Enrichment of surface-active compounds in coalescing cloud drops (original) (raw)
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Effect of Surfactant on Cloud Formation
2022
Atmospheric aerosols can consist of inorganic and organic substances, including surfactants at a significant concentration. Importantly, the latter can reduce the surface tension at the liquid-vapor surfaces, where they preferentially adsorb due to their amphiphilic structure. As a result, processes such as droplet coalescence, development of precipitation and ultimately cloud lifetime, may depend on the presence of surfactants in the aerosols. Here, we present a numerical model for cloud droplet formation, which is based on the Lagrangian particle-based microphysics-scheme super-droplet method and takes into account the presence of surfactant in the droplets. Our results show that surfactant facilitates cloud formation by increasing the number and size of activated droplets, which concentrate at the bottom of the cloud, while the largest droplets are concentrated at the top of the cloud. This indicates a circulation of droplets that involves activation and growth processes from the bottom of the cloud towards the top. Moreover, our conclusions are independent of the particular approach used for modeling the diffusion of Eulerian variables due to the subgrid-scale turbulence. We anticipate that our results will enrich our understanding of the role of surfactants in the behavior of atmospheric aerosols and, importantly, will pave the way for further developments in the numerical modeling of systems with surfactants at macroscopic scales.
Surfactants in cloud droplet activation: mixed organic-inorganic particles
Atmospheric Chemistry and Physics, 2010
Organic compounds with surfactant properties are commonly found in atmospheric aerosol particles. Surface activity can significantly influence the cloud droplet forming ability of these particles. We have studied the cloud droplet formation by two-component particles comprising one of the organic surfactants sodium octanoate, sodium decanoate, sodium dodecanoate, and sodium dodecyl sulfate, mixed with sodium chloride. Critical supersaturations were measured with a static diffusion cloud condensation nucleus counter (Wyoming CCNC-100B). Results were modeled from Köhler theory applying three different representations of surfactant properties in terms of surfactant surface partitioning and reduced droplet surface tension. We here confirm previous results for single-component organic surfactant particles, that experimental critical supersaturations are greatly underpredicted, if reduced surface tension is used while ignoring the effects of surface partitioning in droplets. Furthermore, disregarding surfactant properties by ignoring surface partitioning and assuming the constant surface tension of pure water can also lead to significant underpredictions of experimental critical supersaturations. For the mixed particles comprising less than 50% by mass of surfactant, this approach however still provides a good description of the observed droplet activation. A comprehensive account for surfactant properties, including both surface tension reduction and effects of surface partitioning in activating droplets, generally predicts experimental critical supersaturations well.
Nature Communications, 2014
The activation of aerosol particles into cloud droplets in the Earth's atmosphere is both a key process for the climate budget and a main source of uncertainty. Its investigation is facing major experimental challenges, as no technique can measure the main driving parameters, the Raoult's term and surface tension, s, for sub-micron atmospheric particles. In addition, the surfactant fraction of atmospheric aerosols could not be isolated until recently. Here we present the first dynamic investigation of the total surfactant fraction of atmospheric aerosols, evidencing adsorption barriers that limit their gradient (partitioning) in particles and should enhance their cloud-forming efficiency compared with current models. The results also show that the equilibration time of surfactants in sub-micron atmospheric particles should be beyond the detection of most on-line instruments. Such instrumental and theoretical shortcomings would be consistent with atmospheric and laboratory observations and could have limited the understanding of cloud activation until now.
Journal of the Atmospheric Sciences, 2002
Decades of cloud microphysical research have not provided conclusive understanding of the physical processes responsible for droplet spectral broadening. Numerous mechanisms have been proposed-for example, entrainment mixing, vortex shedding, giant cloud condensation nuclei (CCN), chemical processing of CCN, and radiative cooling-all of which are likely candidates under select conditions. In this paper it is suggested that variability in the composition of CCN, and in particular, the existence of condensation inhibiting compounds, is another possible candidate. The inferred potential abundance of these amphiphilic film-forming compounds (FFCs) suggests that their effect may be important. Using a cloud parcel model with a simplified treatment of the effect of FFCs, it is shown that modest concentrations of FFCs (on the order of 5% of the total aerosol mass) can have a marked effect on drop growth and can cause significant increases in spectral dispersions. Moreover, it is shown that FFCs may, in some cases, reduce the number concentration of cloud droplets, with implications for cloud-climate feedbacks. This trend is at least in qualitative agreement with results from a recent field campaign.
A simple representation of surface active organic aerosol in cloud droplet formation
Atmospheric Chemistry and Physics, 2011
Atmospheric aerosols often contain surface active organics. Surface activity can affect cloud droplet formation through both surface partitioning and surface tension reduction in activating droplets. However, a comprehensive thermodynamic account for these effects in Köhler modeling is computationally demanding and requires knowledge of both droplet composition and component molecular properties, which is generally unavailable. Here, a simple representation of activation properties for surface active organics is introduced and compared against detailed model predictions and laboratory measurements of CCN activity for mixed surfactant-salt particles from the literature. This simple organic representation is seen to work well for aerosol organic-inorganic composition ranges typically found in the atmosphere, and agreement with both experiments and detailed model predictions increases with surfactant strength. The simple representation does not require resolution of the organic aerosol composition and relies solely on properties of the organic fraction that can be measured directly with available techniques. It can thus potentially be applied to complex and ambient surface active aerosols. It is not computationally demanding, and therefore has high potential for implementation to atmospheric models accounting for cloud microphysics.
Surface tension prevails over solute effect in organic-influenced cloud droplet activation
Nature, 2017
The spontaneous growth of cloud condensation nuclei (CCN) into cloud droplets under supersaturated water vapour conditions is described by classic Köhler theory. This spontaneous activation of CCN depends on the interplay between the Raoult effect, whereby activation potential increases with decreasing water activity or increasing solute concentration, and the Kelvin effect, whereby activation potential decreases with decreasing droplet size or increases with decreasing surface tension, which is sensitive to surfactants. Surface tension lowering caused by organic surfactants, which diminishes the Kelvin effect, is expected to be negated by a concomitant reduction in the Raoult effect, driven by the displacement of surfactant molecules from the droplet bulk to the droplet-vapour interface. Here we present observational and theoretical evidence illustrating that, in ambient air, surface tension lowering can prevail over the reduction in the Raoult effect, leading to substantial increa...
Formation of cloud droplets by multicomponent organic particles
Journal of Geophysical Research, 2003
1] Cloud condensation nuclei (CCN) in the atmosphere are usually composed of multiple inorganic and organic chemical species. Determining the ability of these multicomponent particles to activate into cloud droplets is necessary for understanding and quantifying the effect of aerosols on cloud formation and properties. Internally mixed, multicomponent particles as well as particles consisting of a core coated with hexadecane were used in the present study. Laboratory experiments were performed using combinations of sodium chloride, ammonium sulfate, pinonic acid, pinic acid, norpinic acid, glutamic acid, leucine, and hexadecane. Activation diameters were determined combining a Tandem Differential Mobility Analyzer (TDMA) with a thermal diffusion Cloud Condensation Nucleus Counter (CCNC). Studies were performed at supersaturations of 0.3% and 1% with dry particle diameters ranging between 0.02 and 0.2 micrometers. The results were compared to a theory assuming additive behavior of the constituent species. This assumption was sufficient for the prediction of the CCN activation diameter of the mixed particles.
Solubility properties of surfactants in atmospheric aerosol and cloud/fog water samples
Journal of Geophysical Research, 2003
1] Organic films on deliquesced aerosols and cloud droplets lower the surface tension of water and may inhibit the exchange of water vapor and gases between the gas and the liquid phase, with important implications for aerosol and cloud microphysics and heterogeneous chemistry. This study provides an estimate of the solubility properties of surfactants in aerosol and fog/cloud water samples on the basis of the dilatational rheological properties of the surface films. The variations of surface tension induced by the fast expansion/compression of the films were measured by means of a drop shape tensiometer and were linked to the capacity of surfactants to exchange between the surface layer and bulk solution, and ultimately to their water-solubility. The results are in agreement with the properties of standards of soluble surfactants and can be interpreted by the theory of formation of hydrophilic adsorption layers. These findings suggest that the water-soluble organic compounds (WSOC) are the main contributors to the formation of films on cloud/fog droplets. It follows that the surface coverage of film-forming compounds is mainly controlled by the bulk concentration of WSOC, regardless of the available surface area. This also supports that the surface tension decreases observed under laboratory conditions actually occur in the atmosphere. Citation: Decesari, S., M. C. Facchini, M. Mircea, F. Cavalli, and S. Fuzzi, Solubility properties of surfactants in atmospheric aerosol and cloud/fog water samples,
Tellus B, 2008
A B S T R A C T We have measured critical supersaturations of dried single-component particles of sodium caprylate [CH 3 (CH 2 ) 6 COONa], sodium caprate [CH 3 (CH 2 ) 8 COONa], sodium laurate [CH 3 (CH 2 ) 10 COONa] and sodium myristate [CH 3 (CH 2 ) 12 COONa] in the diameter range 33-140 nm at 296 K using a static thermal gradient diffusion cloud condensation nucleus counter. These fatty acid sodium salts are surface active molecules which have all been identified in atmospheric aerosol particles. Experimental critical supersaturations increased systematically with increasing carbon chain length and were in the range 0.96-1.34% for particles with a dry diameter of 40 nm. The experimental data were modelled using Köhler theory modified to account for partitioning of the surface active fatty acid sodium salts between the droplet bulk and surface as well as Köhler theory including surface tension reduction without accounting for surfactant partitioning and Köhler theory using the surface tension of pure water. It was found that Köhler theory using the reduced surface tension with no account for surfactant partitioning underpredicts experimental critical supersaturations significantly, whereas Köhler theory modified to account for surfactant partitioning and Köhler theory using the surface tension of pure water reproduced the experimental data well.
An interfacial mechanism for cloud droplet formation on organic aerosols
Science (New York, N.Y.), 2016
Accurate predictions of aerosol/cloud interactions require simple, physically accurate parameterizations of the cloud condensation nuclei (CCN) activity of aerosols. Current models assume that organic aerosol species contribute to CCN activity by lowering water activity. We measured droplet diameters at the point of CCN activation for particles composed of dicarboxylic acids or secondary organic aerosol and ammonium sulfate. Droplet activation diameters were 40 to 60% larger than predicted if the organic was assumed to be dissolved within the bulk droplet, suggesting that a new mechanism is needed to explain cloud droplet formation. A compressed film model explains how surface tension depression by interfacial organic molecules can alter the relationship between water vapor supersaturation and droplet size (i.e., the Köhler curve), leading to the larger diameters observed at activation.