Surfaces of Atmospheric Droplet Models Probed with Synchrotron XPS on a Liquid Microjet.

ConspectusThe atmosphere is a key part of the earth system comprising myriad chemical species in all basic forms of matter. Ubiquitous nano- and microscopic aerosol particles and cloud droplets suspended in the air play crucial roles in earth's climate and the formation of air pollution. Surfaces are a prominent part of aerosols and droplets, due to the high surface area to bulk volume ratios, but very little is known about their specific properties. Many atmospheric compounds are surface-active, leading to enhanced surface concentrations in aqueous solutions. Their distribution between the surface and bulk may determine heterogeneous chemistry and many other properties of aerosol and cloud droplets, but has not been directly observed.We used X-ray photoelectron spectroscopy (XPS) to obtain direct molecular-level information on the surface composition and structure of aqueous solutions of surface-active organics as model systems for atmospheric aerosol and cloud droplets. XPS is a vacuum-based technique enabled for volatile aqueous organic samples by the application of a high-speed liquid microjet. In combination with brilliant synchrotron X-rays, the chemical specificity of XPS allows distinction between elements in different chemical states and positions within molecular structures. We used core-level C 1s and N 1s signals to identify the alkyl and hydrophilic groups of atmospheric carboxylic acids, alkyl-amines, and their conjugate acids and bases. From this, we infer changes in the orientation of surface-adsorbed species and quantify their relative abundances in the surface. XPS-derived surface enrichments of the organics follow trends expected from their surface activities and we observed a preferential orientation at the surface with the hydrophobic alkyl chains pointing increasingly outward from the solution at higher concentrations. This provides a first direct experimental observation of well-established concepts of surface adsorption and confirms the soundness of the method.We mapped relative abundances of conjugate acid-base pairs in the aqueous solution surfaces from the respective intensities of distinctive XPS signals. For each pair, the protonation equilibrium was significantly shifted toward the neutral form in the surface, compared to the bulk solution, across the full pH range. This represents an apparent shift of the pKa in the surface, which may be toward either higher or lower pH, depending on whether the acid or base form of the pair is the neutral species. The surface shifts are broadly consistent with the relative differences in surface enrichment of the individual acid and base conjugates in binary aqueous solutions, with additional contributions from nonideal interactions in the surface. In aqueous mixtures of surface-active carboxylate anions with ammonium salts at near-neutral pH, we found that the conjugate carboxylic acids were further strongly enhanced. This occurs as the coadsorption of weakly basic carboxylate anions and weakly acidic ammonium cations forms ion-pair surface layers with strongly enhanced local abundances, increasing the probability of net proton transfer according to Le Chatelier's principle. The effect is stronger when the evaporation of ammonia from the surface further contributes to irreversibly perturb the protonation equilibrium, leaving a surplus of carboxylic acid. These surface-specific effects may profoundly influence atmospheric chemistry mediated by aqueous aerosols and cloud droplets but are currently not taken into account in atmospheric models.

Surface-Area-to-Volume Ratio Determines Surface Tensions in Microscopic, Surfactant-Containing Droplets.

The surface composition of aerosol droplets is central to predicting cloud droplet number concentrations, understanding atmospheric pollutant transformation, and interpreting observations of accelerated droplet chemistry. Due to the large surface-area-to-volume ratios of aerosol droplets, adsorption of surfactant at the air-liquid interface can deplete the droplet's bulk concentration, leading to droplet surface compositions that do not match those of the solutions that produced them. Through direct measurements of individual surfactant-containing, micrometer-sized droplet surface tensions, and fully independent predictive thermodynamic modeling of droplet surface tension, we demonstrate that, for strong surfactants, the droplet's surface-area-to-volume ratio becomes the key factor in determining droplet surface tension rather than differences in surfactant properties. For the same total surfactant concentration, the surface tension of a droplet can be >40 mN/m higher than that of the macroscopic solution that produced it. These observations indicate that an explicit consideration of surface-area-to-volume ratios is required when investigating heterogeneous chemical reactivity at the surface of aerosol droplets or estimating aerosol activation to cloud droplets.

Inversion model for extracting chemically resolved depth profiles across liquid interfaces of various configurations from XPS data: PROPHESY.

PROPHESY, a technique for the reconstruction of surface-depth profiles from X-ray photoelectron spectroscopy data, is introduced. The inversion methodology is based on a Bayesian framework and primal-dual convex optimization. The acquisition model is developed for several geometries representing different sample types: plane (bulk sample), cylinder (liquid microjet) and sphere (droplet). The methodology is tested and characterized with respect to simulated data as a proof of concept. Possible limitations of the method due to uncertainty in the attenuation length of the photo-emitted electron are illustrated.

Quantitative alignment parameter estimation for analyzing X-ray photoelectron spectra.

The interpretation of X-ray photoelectron spectroscopy (XPS) data relies on measurement models that depend on several parameters, including the photoelectron attenuation length and X-ray photon flux. However, some of these parameters are not known, because they are not or cannot be measured. The unknown geometrical parameters can be lumped together in a multiplicative factor, the alignment parameter. This parameter characterizes the ability of the exciting light to interact with the sample. Unfortunately, the absolute value of the alignment parameter cannot be measured directly, in part because it depends on the measurement model. Instead, a proxy for the experimental alignment is often estimated, which is closely related to the alignment parameter. Here, a method for estimating the absolute value of the alignment parameter based on the raw XPS spectra (i.e. non-processed photoelectron counts), the geometry of the sample and the photoelectron attenuation length is presented. The proposed parameter estimation method enables the quantitative analysis of XPS spectra using a simplified measurement model. All computations can be executed within the open and free Julia language framework PROPHESY. To demonstrate feasibility, the alignment parameter estimation method is first tested on simulated data with known acquisition parameters. The method is then applied to experimental XPS data and a strong correlation between the estimated alignment parameter and the typically used alignment proxy is shown.

Missed Evaporation from Atmospherically Relevant Inorganic Mixtures Confounds Experimental Aerosol Studies.

Sea salt aerosol particles are highly abundant in the atmosphere and play important roles in the global radiative balance. After influence from continental air, they are typically composed of Na+, Cl-, NH4+, and SO42- and organics. Analogous particle systems are often studied in laboratory settings by atomizing and drying particles from a solution. Here, we present evidence that such laboratory studies may be consistently biased in that they neglect losses of solutes to the gas phase. We present experimental evidence from a hygroscopic tandem differential mobility analyzer and an aerosol mass spectrometer, further supported by thermodynamic modeling. We show that, at normally prevailing laboratory aerosol mass concentrations, for mixtures of NaCl and (NH4)2SO4, a significant portion of the Cl- and NH4+ ions are lost to the gas phase, in some cases, leaving mainly Na2SO4 in the dry particles. Not considering losses of solutes to the gas phase during experimental studies will likely result in misinterpretation of the data. One example of such data is that from particle water uptake experiments. This may bias the explanatory models constructed from the data and introduce errors inte predictions made by air quality or climate models.

Dynamics and Sorption Kinetics of Methanol Monomers and Clusters on Nopinone Surfaces.

Organic-organic interactions play important roles in secondary organic aerosol formation, but the interactions are complex and poorly understood. Here, we use environmental molecular beam experiments combined with molecular dynamics simulations to investigate the interactions between methanol and nopinone, as atmospheric organic proxies. In the experiments, methanol monomers and clusters are sent to collide with three types of surfaces, i.e., graphite, thin nopinone coating on graphite, and nopinone multilayer surfaces, at temperatures between 140 and 230 K. Methanol monomers are efficiently scattered from the graphite surface, whereas the scattering is substantially suppressed from nopinone surfaces. The thermal desorption from the three surfaces is similar, suggesting that all the surfaces have weak or similar influences on methanol desorption. All trapped methanol molecules completely desorb within a short experimental time scale at temperatures of 180 K and above. At lower temperatures, the desorption rate decreases, and a long experimental time scale is used to resolve the desorption, where three desorption components are identified. The fast component is beyond the experimental detection limit. The intermediate component exhibits multistep desorption character and has an activation energy of Ea = 0.18 ± 0.03 eV, in good agreement with simulation results. The slow desorption component is related to diffusion processes due to the weak temperature dependence. The molecular dynamics results show that upon collisions the methanol clusters shatter, and the shattered fragments quickly diffuse and recombine to clusters. Desorption involves a series of processes, including detaching from clusters and desorbing as monomers. At lower temperatures, methanol forms compact cluster structures while at higher temperatures, the methanol molecules form layered structures on the nopinone surface, which are visible in the simulation. Also, the simulation is used to study the liquid-liquid interaction, where the methanol clusters completely dissolve in liquid nopinone, showing ideal organic-organic mixing.

Gas-to-Particle Partitioning of Cyclohexene- and α-Pinene-Derived Highly Oxygenated Dimers Evaluated Using COSMOtherm.

Oxidized organic compounds are expected to contribute to secondary organic aerosol (SOA) if they have sufficiently low volatilities. We estimated saturation vapor pressures and activity coefficients (at infinite dilution in water and a model water-insoluble organic phase) of cyclohexene- and α-pinene-derived accretion products, "dimers", using the COSMOtherm19 program. We found that these two property estimates correlate with the number of hydrogen bond-donating functional groups and oxygen atoms in the compound. In contrast, when the number of H-bond donors is fixed, no clear differences are seen either between functional group types (e.g., OH or OOH as H-bond donors) or the formation mechanisms (e.g., gas-phase radical recombination vs liquid-phase closed-shell esterification). For the cyclohexene-derived dimers studied here, COSMOtherm19 predicts lower vapor pressures than the SIMPOL.1 group-contribution method in contrast to previous COSMOtherm estimates using older parameterizations and nonsystematic conformer sampling. The studied dimers can be classified as low, extremely low, or ultra-low-volatility organic compounds based on their estimated saturation mass concentrations. In the presence of aqueous and organic aerosol particles, all of the studied dimers are likely to partition into the particle phase and thereby contribute to SOA formation.

SO2 formation and peroxy radical isomerization in the atmospheric reaction of OH radicals with dimethyl disulfide.

The atmospheric reaction of OH radicals with dimethyl disulfide, CH3SSCH3, proceeds primarily via OH addition forming CH3S and CH3SOH as reactive intermediates, and to a lesser extent via H-abstraction resulting in the peroxy radical CH3SSCH2OO in the presence of O2. The latter undergoes a fast two-step isomerization process leading to HOOCH2SSCHO. CH3S and CH3SOH are both converted to SO2 and CH3O2 with near unity yields under atmospheric conditions.

Is Decreased Xylem Sap Surface Tension Associated With Embolism and Loss of Xylem Hydraulic Conductivity in Pathogen-Infected Norway Spruce Saplings?

Increased abiotic stress along with increasing temperatures, dry periods and forest disturbances may favor biotic stressors such as simultaneous invasion of bark beetle and ophiostomatoid fungi. It is not fully understood how tree desiccation is associated with colonization of sapwood by fungi. A decrease in xylem sap surface tension (σxylem) as a result of infection has been hypothesized to cause xylem embolism by lowering the threshold for air-seeding at the pits between conduits and disruptions in tree water transport. However, this hypothesis has not yet been tested. We investigated tree water relations by measuring the stem xylem hydraulic conductivity (Kstem), σxylem, stem relative water content (RWCstem), and water potential (Ψstem), and canopy conductance (gcanopy), as well as the compound composition in xylem sap in Norway spruce (Picea abies) saplings. We conducted our measurements at the later stage of Endoconidiophora polonica infection when visible symptoms had occurred in xylem. Saplings of two clones (44 trees altogether) were allocated to treatments of inoculated, wounded control and intact control trees in a greenhouse. The saplings were destructively sampled every second week during summer 2016. σxylem, Kstem and RWCstem decreased following the inoculation, which may indicate that decreased σxylem resulted in increased embolism. gcanopy did not differ between treatments indicating that stomata responded to Ψstem rather than to embolism formation. Concentrations of quinic acid, myo-inositol, sucrose and alkylphenol increased in the xylem sap of inoculated trees. Myo-inositol concentrations also correlated negatively with σxylem and Kstem. Our study is a preliminary investigation of the role of σxylem in E. polonica infected trees based on previous hypotheses. The results suggest that E. polonica infection can lead to a simultaneous decrease in xylem sap surface tension and a decline in tree hydraulic conductivity, thus hampering tree water transport.

Ultrafast diffusion exchange nuclear magnetic resonance.

The exchange of molecules between different physical or chemical environments due to diffusion or chemical transformations has a crucial role in a plethora of fundamental processes such as breathing, protein folding, chemical reactions and catalysis. Here, we introduce a method for a single-scan, ultrafast NMR analysis of molecular exchange based on the diffusion coefficient contrast. The method shortens the experiment time by one to four orders of magnitude. Consequently, it opens the way for high sensitivity quantification of important transient physical and chemical exchange processes such as in cellular metabolism. As a proof of principle, we demonstrate that the method reveals the structure of aggregates formed by surfactants relevant to aerosol research.

Improving Solubility and Activity Estimates of Multifunctional Atmospheric Organics by Selecting Conformers in COSMOtherm.

We estimated aqueous solubilities and activity coefficients of atmospherically relevant highly oxidized multifunctional organic compounds in binary mixtures with water at temperatures between 278.15 and 338.15 K, using the COSMOtherm program. Physicochemical properties of organic aerosol constituents are needed in the modeling of atmospheric aerosol processes. As experimental data are often impossible to obtain, reliable estimates from theoretical approaches are a promising path to fill this gap. We investigated the effect of intramolecular hydrogen bonds on the estimation of these condensed-phase properties, attempting to improve the agreement between experimental and estimated values. Citric, tartaric, malic, and maleic acids, which are often used in atmospheric models as representatives of oxidized compounds, were selected to benchmark our calculations. In addition, we estimated aqueous solubilities and activity coefficients of α-pinene-derived organosulfates and highly oxidized isoprene-derived organic compounds, for which no experimental data are available. Our results indicate that the absolute aqueous solubility and activity coefficient estimates of citric, tartaric, malic, and maleic acids, and likely other multifunctional organics, can be improved significantly by selecting conformers on the basis of their intramolecular hydrogen bonding in COSMOtherm calculations.

The surface tension of surfactant-containing, finite volume droplets.

Surface tension influences the fraction of atmospheric particles that become cloud droplets. Although surfactants are an important component of aerosol mass, the surface tension of activating aerosol particles is still unresolved, with most climate models assuming activating particles have a surface tension equal to that of water. By studying picoliter droplet coalescence, we demonstrate that surfactants can significantly reduce the surface tension of finite-sized droplets below the value for water, consistent with recent field measurements. Significantly, this surface tension reduction is droplet size-dependent and does not correspond exactly to the macroscopic solution value. A fully independent monolayer partitioning model confirms the observed finite-size-dependent surface tension arises from the high surface-to-volume ratio in finite-sized droplets and enables predictions of aerosol hygroscopic growth. This model, constrained by the laboratory measurements, is consistent with a reduction in critical supersaturation for activation, potentially substantially increasing cloud droplet number concentration and modifying radiative cooling relative to current estimates assuming a water surface tension. The results highlight the need for improved constraints on the identities, properties, and concentrations of atmospheric aerosol surfactants in multiple environments and are broadly applicable to any discipline where finite volume effects are operative, such as studies of the competition between reaction rates within the bulk and at the surface of confined volumes and explorations of the influence of surfactants on dried particle morphology from spray driers.

The atmospheric impacts of initiatives advancing shifts towards low-emission mobility: A scoping review.

In an urban environment, people's daily traffic choices are reflected in emissions and the resulting local air composition, or air quality. Traffic contributes to the emissions of both carbon dioxide (CO2), affecting climate, and particulate matter (PM), affecting atmospheric chemistry and human health. While the development of city infrastructure is not in the hands of individuals, it is their transport mode choices that constitute traffic. In this scoping review we analyse 108 initiatives from around the world potentially influencing individual travel behaviour and producing changes in the shares of different transport modes (modal shifts). The targets, types and techniques of initiatives are identified. Examples of economic, regulative, structural and persuasive initiatives are included. Special focus is on whether the impacts on CO2 emissions, PM emissions and/or PM concentrations have been quantitatively evaluated, and on the quality and results of the evaluations. We observe that a variety of targets can motivate actions that lead to modal shifts and emission reductions. The results indicate that the level of atmospheric evaluations is low: absolute or relative changes in emissions and/or concentrations had been evaluated for only 31% (N = 34) of the reviewed initiatives, with substantial heterogeneity in quality. Sanctions, such as congestion charge and restrictions, have more likely been evaluated in peer reviewed analyses than incentives. Scientific evaluations of impacts on ambient PM concentrations are especially scarce (N = 4), although Air Quality is the primary target of 13% of actions and secondary target for at least 12%. We discuss the determinants of success and failure, when it comes to different types of initiatives, emission reductions and evaluations. A high-quality evaluation of atmospheric impacts captures the following: correct data about the modal shift (rate and direction), exclusion of external factors affecting the shift and emissions, and possible indirect impacts of the shift.

Effects of surface tension time-evolution for CCN activation of a complex organic surfactant.

The physical processes and time scales underlying the evolution of surface tension in atmospheric solution droplets are largely unaccounted for in present models describing cloud droplet formation. Adsorption of surface-active molecules at the surface of a solution droplet depresses the droplet surface tension but also depletes solute from the droplet bulk, which have opposing and sometimes canceling effects in cloud droplet formation. In this work, we study the effect of time-evolving surface tension for cloud droplet activation of particles composed of Nordic Aquatic Fulvic Acid (NAFA) mixed with sodium chloride (NaCl). We model the formation of cloud droplets using Köhler theory with surface tension depression and bulk/surface partitioning evaluated from two different thermodynamic surface models. Continuous ternary parameterizations were constructed from surface tension measurements of macroscopic droplets at different time steps after the formation of a droplet surface. The predicted results are compared to previous measurements of mixed NAFA-NaCl cloud condensation nuclei (CCN) activity and a bulk solution model that does not take the NAFA bulk/surface partitioning equilibrium into account. Whereas the bulk model shows a trend in cloud droplet formation following that of macroscopic surface tension depression with time, the variation with time essentially disappears when bulk/surface partitioning is taken explicitly into account during droplet activation. For all equilibrium time steps considered, the effect of surface tension depression in the NAFA-NaCl system is counteracted by the depletion of solute from the finite-sized droplet bulk phase. Our study highlights that a comprehensive data set is necessary to obtain continuous parameterizations of surface tension and other solution properties required to fully account for the bulk/surface partitioning in growing droplets. To our knowledge, no similar data set currently exists for other aqueous organic systems of atmospheric interest. Additional work is necessary to deconvolve the effects of bulk/surface partitioning in the context of time-evolution on cloud droplet activation and to determine whether the results presented here can be further generalized.

Solubility and Activity Coefficients of Atmospheric Surfactants in Aqueous Solution Evaluated Using COSMOtherm.

Fatty acids (CH3(CH2)n-2COOH) and their salts are an important class of atmospheric surfactants. Here, we use COSMOtherm to predict solubility and activity coefficients for C2-C12 fatty acids with even number of carbon atoms and their sodium salts in binary water solutions and also in ternary water-inorganic salt solutions. COSMOtherm is a continuum solvent model implementation which can calculate properties of complex systems using quantum chemistry and thermodynamics. Calculated solubility values of the organic acids in pure water are in good agreement with reported experimental values. The comparison of the COSMOtherm-derived Setschenow constants for ternary solutions comprising NaCl with the corresponding experimental values from the literature shows that COSMOtherm overpredicts the salting out effect in all cases except for the solutions of acetic acid. The calculated activity and mean activity coefficients of fatty acids and fatty acid sodium salts, respectively, show deviation of the systems from ideal solution. The computed mean activity coefficients of the fatty acid salts in binary systems are in better agreement with experimentally derived values for the organic salts with longer aliphatic chain (C8-C10). The deviation of the solutions from ideality could lead to biased estimations of cloud condensation nuclei number concentrations if not considered in Köhler calculations and cloud microphysics.

Strong Even/Odd Pattern in the Computed Gas-Phase Stability of Dicarboxylic Acid Dimers: Implications for Condensation Thermodynamics.

The physical properties of small straight-chain dicarboxylic acids are well known to exhibit even/odd alternations with respect to the carbon chain length. For example, odd numbered diacids have lower melting points and higher saturation vapor pressures than adjacent even numbered diacids. This alternation has previously been explained in terms of solid-state properties, such as higher torsional strain of odd number diacids. Using quantum chemical methods, we demonstrate an additional contribution to this alternation in properties resulting from gas-phase dimer formation. Due to a combination of hydrogen bond strength and torsional strain, dimer formation in the gas phase occurs efficiently for glutaric acid (C5) and pimelic acid (C7) but is unfavorable for succinic acid (C4) and adipic acid (C6). Our results indicate that a significant fraction of the total atmospheric gas-phase concentration of glutaric and pimelic acid may consist of dimers.

Cloud droplet activation of organic-salt mixtures predicted from two model treatments of the droplet surface.

The droplet surface plays important roles in the interaction between organic aerosols with clouds and climate. Surface active organic compounds can partition to the droplet surface, depleting the solute from the droplet bulk or depressing the droplet surface tension. This may in turn affect the shape of the droplet growth curve, threshold of aerosol activation into cloud droplets, activated droplet size distributions, and cloud radiative effects. In this work, a new monolayer model along with a traditional Gibbs adsorption isotherm model was used in conjunction with equilibrium Köhler theory to predict cloud condensation nuclei (CCN) activation of both simple and complex surface active model aerosol systems. For the surface active aerosol considered, the monolayer droplet model produces similar results to the Gibbs model as well as comparable results to CCN measurements from the literature, even for systems where specific molecular identities and aqueous properties are unknown. The monolayer model is self-contained and fully prognostic, and provides a versatile, conceptually simple, yet physically based model for understanding the role of organic surfactants in cloud droplet formation.

Shifted equilibria of organic acids and bases in the aqueous surface region.

Acid-base equilibria of carboxylic acids and alkyl amines in the aqueous surface region were studied using surface-sensitive X-ray photoelectron spectroscopy and molecular dynamics simulations. Solutions of these organic compounds were examined as a function of pH, concentration and chain length to investigate the distribution of acid and base form in the surface region as compared to the aqueous bulk. Results from these experiments show that the neutral forms of the studied acid-base pairs are strongly enriched in the aqueous surface region. Moreover, we show that for species with at least four carbon atoms in their alkyl-chain, their charged forms are also found to be abundant in the surface region. Using a combination of XPS and MD results, a model is proposed that effectively describes the surface composition. Resulting absolute surface concentration estimations show clearly that the total organic mole fractions in the surface region change drastically as a function of solution pH. The origin of the observed surface phenomena, hydronium/hydroxide concentrations in the aqueous surface region and why standard chemical equations, used to describe equilibria in dilute bulk solution are not valid in the aqueous surface region, are discussed in detail. The reported results are of considerable importance especially for the detailed understanding of properties of small aqueous droplets that can be found in the atmosphere.

Statistical Mechanics of Multilayer Sorption: Surface Concentration Modeling and XPS Measurement.

The concentration of solute molecules at the surface of a liquid is a factor in heterogeneous reactions, surface tension, and Marangoni-effect-driven surface flows. Increasingly, X-ray photoelectron spectroscopy (XPS) has enabled surface concentrations to be measured. In prior work, we employed statistical mechanics to derive expressions for surface tension as a function of solute activity in a binary solution. Here we use a Gibbs relation to derive concomitant expressions for surface concentration. Surface tension data from the literature for five alcohols are used to identify parameters in the surface tension equation. These parameters are then used in the surface concentration equation to predict surface concentrations. Comparison of these predictions to those measured with XPS shows a factor of three difference between measured and predicted surface concentrations. Potential reasons for the discrepancy are discussed, including lack of surface-bulk equilibrium in the measurements.