Currently, the impact of various phase states of aerosols on the climate is not well understood, especially for submicrometer sized aerosol particles that typically have extended lifetime in the atmosphere. This is largely due to the inherent size limitations present in current experimental techniques that aim to directly assess the phase states of fine aerosol particles. Herein we present a technique that uses atomic force microscopy to probe directly for the phase states of individual, submicrometer particles by using nanoindentation and nano-Wilhelmy methodologies as a function of relative humidity (RH) and ambient temperature conditions. When using these methodologies for substrate deposited individual sucrose particles, Young's modulus and surface tension can be quantified as a function of RH. We show that the force profiles collected to measure Young's modulus and surface tension can also provide both qualitative and quantitative assessments of phase states that accompany solid, semisolid, and liquid particle phases. Specifically, we introduce direct measurements of relative indentation depth and viscoelastic response distance on a single particle basis at a given applied force to quantitatively probe for the phase state as a function of RH and corresponding viscosity. Thus, we show that the three phase states and phase state transitions of sucrose can be identified and ultimately propose that this technique may also be used to study other atmospherically relevant systems.

Sea spray aerosols (SSAs) affect the Earth’s climate directly by scattering solar radiation and indirectly by acting as ice and cloud condensation nuclei. The relative magnitude of these effects remains uncertain, in part, from substantial compositional and morphological variability between individual particles. Here, the evolving heterogeneity within populations of primary SSAs produced from wave breaking of natural seawater within a wave flume is investigated. Over the course of the study, two successive phytoplankton blooms were induced in the seawater. The morphology, organic volume fraction, hygroscopicity, phase state, and surface tension of individual SSAs collected via deposition on a substrate were characterized using atomic force microscopy. Particles between ca. 0.3 and 1 μm in volume equivalent diameter displayed a distinctive morphology revealing an inorganic core coated with an organic shell. The inferred organic volume fraction was the largest at the peak of the first bloom. The corresponding shell thicknesses ranged from 21 to 40 nm at 20% relative humidity (RH). The organic shell phase state of the majority of the particles during both blooms was semisolid at 20% and 60% RH. At 20% RH, a minor fraction of the organic shells behaved as a solid, while at 60% RH some behaved as liquids during the first bloom. Similar results were evident at 20% RH for the second bloom but with no observed liquid particles at 60% RH. The thick, semisolid organic coatings could potentially reduce atmospheric water and gas uptake efficiencies onto SSAs at lower RH, along with the potential for ice nucleating activity. However, at 80% RH, the SSAs deliquesced and exhibited liquid-like behavior with surface tension values measured over individual particles of 41–87 mN m–1, demonstrating high particle-to-particle variability. The suppressed surface tension at 80% RH relative to pure water is attributed to the high concentrations of surface-active organic compounds, potentially further limiting the diffusion rate of gas molecules through the interface.

The impact of sea spray aerosols (SSAs) on Earth's climate remains uncertain in part due to size-dependent particle-to-particle variability in SSA physicochemical properties such as morphology, composition, phase state, and water uptake that can be further modulated by the environment relative humidity (RH). The current study investigates these properties as a function of particle size and RH, while focusing on submicrometer nascent SSA (0.1−0.6 μm) collected throughout a phytoplankton bloom. Filter-based thermal optical analysis, atomic force microscopy (AFM), and AFM photothermal infrared spectroscopy (AFM−PTIR) were utilized in this regard. AFM imaging at 20% RH identified five main SSA morphologies: prism-like, core−shell, rounded, rod, and aggregate. The majority of smaller SSAs throughout a bloom were rounded, while larger SSAs were core−shell. Filter-based measurements revealed an increasing organic mass fraction with decreasing SSA size. The organic matter is shown to primarily reside in a rounded and core−shell SSA, while the prism-like and rod SSA are predominantly inorganic salts (i.e., sodium chloride, nitrates, and sulfates) with relatively low organic content, as determined by AFM−PTIR spectroscopy. AFM phase state measurements at 20% RH revealed an increasing abundance of core−shell SSA with semisolid shells and rounded SSA with a solid phase state, as the particle size decreases. At 60% RH, shells of core−shell and rounded SSA uptake water, become less viscous, and their phase states change into either semisolid or liquid. Collectively, findings reveal the dynamic and size-dependent nature of SSA's morphology, composition, phase states, and water uptake, which should be considered to accurately predict their climate-related effects.

Individual airborne sea spray aerosol (SSA) particles show diversity in their morphologies and water uptake properties that are highly dependent on the biological, chemical, and physical processes within the sea subsurface and the sea surface microlayer. In this study, hygroscopicity data for model systems of organic compounds of marine origin mixed with NaCl are compared to data for authentic SSA samples collected in an ocean-atmosphere facility providing insights into the SSA particle growth, phase transitions and interactions with water vapor in the atmosphere. In particular, we combine single particle morphology analyses using atomic force microscopy (AFM) with hygroscopic growth measurements in order to provide important insights into particle hygroscopicity and the surface microstructure. For model systems, a range of simple and complex carbohydrates were studied including glucose, maltose, sucrose, laminarin, sodium alginate, and lipopolysaccharides. The measured hygroscopic growth was compared with predictions from the Extended-Aerosol Inorganics Model (E-AIM). It is shown here that the E-AIM model describes well the deliquescence transition and hygroscopic growth at low mass ratios but not as well for high ratios, most likely due to a high organic volume fraction. AFM imaging reveals that the equilibrium morphology of these single-component organic particles is amorphous. When NaCl is mixed with the organics, the particles adopt a core-shell morphology with a cubic NaCl core and the organics forming a shell similar to what is observed for the authentic SSA samples. The observation of such core-shell morphologies is found to be highly dependent on the salt to organic ratio and varies depending on the nature and solubility of the organic component. Additionally, single particle organic volume fraction AFM analysis of NaCl : glucose and NaCl : laminarin mixtures shows that the ratio of salt to organics in solution does not correspond exactly for individual particles -showing diversity within the ensemble of particles produced even for a simple two component system. IntroductionCovering a substantial area of the Earth's surface, the ocean serves as one of the main sources of particulate matter in the atmosphere. Sea spray aerosols (SSAs) are generated by breaking waves in marine environments and account for the largest atmospheric aerosol flux. it has a profound influence on the composition of aerosol particles as they escape across the interface. The SSML is a rich mixture of organic compounds such as fatty acids, fatty alcohols, sterols, carbohydrates, proteins and more complex colloids and aggregates exuded by phytoplankton such as lipopolysaccharides (LPSs). 11,12In the authentic samples collected from a pristine region of the Pacific Ocean, Gagosian and coworkers 13 detected alcohols, fatty acid salts and esters as specific tracers of ocean-derived organic compounds in atmospheric aerosols. The organic compounds include saccharides, fatty acids, and a few other organic classes. 13 Progress in analytic...

Understanding the role of sea spray aerosol (SSA) on climate and the environment is of great interest due to their high number concentration throughout the Earth's atmosphere. Despite being of fundamental importance, direct surface tension measurements of SSA relevant sub-micrometer particles are rare, largely due to their extremely small volumes. Herein, atomic force microscopy (AFM) is used to directly measure the surface tension of individual sub-micrometer SSA particle mimics at ambient temperature and varying relative humidity (RH). Specifically, we probed both atmospherically relevant and fundamentally important model systems including electrolyte salts, dicarboxylic acids, and saccharides as single components and mixtures. Our results show that the single particle surface tension depends on RH or solute mole percentage and chemical composition. Moreover, for liquid droplets at and below 100 Pa s in viscosity, or at corresponding RH, we show good agreement between the AFM single particle and the bulk solution surface tension measurements at overlapping concentration ranges. Thus, direct surface tension measurements of individual particles using AFM is shown over a wide range of chemical systems as a function of RH, solute mole percentage, and viscosity than previously reported.