Collision dynamics and uptake of water on alcohol-covered ice (original) (raw)
Related papers
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.
Enrichment of surface-active compounds in coalescing cloud drops
Geophysical Research Letters, 2008
Surfactants often found in tropospheric aerosols, can affect the onset and development of clouds. Due to high dilution during droplet growth, the effects of surfactants on cloud microphysical processes have been mostly neglected. However, while cloud growth by coalescence conserves the combined volume of all cloud droplets, it reduces the combined surface area. This could lead to enrichment of water-insoluble surfactants (WIS) and to reduced surface tension of droplets forming in warm processes. Measurements of individual raindrops reveal the presence of water insoluble surfactants. Our field and laboratory studies as well as simple theoretical arguments suggest that by causing varying and size-dependent surface tension, WIS can affect cloud microphysics.
The influence of small aerosol particles on the properties of water and ice clouds
Faraday Discussions, 2008
In this paper, results are presented of the influence of small organic-and soot-containing particles on the formation of water and ice clouds. There is strong evidence that these particles have grown from nano particle seeds produced by the combustion of oil products. Two series of field experiments are selected to represent the observations made. The first is the CLoud-Aerosol Characterisation Experiment (CLACE) series of experiments performed at a high Alpine site (Jungfraujoch), where cloud was in contact with the ground and the measuring station. Both water and ice clouds were examined at different times of the year. The second series of experiments is the CLOud Processing of regional Air Pollution advecting over land and sea (CLOPAP) series, where ageing pollution aerosol from UK cities was observed, from an airborne platform, to interact with warm stratocumulus cloud in a cloud-capped atmospheric boundary layer. Combining the results it is shown that aged pollution aerosol consists of an internal mixture of organics, sulfate, nitrate and ammonium, the organic component is dominated by highly oxidized secondary material. The relative contributions and absolute loadings of the components vary with location and season. However, these aerosols act as Cloud Condensation Nuclei (CCN) and much of the organic material, along with the other species, is incorporated into cloud droplets. In ice and mixed phase cloud, it is observed that very sharp transitions (extending over just a few metres) are present between highly glaciated regions and regions consisting of supercooled water. This is a unique finding; however, aircraft observations in cumulus suggest that this kind of structure may be found in these cloud types too. It is suggested that this sharp transition is caused by ice nucleation initiated by oxidised organic aerosol coated with sulfate in more polluted regions of cloud, sometimes enhanced by secondary ice particle production in these regions.
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,
Dependence of in-cloud scavenging of polar organic aerosol compounds on the water solubility
Journal of Geophysical Research, 2000
In spring 1997 at the Sonnblick Observatory, located at 3106 rn elevation in the Austrian Alps, interstitial aerosol and cloud water samples were simultaneously collected in supercooled convective clouds. These samples were analyzed for their polar organic composition using a newly developed analytical method that allows the simultaneous determination of dicarboxylic acids, monocarboxylic acids, and other polar organic constituents. Using the obtained data set, in-cloud scavenging efficiencies (•) for individual polar organic compounds were calculated. For the different organic substances, scavenging efficiencies ranged from 0.16 to 0.98, compared with sulfate, which exhibited an average scavenging efficiency of 0.94. For dicarboxylic acids, scavenging efficiencies (average of about 0.8) were of the same order as for sulfate. Distinctly lower values (average of about 0.6) were achieved for polar aromatic compounds like phthalic acid or diisobutylphenol. The lowest scavenging efficiencies (average about 0.4) were found for alcohols and monocarboxylic acids. Thus we found in the Sonnblick cloud experiment that more polar organic aerosol constituents are more efficiently scavenged into cloud droplets than less polar compounds. In addition, the scavenging efficiencies exhibited a dependence on the solubilities of the examined compounds. For highly water soluble compounds (1-1000 g L -1) a decrease of the water solubility for an individual compound leads to a decrease in the scavenging efficiency for this compound. For "poorly soluble" substances with water solubilities below 1 g L -1, a near-constant value for the scavenging efficiency was found, indicating that their scavenging behavior is then dominated by the scavenging of the bulk noncarbonate carbon independent of the physical and chemical properties of the individual substances. tions similar to those occurring in the atmosphere. In addiing species in aerosols, Novakov and Penner [ 1993] pointed out the importance of organic aerosol constituents in cloud tion, Corrigan and Novakov [ 1999] demonstrated that highly processes. Their measurements in Puerto Rico revealed that water soluble organic compounds exhibit critical diameters organic aerosols may often account for the majority (about that approach that of ammonium sulfate. Very little, how-75 %) of CCN. This large fraction indicates that organic ever, is known yet about the scavenging behavior of the bulk carbonaceous material in the atmospheric aerosol. A first
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.
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.
Importance of submicron surface-active organic aerosols for pristine Arctic clouds
Tellus B, 2005
A B S T R A C T Recent results from summer measurement campaigns over the partly ice covered central Arctic Ocean show that the high Arctic aerosol has a larger organic fraction than previously thought. We use a Lagrangian parcel model to infer the properties of the unexplained organic aerosol fraction that is necessary for reproducing the observed concentrations of cloud condensation nuclei (CCN). With increasing distance from the open ocean a highly surface-active Aitken mode, associated with particles found in the open lead surface microlayer, becomes increasingly important for cloud droplet formation. The presence of such an Aitken mode population increases the high Arctic indirect aerosol effect (added cooling) relative to just a marine source of CCN from oxidation products of dimethyl sulfide (DMS) released from phytoplankton.
Surface active compounds present in aerosols can increase their cloud condensation nuclei (CCN) activation efficiency by reducing the surface tension (σ) in the growing droplets. However, the importance of this effect is poorly constrained by measurements. Here we present estimates of droplet surface tension near the point of activation derived from direct measurement 15 of droplet diameters using a continuous flow stream-wise thermal gradient chamber (CFSTGC). The experiments used sea spray aerosol mimics composed of NaCl coated by varying amounts of (i) oleic acid, palmitic acid or myristic acid, (ii) mixtures of palmitic acid and oleic acid, and (iii) oxidized oleic acid. Significant reductions in σ relative to that for pure water were observed for these mimics at relative humidity (RH) near activation (~99.9%) when the coating was 20 sufficiently thick. The calculated surface pressure (π = σH2O-σobserved) values for a given organic compound or mixture collapse onto one curve when plotted as a function of molecular area for different NaCl seed sizes and measured RH. The observed critical molecular area (A0) for oleic acid determined from droplet growth was similar to that from bulk experiments conducted in a Langmuir trough. However, the observations presented here suggest that oleic acid in 25 microscopic droplets may exhibit larger π values during monolayer compression. For myristic acid, the observed A0 compared well to bulk experiments on a fresh subphase, for which dissolution has an important impact. A significant kinetic limitation to water uptake was observed for NaCl particles coated with pure palmitic acid, likely as a result of palmitic acid