Effects of Wind Speed on Size-Dependent Morphology and Composition of Sea Spray Aerosols

Variable wind speeds over the ocean can have a significant impact on the formation mechanism and physical-chemical properties of sea spray aerosols (SSA), which in turn influence their climate-relevant impacts. Herein, for the first time, we investigate the effects of wind speed on size-dependent morphology and composition of individual nascent SSA generated from wind-wave interactions of natural seawater within a wind-wave channel as a function of size and their particle-to-particle variability. Filter-based thermal optical analysis, atomic force microscopy (AFM), AFM infrared spectroscopy (AFM-IR), and scanning electron microscopy (SEM) were employed in this regard. This study focuses on SSA with sizes within 0.04-1.8 μm generated at two wind speeds: 10 m/s, representing a wind lull scenario over the ocean, and 19 m/s, indicative of the wind speeds encountered in stormy conditions. Filter-based measurements revealed a reduction of the organic mass fraction as the wind speed increases. AFM imaging at 20% relative humidity of individual SSA identified six main morphologies: prism-like, rounded, core-shell, rod, rod inclusion core-shell, and aggregates. At 10 m/s, most SSA were rounded, while at 19 m/s, core-shells became predominant. Based on AFM-IR, rounded SSA at both wind speeds had similar composition, mainly composed of aliphatic and oxygenated species, whereas the shells of core-shells displayed more oxygenated organics at 19 m/s and more aliphatic organics at 10 m/s. Collectively, our observations can be attributed to the disruption of the sea surface microlayer film structure at higher wind speeds. The findings reveal a significant impact of wind speed on morphology and composition of SSA, which should be accounted for accurate assessment of their climate 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.

Investigating the effect of volatility on the hygroscopicities of acetate nanoparticle aerosols by surface plasmon resonance microscopy

Under high relative humidity (RH) conditions, the release of volatile components (such as acetate) has a significant impact on the aerosol hygroscopicity. In this work, one surface plasmon resonance microscopy (SPRM) measurement system was introduced to determine the hygroscopic growth factors (GFs) of three acetate aerosols separately or mixed with glucose at different RHs. For Ca(CH3COO)2 or Mg(CH3COO)2 aerosols, the hygroscopic growth trend of each time was lower than that of the previous time in three cyclic humidification from 70% RH to 90% RH, which may be due to the volatility of acetic acid leading to the formation of insoluble hydroxide (Ca(OH)2 or Mg(OH)2) under high RH conditions. Then the third calculated GF (using the Zdanovskii-Stokes-Robinson method) for Ca(CH3COO)2 or Mg(CH3COO)2 in bicomponent aerosols with 1:1 mass ratio were 3.20% or 5.33% lower than that of the first calculated GF at 90% RH. The calculated results also showed that the hygroscopicity change of bicomponent aerosol was negatively correlated with glucose content, especially when the mass ratio of Mg(CH3COO)2 to glucose was 1:2, the GF at 90% RH only decreased by 4.67% after three cyclic humidification. Inductively coupled plasma atomic emission spectrum (ICP-AES) based measurements also indicated that the changes of Mg2+concentration in bicomponent was lower than that of the single-component. The results of this study reveal thatduring the efflorescence transitions of atmospheric nanoparticles, the organic acids diffusion rate may be inhibited by the coating effect of neutral organic components, and the particles aging cycle will be prolonged.

Water Adsorption on Hydrophilic Fibers and Porous and Deliquescent Materials: Cellulose, Polysaccharide, Silica, Inorganic Salt, Sugar Alcohol, and Amino Acid

Water adsorption isotherms are systematically summarized by using celluloses and polysaccharides as hydrophilic crystal/amorphous materials with functional groups, silicas as hydrophilic porous materials, and inorganic salts, sugar alcohols, and amino acids as hygroscopic deliquescent materials. For hydrophilic fibers such as celluloses and polysaccharides, water was adsorbed on amorphous solids, and water clusters were formed around functional groups. For porous materials such as silicas, capillary condensation occurred in the micropores of silicas. For deliquescent materials such as inorganic salts, sugar alcohols, and amino acids, water adsorption rapidly increased stepwise at a specific threshold relative humidity, accompanied with a structure transformation to a liquid state. In addition, the water activity (Aw) of materials used in packed products was able to be estimated from the water adsorption isotherms of the pure component. This indicated that the deliquescent materials have a great effect on the depression of Aw for the suppression of microbial growth at an extremely high water content. The deliquescent materials could be useful to develop new environmentally and sustainable products and technologies with the mediation of water vapor and/or hydration.

Physical properties of short chain aqueous organosulfate aerosol

Organosulfates comprise up to 30% of the organic fraction of aerosol. Organosulfate aerosol physical properties, such as water activity, density, refractive index, and surface tension, are key to predicting their impact on global climate. However, current understanding of these properties is limited. Here, we measure the physical properties of aqueous solutions containing sodium methyl or ethyl sulfate and parameterise the data as a function of solute concentration. The experimental data are compared to available literature data for organosulfates, as well as salts (sodium sulfate and sodium bisulfate) and organics (short alkyl chain length alcohols and carboxylic acids) to determine if the physical properties of organosulfates can be approximated by molecules of similar functionality. With the exception of water activity, we find that organosulfates have intermediate physical properties between those of the salts and short alkyl chain organics. This work highlights the importance of measuring and developing models for the physical properties of abundant atmospheric organosulfates in order to better describe aerosol's impact on climate.

Investigating the hygroscopicities of calcium and magnesium salt particles aged with SO2 using surface plasmon resonance microscopy

The hygroscopicities of calcium and magnesium salts strongly affect the environment and climate, but the aging products of these salts at high relative humidities (RHs) are still poorly understood. In this study, surface plasmon resonance microscopy (SPRM) was used to determine the hygroscopic growth factors (GFs) of Ca(NO₃)₂ and Mg(NO₃)₂ separately or mixed with galactose at different mass ratios at different RHs before and after aging. For all particles, the measured GFs showed no indication of deliquescence across the range of RHs tested, and overall hygroscopicity was clearly lower after than before aging. The Ca(NO₃)₂ and Mg(NO₃)₂ GFs at 90 % RH were 1.80 and 1.66, respectively, before aging and 1.33 and 1.42, respectively, after 4 h aging, meaning aging decreased the GFs by 26.11 % and 14.46 %, respectively. Aging decreased the hygroscopicity because insoluble or sparingly soluble substances (CaSO₃, CaSO₄, MgSO₃) formed and strongly changed the overall hygroscopicity. For bicomponent aerosols with different mass ratios, the GFs (calculated using the Zdanovskii-Stokes-Robinson method) of the other components except galactose at 90 % RH after 1 h aging were all lower, respectively, than the measured GFs of pure Ca(NO₃)₂ and Mg(NO₃)₂ after aging for 1 h, especially with the mass ratio of 1:2, their GFs have decreased by 14.63 % and 7.50 %, respectively. Subsequently, Ion chromatograms indicated that the peak area ratio of SO₄^2- to NO₃^- ratios were higher for the aged bicomponent particles than aged single-component particles, possibly because adding galactose improved the gas-liquid state stability during drying after the aging process and therefore promoted nitrate consumption and sulfate formation. The results indicated that organic components may play important roles in heterogeneous reactions between trace gases and multicomponent aerosols and should be considered in evaluating the impacts on submicron aerosol composition of high atmospheric SO₂ concentrations at high humidities.

Measurement of atmospheric nanoparticles: Bridging the gap between gas-phase molecules and larger particles

Atmospheric nanoparticles are crucial components contributing to fine particulate matter (PM_2.5), and therefore have significant effects on visibility, climate, and human health. Due to the unique role of atmospheric nanoparticles during the evolution process from gas-phase molecules to larger particles, a number of sophisticated experimental techniques have been developed and employed for online monitoring and characterization of the physical and chemical properties of atmospheric nanoparticles, helping us to better understand the formation and growth of new particles. In this paper, we firstly review these state-of-the-art techniques for investigating the formation and growth of atmospheric nanoparticles (e.g., the gas-phase precursor species, molecular clusters, physicochemical properties, and chemical composition). Secondly, we present findings from recent field studies on the formation and growth of atmospheric nanoparticles, utilizing several advanced techniques. Furthermore, perspectives are proposed for technique development and improvements in measuring atmospheric nanoparticles.

Effects of Atmospheric Aging Processes on Nascent Sea Spray Aerosol Physicochemical Properties

The effects of atmospheric aging on single-particle nascent sea spray aerosol (nSSA) physicochemical properties, such as morphology, composition, phase state, and water uptake, are important to understanding their impacts on the Earth's climate. The present study investigates these properties by focusing on the aged SSA (size range of 0.1-0.6 μm) and comparing with a similar size range nSSA, both generated at a peak of a phytoplankton bloom during a mesocosm study. The aged SSAs were generated by exposing nSSA to OH radicals with exposures equivalent to 4-5 days of atmospheric aging. Complementary filter-based thermal optical analysis, atomic force microscopy (AFM), and AFM photothermal infrared spectroscopy were utilized. Both nSSA and aged SSA showed an increase in the organic mass fraction with decreasing particle sizes. In addition, aging results in a further increase of the organic mass fraction, which can be attributed to new particle formation and oxidation of volatile organic compounds followed by condensation on pre-existing particles. The results are consistent with single-particle measurements that showed a relative increase in the abundance of aged SSA core-shells with significantly higher organic coating thickness, relative to nSSA. Increased hygroscopicity was observed for aged SSA core-shells, which had more oxygenated organic species. Rounded nSSA and aged SSA had similar hygroscopicity and no apparent changes in the composition. The observed changes in aged SSA physicochemical properties showed a significant size-dependence and particle-to-particle variability. Overall, results showed that the atmospheric aging can significantly influence the nSSA physicochemical properties, thus altering the SSA effects on the climate.

Ice-nucleating agents in sea spray aerosol identified and quantified with a holistic multimodal freezing model

Sea spray aerosol (SSA) is a widely recognized important source of ice-nucleating particles (INPs) in the atmosphere. However, composition-specific identification, nucleation processes, and ice nucleation rates of SSA-INPs have not been well constrained. Microspectroscopic characterization of ambient and laboratory-generated SSA confirms that water-borne exudates from planktonic microorganisms composed of a mixture of proteinaceous and polysaccharidic compounds act as ice-nucleating agents (INAs). These data and data from previously published mesocosm and wave channel studies are subsequently used to further develop the stochastic freezing model (SFM) producing ice nucleation rate coefficients for SSA-INPs. The SFM simultaneously predicts immersion freezing and deposition and homogeneous ice nucleation by SSA particles under tropospheric conditions. Predicted INP concentrations agree with ambient and laboratory measurements. In addition, this holistic freezing model is independent of the source and exact composition of the SSA particles, making it well suited for implementation in cloud and climate models.

The Sea Spray Chemistry and Particle Evolution study (SeaSCAPE): overview and experimental methods

Marine aerosols strongly influence climate through their interactions with solar radiation and clouds. However, significant questions remain regarding the influences of biological activity and seawater chemistry on the flux, chemical composition, and climate-relevant properties of marine aerosols and gases. Wave channels, a traditional tool of physical oceanography, have been adapted for large-scale ocean-atmosphere mesocosm experiments in the laboratory. These experiments enable the study of aerosols under controlled conditions which isolate the marine system from atmospheric anthropogenic and terrestrial influences. Here, we present an overview of the 2019 Sea Spray Chemistry and Particle Evolution (SeaSCAPE) study, which was conducted in an 11 800 L wave channel which was modified to facilitate atmospheric measurements. The SeaSCAPE campaign sought to determine the influence of biological activity in seawater on the production of primary sea spray aerosols, volatile organic compounds (VOCs), and secondary marine aerosols. Notably, the SeaSCAPE experiment also focused on understanding how photooxidative aging processes transform the composition of marine aerosols. In addition to a broad range of aerosol, gas, and seawater measurements, we present key results which highlight the experimental capabilities during the campaign, including the phytoplankton bloom dynamics, VOC production, and the effects of photochemical aging on aerosol production, morphology, and chemical composition. Additionally, we discuss the modifications made to the wave channel to improve aerosol production and reduce background contamination, as well as subsequent characterization experiments. The SeaSCAPE experiment provides unique insight into the connections between marine biology, atmospheric chemistry, and climate-relevant aerosol properties, and demonstrates how an ocean-atmosphere-interaction facility can be used to isolate and study reactions in the marine atmosphere in the laboratory under more controlled conditions.

Organic Coating Reduces Hygroscopic Growth of Phase-Separated Aerosol Particles

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

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.

Calcium bridging drives polysaccharide co-adsorption to a proxy sea surface microlayer

Saccharides comprise a significant mass fraction of organic carbon in sea spray aerosol (SSA), but the mechanisms through which saccharides are transferred from seawater to the ocean surface and eventually into SSA are unclear. It is hypothesized that saccharides cooperatively adsorb to other insoluble organic matter at the air/sea interface, known as the sea surface microlayer (SSML). Using a combination of surface-sensitive infrared reflection-absorption spectroscopy and all-atom molecular dynamics simulations, we demonstrate that the marine-relevant, anionic polysaccharide alginate co-adsorbs to an insoluble palmitic acid monolayer via divalent cationic bridging interactions. Ca2+ induces the greatest extent of alginate co-adsorption to the monolayer, evidenced by the ∼30% increase in surface coverage, whereas Mg2+ only facilitates one-third the extent of co-adsorption at seawater-relevant cation concentrations due to its strong hydration propensity. Na+ cations alone do not facilitate alginate co-adsorption, and palmitic acid protonation hinders the formation of divalent cationic bridges between the palmitate and alginate carboxylate moieties. Alginate co-adsorption is largely confined to the interfacial region beneath the monolayer headgroups, so surface pressure, and thus monolayer surface coverage, only changes the amount of alginate co-adsorption by less than 5%. Our results provide physical and molecular characterization of a potentially significant polysaccharide enrichment mechanism within the SSML.

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.

Cation-Driven Lipopolysaccharide Morphological Changes Impact Heterogeneous Reactions of Nitric Acid with Sea Spray Aerosol Particles

Lipopolysaccharides (LPS) in sea spray aerosol (SSA) particles have recently been shown to undergo heterogeneous reactions with HNO₃ in the atmosphere. Here, we integrate theory and experiment to further investigate how the most abundant sea salt cations, Na⁺, Mg²⁺, and Ca²⁺, impact HNO₃ reactions with LPS-containing SSA particles. Aerosol reaction flow tube studies show that heterogeneous reactions of SSA particles with divalent cation (Mg²⁺ and Ca²⁺) and LPS signatures were less reactive with HNO₃ than those dominated by monovalent cations (Na⁺). All-atom molecular dynamics simulations of model LPS aggregates suggest that divalent cations cross-link the oligosaccharide chains to increase molecular aggregation and rigidity, which changes the particle phase and morphology, decreases water diffusion, and consequently decreases the reactive uptake of HNO₃. This study provides new insight into how complex chemical interactions between ocean-derived salts and biogenic organic species can impact the heterogeneous reactivity of SSA particles.

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.

In situ measurements of human cough aerosol hygroscopicity

The airborne dynamics of respiratory droplets, and the transmission routes of pathogens embedded within them, are governed primarily by the diameter of the particles. These particles are composed of the fluid which lines the respiratory tract, and is primarily mucins and salts, which will interact with the atmosphere and evaporate to reach an equilibrium diameter. Measuring organic volume fraction (OVF) of cough aerosol has proved challenging due to large variability and low material volume produced after coughing. Here, the diametric hygroscopic growth factors (GF) of the cough aerosol produced by healthy participants were measured in situ using a rotating aerosol suspension chamber and a humidification tandem differential mobility analyser. Using hygroscopicity models, it was estimated that the average OVF in the evaporated cough aerosol was 0.88 ± 0.07 and the average GF at 90% relative humidity (RH) was 1.31 ± 0.03. To reach equilibrium in dry air the droplets will reduce in diameter by a factor of approximately 2.8 with an evaporation factor of 0.36 ± 0.05. Hysteresis was observed in cough aerosol at RH = ∼35% and RH = ∼65% for efflorescence and deliquescence, respectively, and may depend on the OVF. The same behaviour and GF were observed in nebulized bovine bronchoalveolar lavage fluid.

Atomic Force Microscopy: An Emerging Tool in Measuring the Phase State and Surface Tension of Individual Aerosol Particles

Atmospheric aerosols are suspended particulate matter of varying composition, size, and mixing state. Challenges remain in understanding the impact of aerosols on the climate, atmosphere, and human health. The effect of aerosols depends on their physicochemical properties, such as their hygroscopicity, phase state, and surface tension. These properties are dynamic with respect to the highly variable relative humidity and temperature of the atmosphere. Thus, experimental approaches that permit the measurement of these dynamic properties are required. Such measurements also need to be performed on individual, submicrometer-, and supermicrometer-sized aerosol particles, as individual atmospheric particles from the same source can exhibit great variability in their form and function. In this context, this review focuses on the recent emergence of atomic force microscopy as an experimental tool in physical, analytical, and atmospheric chemistry that enables such measurements. Remaining challenges are noted and suggestions for future studies are offered.

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.

Quantifying the hygroscopic properties of cyclodextrin containing aerosol for drug delivery to the lungs

Aerosol dynamics is important to quantify in drug delivery to the lungs with the aim of delivering therapeutics to a target location and optimising drug efficacy. The macrocycle (2-hydroxypropyl)-β-cyclodextrin (2-HP-β-CD) is thought to alleviate symptoms associated with neurodegenerative diseases when inhaled but the hygroscopic response is not well understood. Here we measure the hygroscopic growth of individual aqueous aerosol containing 2-HP-β-CD in optical tweezers through analysis of morphology-dependent resonances arising in Raman spectra. Droplets are analysed in the size range of 3-5 μm in radius. The evolving radius and refractive index of each droplet are measured in response to change in relative humidity from 98-20% to determine mass and radius based hygroscopic growth factors, and compared with dynamic vapour sorption measurements. Bulk solution refractive index and density measurements were used in accordance with the self-consistent Lorenz-Lorentz rule to determine melt solute and droplet properties. The refractive index of 2-HP-β-CD was determined to be 1.520 ± 0.002 with a density of 1.389 ± 0.005 g cm^-3. To our knowledge, we show the first aerosol measurements of 2-HP-β-CD and determine hygroscopicity. By quantifying the hygroscopic growth and physicochemical properties of 2-HP-β-CD, the impact of aerosol dynamics can be accounted for in tailoring drug formulations and informing models used to predict drug deposition patterns within the respiratory system.

Aqueous solvation of the chloride ion revisited with density functional theory: impact of correlation and exchange approximations

Aqueous chloride is simulated using PBE-D3, PBE0-D3, and SCAN to investigate the impact of exchange and correlation approximations; we find the exact exchange fraction strongly impacts the energetics and polarizability of solvated chloride.

Measurements of Immersion Freezing and Heterogeneous Chemistry of Atmospherically Relevant Single Particles with Micro-Raman Spectroscopy

In the atmosphere, there are several different trajectories by which particles can nucleate ice; two of the major pathways are deposition and immersion freezing. Single particle depositional freezing has been widely studied with spectroscopic methods while immersion freezing has been predominantly studied either for particles within bulk aqueous solutions or using optical imaging of single particles. Of the few existing spectroscopic methods that monitor immersion freezing, there are limited opportunities for investigating the impact of heterogeneous chemistry on freezing. Herein, we describe a method that couples a confocal Raman spectrometer with an environmental cell to investigate single particle immersion freezing along with the capability to investigate in situ the impact of heterogeneous reactions with ozone and other trace gases on ice nucleation. This system, which has been rigorously calibrated (temperature and relative humidity) across a large dynamic range, is used to investigate low temperature water uptake and heterogeneous ice nucleation of atmospherically relevant single particles deposited on a substrate. The use of Raman spectroscopy provides important insights into the phase state and chemical composition of ice nuclei and, thus, insights into cloud formation.

Windows into Microbial Seascapes: Advances in Nanoscale Imaging and Application to Marine Sciences

Geochemical cycles of all nonconservative elements are mediated by microorganisms over nanometer spatial scales. The pelagic seascape is known to possess microstructure imposed by heterogeneous distributions of particles, polymeric gels, biologically important chemicals, and microbes. While indispensable, most traditional oceanographic observational approaches overlook this heterogeneity and ignore subtleties, such as activity hot spots, symbioses, niche partitioning, and intrapopulation phenotypic variations, that can provide a deeper mechanistic understanding of planktonic ecosystem function. As part of the movement toward cultivation-independent tools in microbial oceanography, techniques to examine the ecophysiology of individual populations and their role in chemical transformations at spatial scales relevant to microorganisms have been developed. This review presents technologies that enable geochemical and microbiological interrogations at spatial scales ranging from 0.02 to a few hundred micrometers, particularly focusing on atomic force microscopy, nanoscale secondary ion mass spectrometry, and confocal Raman microspectroscopy and introducing promising approaches for future applications in marine sciences.

Sea Spray Aerosol: Where Marine Biology Meets Atmospheric Chemistry

Atmospheric aerosols have long been known to alter climate by scattering incoming solar radiation and acting as seeds for cloud formation. These processes have vast implications for controlling the chemistry of our environment and the Earth's climate. Sea spray aerosol (SSA) is emitted over nearly three-quarters of our planet, yet precisely how SSA impacts Earth's radiation budget remains highly uncertain. Over the past several decades, studies have shown that SSA particles are far more complex than just sea salt. Ocean biological and physical processes produce individual SSA particles containing a diverse array of biological species including proteins, enzymes, bacteria, and viruses and a diverse array of organic compounds including fatty acids and sugars. Thus, a new frontier of research is emerging at the nexus of chemistry, biology, and atmospheric science. In this Outlook article, we discuss how current and future aerosol chemistry research demands a tight coupling between experimental (observational and laboratory studies) and computational (simulation-based) methods. This integration of approaches will enable the systematic interrogation of the complexity within individual SSA particles at a level that will enable prediction of the physicochemical properties of real-world SSA, ultimately illuminating the detailed mechanisms of how the constituents within individual SSA impact climate.

Collapse Mechanisms of Nascent and Aged Sea Spray Aerosol Proxy Films

Sea spray aerosol (SSA) is highly enriched in marine-derived organic compounds during seasons of high biological productivity, and saturated fatty acids comprise one of the most abundant classes of molecules. Fatty acids and other organic compounds form a film on SSA surfaces, and SSA particle surface-area-to-volume ratios are altered during aging in the marine boundary layer (MBL). To understand SSA surface organization and its role during dynamic atmospheric conditions, an SSA proxy fatty acid film and its individual components stearic acid (SA), palmitic acid (PA), and myristic acid (MA) are studied separately using surface pressure–area ( Π − A ) isotherms and Brewster angle microscopy (BAM). The films were spread on an aqueous NaCl subphase at pH 8.2, 5.6, and 2.0 to mimic nascent to aged SSA aqueous core composition in the MBL, respectively. We show that the individual fatty acid behavior differs from that of the SSA proxy film, and at nascent SSA pH the mixture yields a monolayer with intermediate rigidity that folds upon film compression to the collapse state. Acidification causes the SSA proxy film to become more rigid and form 3D nuclei. Our results reveal film morphology alterations, which are related to SSA reflectivity, throughout various stages of SSA aging and provide a better understanding of SSA impacts on climate.

The Hydropathy Scale as a Gauge of Hygroscopicity in Sub-Micron Sodium Chloride-Amino Acid Aerosols

Sodium chloride, NaCl, is commonly used as a proxy for sea spray aerosols. However, field work has demonstrated that sea spray aerosols also often contain a significant organic component. In this work, we examine the effect of amino acids on the hygroscopic properties of NaCl aerosols using a Fourier transform infrared spectrometer coupled to a flow-cell apparatus. It is found that the effect can be drastically different depending on the nature of the amino acid. More hydrophilic amino acids such as glycine lead to continuous hygroscopic growth of internally mixed NaCl-amino acid aerosols generated from an equimolar precursor solution. However, more hydrophobic amino acids such as alanine do not significantly alter the hygroscopicity of NaCl aerosols. The hydropathy scale is found to be a good qualitative diagnostic for the effect that an amino acid will have on the hygroscopicity of NaCl.

Investigation on the hygroscopicity of oxalic acid and atmospherically relevant oxalate salts under sub- and supersaturated conditions

Oxalic acid (OxA) is an end product in the oxidation of many organic compounds, and therefore is ubiquitous in the atmosphere and is often the most abundant organic species in ambient aerosols. To better understand the hygroscopic properties of OxA under sub- and supersaturated conditions in the atmosphere, we investigated the hygroscopic growth and cloud condensation nuclei (CCN) activation ability of pure OxA and its salts using a hygroscopic tandem differential mobility analyzer (HTDMA) and cloud condensation nuclei counter (CCNC), respectively. OxA particles absorb water under >45% RH, suggesting that the initial phase state might be an amorphous solid. The measured hygroscopic growth factor (HGF) of OxA at 90% RH was 1.47. We found that the HGF of ammonium oxalate (NH4-Ox) was larger than that of OxA, whereas HGFs of sodium, calcium, and magnesium oxalates (Na-Ox, Ca-Ox, and Mg-Ox) were smaller than that of OxA particles. Potassium oxalate (K-Ox) behaved like a typical water-soluble inorganic salt, exhibiting deliquescence and efflorescence transitions at around 85% and 50% RH, respectively. Na-Ox exhibited strong activation capabilities among all the investigated salts, followed by NH4-Ox and K-Ox as inferred from the activation ratios (CCN/CN) against supersaturations (SS). On the other hand, Ca-Ox showed moderate activation ability and Mg-Ox showed poor CCN activation ability. We also observed significantly higher κCCN values compared to κHTDMA for pure OxA and its salts (NH4-Ox and Na-Ox), suggesting that the condensation of OxA into the aqueous phase occurs during water uptake. These findings improve the fundamental understanding of hygroscopic behaviors and phase states of oxalic acid and its salts under sub- and supersaturated conditions in the atmosphere and impacts of hygroscopicity on the direct and indirect effects of aerosol particles.

Lab on a tip: atomic force microscopy – photothermal infrared spectroscopy of atmospherically relevant organic/inorganic aerosol particles in the nanometer to micrometer size range

AFM-PTIR is utilized to analyze atmospherically relevant multicomponent substrate deposited aerosol particles.

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.