, following the multiphase chemical reactions of IEPOX. These results can be used in future studies to improve model parameterizations of SOA formation from IEPOX and its properties, toward the goal of bridging predictions and atmospheric observations.

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.

Organic deliquescence: organic vapor-induced dissolution of molecular salts

This report demonstrates organic vapor-induced dissolution of several molecular salts (i.e., organic deliquescence), like water vapor-induced deliquescence. Systematic experiments indicate that appropriate organic deliquescent responses to volatile organic compounds can be designed according to the principle, “like dissolves like”. The phenomena will be useful for developing agents to collect various volatile organic compounds.

Several molecular salts exhibit organic deliquescence in response to organic vapors.

Synergistic HNO3-H2SO4-NH3 upper tropospheric particle formation

New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN)^1-4. However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region^5,6. Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles-comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO₃-H₂SO₄-NH₃ nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere.

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.

Estimation of Aerosol Extinction Coefficient Using Camera Images and Application in Mass Extinction Efficiency Retrieval

In this study, we attempted to calculate the extinction parameters of PM2.5 using images from a commercial camera. The photo pixels provided information on the characteristics of the objects (i.e., the reflectivity, transmittance, or extinction efficiency) and ambient brightness. Using the RGB values of pixels, we calculated the extinction coefficient and efficiency applied to the mass concentration of PM2.5. The calculated extinction coefficient of PM2.5 determined from the camera images had a higher correlation with the PM2.5 mass concentration (R2 = 0.7) than with the visibility data, despite the limited mass range. Finally, we identified that the method of calculating extinction parameters using the effective wavelength of RGB images could be applied to studies of changes in the atmosphere and aerosol characteristics. The mass extinction efficiency of PM2.5, derived from images, and the mass concentration of PM2.5 was (10.8 ± 6.9) m2 g−1, which was higher than the values obtained in Northeast Asia by previous studies. We also confirmed that the dry extinction efficiency of PM2.5, applied with a DRH of 40%, was reduced to (6.9 ± 5.0) m2 g−1. The extinction efficiencies of PM2.5, calculated in this study, were higher than those reported in previous other studies. We inferred that high extinction efficiency is related to changes in size or the composition of aerosols; therefore, an additional long-term study must be conducted.

Electric Fields Enhance Ice Formation from Water Vapor by Decreasing the Nucleation Energy Barrier

Video images of ice formation from moist air under temperature and electric potential gradients reveal that ambient electricity enhances ice production rates while changing the habit of ice particles formed under low supersaturation. The crystals formed under an electric field are needles and dendrites instead of the isometric ice particles obtained within a Faraday cage. Both a non-classical mechanism and classical nucleation theory independently explain the observed mutual feedback between ice formation and its electrification. The elongated shapes result from electrostatic repulsion at the crystal surfaces, opposing the attractive intermolecular forces and thus lowering the ice-air interfacial tension. The video images allow for the estimation of ice particle dimensions, weight, and speed within the electric field. Feeding this data on standard equations from electrostatics shows that the ice surface charge density attains 0.62–1.25 × 10−6 C·m−2, corresponding to 73–147 kV·m−1 potential gradients, reaching the range measured within thunderstorms. The present findings contribute to a better understanding of natural and industrial processes involving water phase change by acknowledging the presence and effects of the pervasive electric fields in the ambient environment.

Enhanced secondary aerosol formation driven by excess ammonia during fog episodes in Delhi, India

The Indo-Gangetic Plain (IGP) has high wintertime fine aerosol loadings that significantly modulate the widespread fog formation and sustenance. Here, we investigate the potential formation of secondary inorganic aerosol driven by excess ammonia during winter fog. Physicochemical properties of fine aerosols (PM₁ and PM_2.5) and trace gases (HCl, HONO, HNO₃, SO₂, and NH₃) were simultaneously monitored at hourly resolution using Monitor for AeRosols and Gases in Ambient air (MARGA-2S) for the first time in India. Results showed that four major ions, i.e., Cl^-, NO₃^-, SO₄^2-, and NH₄⁺ contributed approximately 97% of the total measured inorganic ionic mass. The atmosphere was ammonia-rich in winter and ammonium was the dominant neutralizer with aerosol neutralization ratio (ANR) close to unity. The correlation between ammonium and chloride was ≥0.8, implying the significant formation of ammonium chloride during fog in Delhi. Thermodynamical model ISORROPIA-II showed the predicted PM₁ and PM_2.5 pH to be 4.49 ± 0.53, and 4.58 ± 0.48 respectively which were in good agreement with measurements. The ALWC increased from non-foggy to foggy periods and a considerable fraction of fine aerosol mass existed in the supermicron size range of 1-2.5 μm. The sulfur oxidation ratio (SOR) of PM₁, PM_2.5 reached up to 0.60, 0.75 in dense fog and 0.74, 0.87 when ambient RH crossed a threshold of 95%, much higher than non-foggy periods (with confidence level of ≥95%) pointing to enhanced formation of secondary aerosol in fog.

The different sensitivities of aerosol optical properties to particle concentration, humidity, and hygroscopicity between the surface level and the upper boundary layer in Guangzhou, China

This study investigates the impact of aerosol liquid water content (ALWC) and related factors, i.e., relative humidity (RH), aerosol mass concentration (PM_2.5), and aerosol hygroscopicity, on aerosol optical properties, based on field measurements made in the Pearl River Delta (PRD) region of China at the surface (1 November 2019 to 21 January 2020) and in the upper boundary layer (the 532-m Guangzhou tower from 1 February to 21 March 2020). In general, temporal variations in the ambient aerosol backscattering coefficient (β_p) and ALWC followed each other. However, the surface β_p and 532-m β_p had generally opposite diurnal variation patterns, caused by dramatic differences in PM_2.5 and ambient RH between the surface and the upper boundary layer. The ambient 532-m RH was systematically higher than the surface RH, with the latter having a much pronounced diurnal cycle than the former. The surface PM_2.5 concentration was systematically higher than the PM_2.5 concentration at 532 m, and their diurnal cycle patterns were overall opposite. These dramatic differences reveal that the atmospheric variables, i.e., ambient RH and the PM_2.5 concentration in the upper boundary layer, cannot be directly represented by the same variables at the surface. Vertical variability should be considered. Clear differences in the sensitivities of aerosol light scattering to ambient RH, PM_2.5, and aerosol hygroscopicity between the two levels were found and examined. Aerosol chemical composition played a minor role in causing the differences between the two levels. In particular, β_p was more sensitive to PM_2.5 at the surface level but more to the ambient RH in the upper boundary layer. The larger contribution of aerosol loading to the variability in β_p at the surface implies that local emission controls can decrease β_p and further improve atmospheric visibility effectively at the surface during winter in the PRD region.

Advancing the science of dynamic airborne nanosized particles using Nano-DIHM

In situ and real-time characterization of aerosols is vital to several fundamental and applied research domains including atmospheric chemistry, air quality monitoring, or climate change studies. To date, digital holographic microscopy is commonly used to characterize dynamic nanosized particles, but optical traps are required. In this study, a novel integrated digital in-line holographic microscope coupled with a flow tube (Nano-DIHM) is demonstrated to characterize particle phase, shape, morphology, 4D dynamic trajectories, and 3D dimensions of airborne particles ranging from the nanoscale to the microscale. We demonstrate the application of Nano-DIHM for nanosized particles (≤200 nm) in dynamic systems without optical traps. The Nano-DIHM allows observation of moving particles in 3D space and simultaneous measurement of each particle's three dimensions. As a proof of concept, we report the real-time observation of 100 nm and 200 nm particles, i.e. polystyrene latex spheres and the mixture of metal oxide nanoparticles, in air and aqueous/solid/heterogeneous phases in stationary and dynamic modes. Our observations are validated by high-resolution scanning/transmission electron microscopy and aerosol sizers. The complete automation of software (Octopus/Stingray) with Nano-DIHM permits the reconstruction of thousands of holograms within an hour with 62.5 millisecond time resolution for each hologram, allowing to explore the complex physical and chemical processes of aerosols.

Low-Temperature Water Uptake of Individual Marine and Biologically Relevant Atmospheric Particles Using Micro-Raman Spectroscopy

The interaction of water vapor and the water uptake behavior of atmospheric particles are often investigated as a function of relative humidity (0-100% RH) at ambient temperature. However, lower temperature studies are important to understand how atmospheric particles nucleate ice through various mechanisms including immersion freezing. Immersion freezing requires the formation of a condensed water droplet at lower temperatures prior to freezing. To better understand low-temperature water uptake behavior of marine and biologically relevant atmospheric particles, we have investigated water uptake of single atmospheric particles using a micro-Raman spectrometer coupled to an environmental cell for measurements at lower temperatures and as a function of relative humidity. These particles include sodium chloride, sucrose, Snomax, lipopolysaccharide, and laminarin. Particles range in size from 2 to 3 μm in diameter and can be monitored by using optical microscopy and Raman spectroscopy as a function of relative humidity at temperatures between 253 and 298 K. From the Raman spectra collected, we can determine a Raman growth factor defined as an increase in the intensity of the O-H stretch as a measure of the integrated water content of a particle compared to the dry particle. These data show that for lipopolysaccharide, laminarin, and Snomax, unlike simple saccharides such as sucrose and other soluble organics, as temperature decreases, water uptake begins at lower relative humidity and does not follow a solubility temperature dependence. This suggests that at lower temperatures the particles are adsorbing water on the surface rather than dissolving and absorbing water. Furthermore, repeated water uptake cycles cause a change in the morphology of some of these particles, which is shown to promote water uptake at lower relative humidity. These results give new insights into water uptake of these different marine and biologically relevant particles at low temperature at subsaturation relative humidity prior to droplet formation and immersion freezing.

Liquid nucleation around charged particles in the vapor phase

We theoretically investigate the nucleation of liquid droplets from vapor in the presence of a charged spherical particle. Due to field gradients, sufficiently close to the critical point of the vapor-gas system, the charge destabilizes the vapor phase and initiates a phase transition. The fluid's free energy is described by the van der Waals expression augmented by electrostatic energy and a square-gradient term. We calculate the equilibrium density profile at arbitrary temperatures, particle charges, and vapor densities. In contrast to classical nucleation theory, here, both liquid and vapor phases are different from the bulk phases because they are spatially nonuniform. In addition, the theory applies to both sharp and diffuse interfaces and calculates the surface tension self-consistently. We find the composition profiles and integrate them to get the adsorption near the particle. We find that the adsorption changes discontinuously at a first-order phase transition line. This line becomes a second-order phase transition at high enough temperatures. We describe the transition point numerically and provide approximate analytical expressions for it. Similarly to prewetting, the adsorption diverges at the binodal phase boundary. We construct a phase diagram indicating changes in the binodal, spinodal, and critical temperature. It is shown that the field gradient enlarges the range of temperature and vapor density where liquid can nucleate.

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

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

Hygroscopicity and mass transfer limit of mixed glutaric acid/MgSO4/water particles

Tropospheric aerosols are usually complex mixtures of inorganic and organic components, which show non-ideal behavior in hygroscopicity, mass transfer, and partitioning between gas and aerosols. In this study, we applied a novel approach based on a combination of a pulse RH controlling system and a rapid scan vacuum FTIR spectrometer to investigate the mass transfer limit of magnesium sulfate/glutaric acid (GA) mixture aerosol particles. The liquid water band area of the aerosols is used to reveal the mass transfer limit during the rapid pulse RH downward and upward processes. Partitioning equilibrium between the aerosol particles and water gas phase is observed at the higher RH range (73-50%). When the RH is lower than 40%, there is a hysteresis for the liquid water content changing with the RH, indicating the limited water mass transfer in the aerosols.

A review of efflorescence kinetics studies on atmospherically relevant particles

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

Zeolite Lightweight Repair Renders: Effect of Binder Type on Properties and Salt Crystallization Resistance

Rendering mortars with lightweight zeolite aggregates were designed and tested. The effect of the type of binder used was also researched. For the hardened mortars, macrostructural parameters, mechanical characteristics, hygric and thermal properties were assessed. Specific attention was paid to the analysis of the salt crystallization resistance of the developed rendering mortars. Quartz sand was fully replaced in the composition of mortars with zeolite gave materials with low density, high porosity, sufficient mechanical strength, high water vapor permeability and high water absorption coefficient, which are technical parameters required for repair rendering mortars as prescribed in the WTA directive 2-9-04/D and EN 998-1. Moreover, the zeolite enhanced mortars exhibit good thermal insulation performance and high sorption capacity. The examined rendering mortars were found to be well durable against salt crystallization, which supports their applicability in salt-laden masonry. Based on the compatibility of the repair materials with those originally used, the lime and natural hydraulic lime zeolite mortars can be used as rendering mortars for the repair of historical and heritage buildings. The cement-lime zeolite render is applicable for repair purposes only in the case of the renewal of masonry in which Portland cement-based materials were originally used.

Combined analyses of hygroscopic properties of organic and inorganic components of three representative black carbon samples recovered from pyrolysis

Hygroscopicity of black carbon (BC) aerosols is a key factor determining their climate forcing effect and atmospheric lifetime. However, the compositional dependence of BC hygroscopicity is not well understood. Here, a variety of different compositional components were separated from three representative BC samples recovered from pyrolysis (grass and wheat straw derived BC, household soot), including water extracted fraction of BC (WEBC, 9-21 wt%), residue fraction of BC after water extraction (R-WEBC, 79-91 wt%), water extracted minerals (WEM, 9-18 wt%), alkali extracted organic carbon (OC_AE, 1-9 wt%), and elemental carbon (EC, 37-48 wt%). The bulk BC and separated BC components were analyzed in detail by elemental analysis and combined spectroscopic analyses. Their equilibrium hygroscopicity was measured by gravimetric method over a range of relative humidity (RH) levels (10-94%). Compared with the two organic components (OC_AE and EC), the inorganic component (WEM) exhibited much stronger water uptake at all RH levels. At 94% RH level, WEM accounted for 16-139% of the overall water uptake by BC, whereas OC_AE and EC accounted for only 1-3% and 6-26%, respectively. The XRD analysis of WEBC and WEM from household soot at varying RH levels indicated that the enhanced water uptake by these two components as well as that by bulk BC at high RH levels was due to the deliquescent salts (e.g., KCl, NH₄Cl, KNO₃, and NaCl). The strong hysteresis loops observed for bulk BC and WEBC could be attributed to the organic-facilitated drastic structural and morphological rearrangement of mineral particles as evidenced by the optical microscope analysis. The diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis reaffirmed the dominant role played by the inorganic component in the hygroscopic behaviors of BC.

Hygroscopic behavior and chemical reactivity of aerosols generated from mixture solutions of low molecular weight dicarboxylic acids and NaCl

Ambient sea spray aerosols (SSAs) have been reported to undergo reactions with low molecular weight dicarboxylic acids (LMW DCAs). In the present study, the hygroscopic behavior of aerosols generated from NaCl-LMW DCA mixture solutions with different mixing ratios was explained. In situ Raman microspectrometry (RMS) was used to simultaneously monitor the alterations in chemical composition, size, and phase as a function of the relative humidity (RH) for individual aerosols. The observation of individual mixture aerosols revealed chemical reactions on the timescale of one hour in the aqueous phase, mostly during the dehydration process, leading to the formation of sodium salts of DCAs with distinct reactivities among different DCAs and mixing ratios, which in turn exhibited diverse hygroscopic behaviors. The NaCl-DCA mixture aerosols were either in a ternary NaCl-DCA-DCA sodium salt system or a binary NaCl-DCA sodium salt or DCA-DCA sodium salt system, instead of a binary NaCl-DCA system when experiencing the hygroscopic process. The chemical compositional evolution of the NaCl-DCA aerosols during the hygroscopic measurements was examined based on the Raman spectra acquired for aqueous, amorphous, and/or crystalline pure standard aerosols at specific RHs. The different reactivity observed among the DCAs with different mixing ratios suggests that the reactivity driven by the irreversible liberation of HCl is governed mainly by the available aqueous H⁺ because Cl^- is always available in the aqueous NaCl-DCA aerosols until the complete consumption of NaCl.

Multiphase Oxidation of Sulfur Dioxide in Aerosol Particles: Implications for Sulfate Formation in Polluted Environments

Atmospheric oxidation of sulfur dioxide (SO₂) forms sulfate-containing aerosol particles that impact air quality, climate, and human and ecosystem health. It is well-known that in-cloud oxidation of SO₂ frequently dominates over gas-phase oxidation on regional and global scales. Multiphase oxidation involving aerosol particles, fog, and cloud droplets has been generally thought to scale with liquid water content (LWC) so multiphase oxidation would be negligible for aerosol particles due to their low aerosol LWC. However, recent field evidence, particularly from East Asia, shows that fast sulfate formation prevails in cloud-free environments that are characterized by high aerosol loadings. By assuming that the kinetics of cloud water chemistry prevails for aerosol particles, most atmospheric models do not capture this phenomenon. Therefore, the field of aerosol SO₂ multiphase chemistry has blossomed in the past decade, with many oxidation processes proposed to bridge the difference between modeled and observed sulfate mass loadings. This review summarizes recent advances in the fundamental understanding of the aerosol multiphase oxidation of SO₂, with a focus on environmental conditions that affect the oxidation rate, experimental challenges, mechanisms and kinetics results for individual reaction pathways, and future research directions. Compared to dilute cloud water conditions, this paper highlights the differences that arise at the molecular level with the extremely high solute strengths present in aerosol particles.

Coexistence of three liquid phases in individual atmospheric aerosol particles

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

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

Life in a Droplet: Microbial Ecology in Microscopic Surface Wetness

While many natural and artificial surfaces may appear dry, they are in fact covered by thin liquid films and microdroplets invisible to the naked eye known as microscopic surface wetness (MSW). Central to the formation and the retention of MSW are the deliquescent properties of hygroscopic salts that prevent complete drying of wet surfaces or that drive the absorption of water until dissolution when the relative humidity is above a salt-specific level. As salts are ubiquitous, MSW occurs in many microbial habitats, such as soil, rocks, plant leaf, and root surfaces, the built environment, and human and animal skin. While key properties of MSW, including very high salinity and segregation into droplets, greatly affect microbial life therein, it has been scarcely studied, and systematic studies are only in their beginnings. Based on recent findings, we propose that the harsh micro-environment that MSW imposes, which is very different from bulk liquid, affects key aspects of bacterial ecology including survival traits, antibiotic response, competition, motility, communication, and exchange of genetic material. Further research is required to uncover the fundamental principles that govern microbial life and ecology in MSW. Such research will require multidisciplinary science cutting across biology, physics, and chemistry, while incorporating approaches from microbiology, genomics, microscopy, and computational modeling. The results of such research will be critical to understand microbial ecology in vast terrestrial habitats, affecting global biogeochemical cycles, as well as plant, animal, and human health.

Relative humidity in droplet and airborne transmission of disease

A large number of infectious diseases are transmitted by respiratory droplets. How long these droplets persist in the air, how far they can travel, and how long the pathogens they might carry survive are all decisive factors for the spread of droplet-borne diseases. The subject is extremely multifaceted and its aspects range across different disciplines, yet most of them have only seldom been considered in the physics community. In this review, we discuss the physical principles that govern the fate of respiratory droplets and any viruses trapped inside them, with a focus on the role of relative humidity. Importantly, low relative humidity-as encountered, for instance, indoors during winter and inside aircraft-facilitates evaporation and keeps even initially large droplets suspended in air as aerosol for extended periods of time. What is more, relative humidity affects the stability of viruses in aerosol through several physical mechanisms such as efflorescence and inactivation at the air-water interface, whose role in virus inactivation nonetheless remains poorly understood. Elucidating the role of relative humidity in the droplet spread of disease would permit us to design preventive measures that could aid in reducing the chance of transmission, particularly in indoor environment.

Hygroscopicity of Hofmeister Salts and Glycine Aerosols-Salt Specific Interactions

The Hofmeister effect of inorganic ions to precipitate proteins has been used to understand the coagulation phenomenon in colloid and protein science. Herein, for the first time, this effect is studied on the hygroscopicity of aerosols using ATR-FTIR spectroscopy. The representative Hofmeister salts (MgSO₄, KCl, NH₄NO₃) and amino acid (glycine) with different amino acid/salt molar ratios (ASRs) are mixed and atomized into micrometer-sized particles. For mixed kosmotrope (MgSO₄)/glycine and chaotrope (NH₄NO₃)/glycine with an ASR of 1:1, both ERHs (efflorescence relative humidities) and DRHs (deliquescence relative humidities) are absent. However, for the mixtures of glycine and neutral salt (KCl), no DRH is observed while 66.2 and 61.4% ERH of glycine is detected for mixtures with ASRs of 1:1 and 1:3, respectively, which is similar to pure glycine. For the mixture of NH₄NO₃/glycine with an ASR of 1:3, ERH and DRH are found to be 15.4 and 32.2% RH, less than that of pure NH₄NO₃. Further, interactions between glycine-salt and/or water is also studied in the mixtures during hydration and dehydration. Water-mediated ion-glycine interaction is detected based on the two glycine bands merging into one band. Glycine-SO₄^2- interaction is present for glycine/sulfate in all ASRs, while glycine-NO₃^- interaction is only seen for 1:3 glycine/NH₄NO₃ mixtures during hydration. This work opens a window to understand the Hofmeister effect on the hygroscopicity of atmospheric aerosols.

Core level photoelectron spectroscopy of heterogeneous reactions at liquid-vapor interfaces: Current status, challenges, and prospects

Liquid-vapor interfaces, particularly those between aqueous solutions and air, drive numerous important chemical and physical processes in the atmosphere and in the environment. X-ray photoelectron spectroscopy is an excellent method for the investigation of these interfaces due to its surface sensitivity, elemental and chemical specificity, and the possibility to obtain information on the depth distribution of solute and solvent species in the interfacial region. In this Perspective, we review the progress that was made in this field over the past decades and discuss the challenges that need to be overcome for investigations of heterogeneous reactions at liquid-vapor interfaces under close-to-realistic environmental conditions. We close with an outlook on where some of the most exciting and promising developments might lie in this field.

Subsecond measurement on deliquescence kinetics of aerosol particles: Observation of partial dissolution and calculation of dissolution rates

The deliquescence behavior of atmospheric aerosols has significant effects on global climate and atmospheric heterogeneous chemistry but remains largely unclear. The deliquescence kinetics data of micron-sized particles are scarce owing to the difficulty on performing the time-resolved dissolution measurements. In view of this technique bottleneck, an applicable and powerful experimental technique, i. e., vacuum FTIR combining pulsed relative humidity (RH) change technique, is introduced for gaining deliquescence kinetics information of three inorganic salts. For NaCl and (NH₄)₂SO₄ aerosols, a solid-liquid mixing state derived from partial dissolution of NaCl and (NH₄)₂SO₄ crystals is present during deliquescence, and the recrystallization will occur once RH decreases. While for NaNO₃ particles, the recrystallization cannot occur as RH decreases owing to the formed amorphous NaNO₃ solids after dying. The dissolution rates of NaCl, (NH₄)₂SO₄ and NaNO₃ solid particles are calculated, as a first attempt, by the upward pulsed RH mode. The measured rates show a significant dependency on ambient RH with three orders of magnitude. For NaCl particles, the measured J values range from 1.41 × 10^-4 to 7.67 × 10^-1 s^-1 at RH of 73.41-75.15%. The J for (NH₄)₂SO₄ particles is 7.34 × 10^-3 to 2.46 × 10⁰ s^-1 over the RH range of 77.27%-80.13%. The J values for amorphous NaNO₃ solids range from 6.01 × 10^-3 to 2.63 × 10⁰ s^-1 as RH increases from 71.15% to 73.84%. Our results fill in the dataset of atmospheric models describing the kinetics features of deliquescence and provide an insight into dynamic solid-solution transition for PM2.5 particles.

Optical Interrogation of Single Levitated Droplets in a Linear Quadrupole Trap by Cavity Ring-Down Spectroscopy

Optical trapping is a well-established technique to manipulate and levitate micro- and nanoscale particles and droplets. However, optical traps for single aerosol studies are most often limited to trapping spherical nonabsorbing droplets, and a universal optical trap for the stable confinement of particles regardless of their absorption strength and morphology is not established. Instead, new opportunities arise from levitating droplets using electrodynamic traps. Here, using a combined electrodynamic linear quadrupole trap and a cavity ring-down spectrometer, we demonstrate that it is possible to trap single droplets and simultaneously measure their extinction cross sections and elastic scattering phase functions over extended periods of time. To test the novel setup, we evaluated the evaporation of 1,2,6-hexanetriol under low-humidity conditions, and the evolution of aqueous (NH₄)₂SO₄ and NaCl droplets experiencing changing environmental conditions. Our studies extended beyond spherical droplets and we measured particle extinction cross sections after the efflorescence (crystallization) of the inorganic salt particles. Comparison of measured cross sections for crystallized particles with light scattering model predictions (using Mie theory or the T-matrix/extended boundary-condition method (EBCM) implementations for random orientation, with either the spheroid or superellipsoid parameterizations) enables information on particle shape to be inferred. Specifically, we find that cross sections for dry (NH₄)₂SO₄ particles are accounted for by Mie theory and, thus, particle shape is represented well by a sphere. Conversely, the cross sections for dry NaCl particles are only reconciled with light scattering models pertaining to nonspherical shapes. These results will have implications for accurate remote sensing retrievals of dry salt optical properties and for parameterizations implemented in radiative forcing calculations with changing humidity. Moreover, our new platform for precise and accurate measurement of optical properties of micron-scale and sub-micron particles has potential applications in a range of areas of atmospheric science, such as precise light scattering measurements for ice crystals and mineral dust. It represents a promising step toward accurate characterizations of optical properties for nonspherical and light-absorbing aerosols.

Impact of the particle mixing state on the hygroscopicity of internally mixed sodium chloride-ammonium sulfate single droplets: a theoretical and experimental study

Sodium chloride (NaCl) is the main constituent of sea-salt aerosols. During atmospheric transport, sea-salt aerosols can interact with gases and other particles including secondary aerosols containing ammonium sulfate ((NH4)2SO4). This paper reports on the deliquescence relative humidity (DRH) of internally mixed sodium chloride-ammonium sulfate (NaCl/(NH4)2SO4) coarse particles by means of an acoustic levitation system fitted with a confocal Raman microscope (CRM). The chemical composition and physical state of individual levitated particles of different initial NaCl mole fractions were monitored during the deliquescence cycle by CRM. Experimental results were compared to the data predicted by the thermodynamic model E-AIM (Extended-Aerosol Inorganics Model). We demonstrated that NH4Cl, Na2SO4 and NH4NaSO4·2H2O are formed in recrystallized particles and coexist with NaCl and (NH4)2SO4. All these products are randomly distributed within the particles. Deliquescence curves described two or three-stage phase transitions depending on the initial composition of the droplet. Significant discrepancies between the model and the laboratory experiments were observed for NaCl mole fractions varying between 0.40 and 0.77 due to a divergence between the predicted and the truly present products in the particles' solid fraction during the humidification cycle.

Survival of the enveloped bacteriophage Phi6 (a surrogate for SARS-CoV-2) in evaporated saliva microdroplets deposited on glass surfaces

Survival of respiratory viral pathogens in expelled saliva microdroplets is central to their transmission, yet the factors that determine survival in such microdroplets are not well understood. Here we combine microscopy imaging with virus viability assays to study survival of three bacteriophages suggested as good models for respiratory pathogens: the enveloped Phi6 (a surrogate for SARS-CoV-2), and the non-enveloped PhiX174 and MS2. We measured virus viability in human saliva microdroplets, SM buffer, and water following deposition on glass surfaces at various relative humidities (RH). Saliva and water microdroplets dried out rapidly, within minutes, at all tested RH levels (23%, 43%, 57%, and 78%), while SM microdroplets remained hydrated at RH ≥ 57%. Generally, the survival of all three viruses in dry saliva microdroplets was significantly greater than those in SM buffer and water under all RH (except PhiX174 in water under 57% RH survived the best among 3 media). Thus, atmosphere RH and microdroplet hydration state are not sufficient to explain virus survival, indicating that the virus-suspended medium, and association with saliva components in particular, likely play a role in virus survival. Uncovering the exact properties and components that make saliva a favorable environment for the survival of viruses, in particular enveloped ones like Phi6, is thus of great importance for reducing transmission of viral respiratory pathogens including SARS-CoV-2.