Adsorptive uptake of water by semisolid secondary organic aerosols

Aerosol climate effects are intimately tied to interactions with water. Here we combine hygroscopicity measurements with direct observations about the phase of secondary organic aerosol (SOA) particles to show that water uptake by slightly oxygenated SOA is an adsorption-dominated process under subsaturated conditions, where low solubility inhibits water uptake until the humidity is high enough for dissolution to occur. This reconciles reported discrepancies in previous hygroscopicity closure studies. We demonstrate that the difference in SOA hygroscopic behavior in subsaturated and supersaturated conditions can lead to an effect up to about 30% in the direct aerosol forcinghighlighting the need to implement correct descriptions of these processes in atmospheric models. Obtaining closure across the water saturation point is therefore a critical issue for accurate climate modeling.Peer reviewe

On the early studies recognizing the role of sulphuric acid in atmospheric haze and new particle formation

Abstract Atmospheric aerosols have been a subject to scientific interest at least since the Age of Enlightenment, including theories concerning the origins of atmospheric haze and dust. Early studies associated haze with geological sources — earthquakes and volcanism — which were believed to be related to the chemistry of sulphuric compounds. Thus, sulphuric acid became the strongest candidate to explain atmospheric new particle formation. The idea was carried over when the first quantitative studies of condensation nuclei and atmospheric chemistry took place during the later part of the 19th century. Laboratory and field measurements by von Helmholtz, Aitken, Kiessling, and Barus, among others, a century ago led to the conclusion that widespread new particle formation occurs in the atmosphere and is caused by sulphuric acid together with water and ammonia — a viewpoint, which has been rediscovered and expanded during the past 25 years

A monolayer partitioning scheme for droplets of surfactant solutions

Abstract Bulk‐surface partitioning of surface active species affects both cloud droplet activation by aerosol particles and heterogeneous atmospheric chemistry. Various approaches are given in the literature to capture this effect in atmospheric models. Here we present a simple, yet physically self‐contained, monolayer model for prediction of both composition and thickness of the surface layer of an aqueous droplet. The monolayer surface model is based on assuming a finite surface layer and mass balance of all species within the droplet. Model predictions are presented for binary and ternary aqueous surfactant model systems and compared to both experimental and model data from the literature and predictions using a common Gibbsian model approach. Deviations from Gibbsian surface thermodynamics due to volume constraints imposed by the finite monolayer lead to stronger predicted surface tension reduction at smaller droplet sizes with the monolayer model. Process dynamics of the presented monolayer model are also contrasted to other recently proposed approaches to treating surface partitioning in droplets, with different underlying assumptions

Heterogeneous nucleation of water vapor on different types of black carbon particles

Abstract The heterogeneous nucleation of water vapor on insoluble particles affects cloud formation, precipitation, the hydrological cycle, and climate. Despite its importance, heterogeneous nucleation remains a poorly understood phenomenon that relies heavily on empirical information for its quantitative description. Here, we examine the heterogeneous nucleation of water vapor on different types of soots as well as cloud drop activation of different types of soots, including both pure black carbon particles and black carbon particles mixed with secondary organic matter. We show that the recently developed adsorption nucleation theory quantitatively predicts the nucleation of water and droplet formation upon particles of the various soot types. A surprising consequence of this new understanding is that, with sufficient adsorption site density, soot particles can activate into cloud droplets – even when completely lacking any soluble material

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

Abstract 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 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, as well as activated droplet size distributions, and cloud radiative effects. In this work, a new monolayer model along with a traditional Gibbs adsorption isotherm model were used in conjunction with equilibrium Köhler theory to predict 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 literature, even for systems where specific molecular identities and aqueous properties are unknown. The monolayer model is self-contained, fully prognostic, and provides a versatile, conceptually simple, yet physically-based model for understanding the role of organic surfactants in cloud droplet formation

Model for estimating activity coefficients in binary and ternary ionic surfactant solutions

Abstract We introduce the CMC based Ionic Surfactant Activity model (CISA) to calculate activity coefficients in ternary aqueous solutions of an ionic surfactant and an inorganic salt. The surfactant can be either anionic or cationic and in the present development, the surfactant and inorganic salts share a common counterion. CISA incorporates micellization into the Pitzer–Debye–Hückel (PDH) framework for activities of mixed electrolyte solutions. To reduce computing requirements, a parametrization of the critical micelle concentration (CMC) is used to estimate the degree of micellization instead of explicit equilibrium calculations. For both binary and ternary systems, CISA only requires binary experimentally-based parameters to describe water–ion interactions and temperature–composition dependency of the CMC. The CISA model is intended in particular for atmospheric applications, where higher-order solution interaction parameters are typically not constrained by experiments and the description must be reliable across a wide range of compositions. We evaluate the model against experimental activity data for binary aqueous solutions of ionic surfactants sodium octanoate and sodium decanoate, as common components of atmospheric aerosols, and sodium dodecylsulfate, the most commonly used model compound for atmospheric surfactants. Capabilities of the CISA model to describe ternary systems are tested for the water–sodium decanoate–sodium chloride system, a common surrogate for marine background cloud condensation nuclei and to our knowledge the only atmospherically relevant system for which ternary activity data is available. For these systems, CISA is able to provide continuous predictions of activity coefficients both below and above CMC and in all cases gives an improved description of the water activity above the CMC, compared to the alternative model of Burchfield and Wolley [J. Phys. Chem., 88(10), 2149–2155 (1984)]. The water activity is a key parameter governing the formation and equilibrium growth of cloud droplets. The CISA model can be extended from the current form to include the effect of other inorganic salts with the existing database of binary PDH parameters and using appropriate mixing rules to account for ion specificity in the micellization process

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

The surface tension of surfactant-containing, finite volume droplets

Abstract Atmospheric aerosol particles cool Earth’s climate by serving as cloud droplet seeds. This cooling effect represents both the single most uncertain and the largest negative radiative forcing. Cloud droplet activation is strongly influenced by aerosol particle surface tension, which in climate models is assumed equivalent to that of pure water. We directly measure the surface tensions of surfactant-coated, high surface-to-volume ratio droplets, demonstrating that their surface tensions are significantly lower than pure water but do not match the surface tension of the solution from which they were produced and depend on finite droplet size. These results suggest surfactants could potentially significantly modify radiative forcing and highlight the need for a better understanding of atmospheric surfactant concentrations and properties

Thermodynamic properties of isoprene- and monoterpene-derived organosulfates estimated with COSMOtherm

Abstract Organosulfates make significant contributions to atmospheric secondary organic aerosol (SOA), but little is known about the thermodynamic properties of atmospherically relevant organosulfates. We have used the COSMOtherm program to calculate both the gas- and condensed-phase properties of previously identified atmospherically relevant monoterpene- and isoprene-derived organosulfates. Properties include solubilities, activities and saturation vapor pressures, which are critical to the aerosol-phase stability and atmospheric impact of organosulfate SOA. Based on the estimated saturation vapor pressures, the organosulfates of this study can all be categorized as semi-volatile or low-volatile, with saturation vapor pressures 4 to 8 orders of magnitude lower than that of sulfuric acid. The estimated pKa values of all the organosulfates indicate a high degree of dissociation in water, leading in turn to high dissociation-corrected solubilities. In aqueous mixtures with inorganic sulfate, COSMOtherm predicts a salting-out of both the organosulfates and their sodium salts from inorganic co-solutes. The salting-out effect of ammonium sulfate (less acidic) is stronger than of ammonium bisulfate (more acidic). Finally, COSMOtherm predicts liquid–liquid-phase separation in systems containing water and monoterpene-derived organosulfates. The COSMOtherm-estimated properties support the observed stability of organosulfates as SOA constituents and their long-range transport in the atmosphere but also show significant variation between specific compounds and ambient conditions

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

Abstract 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.Environmental significance The time-evolution of aqueous surface tension is a well-known result of partitioning of surface-active compounds from the bulk to the surface. Surfactants are ubiquitous components of atmospheric organic aerosol. Equilibration time scales potentially impact interpretation of all measurements of cloud condensation nuclei (CCN) activity and modeled effects taking surface tension into account, including closure studies between instruments with different residence times and analysis using κ-Köhler theory. Using continuous surface tension parameterizations at different times, we provide a thermodynamically consistent analysis of the impact of time-dependency for predictions of CCN activity. We show that although time-evolution is clearly seen in surface partitioning, its signature decreases in droplet surface tension and we find no trends in CCN activity with time