On the contribution of Aitken mode particles to cloud droplet populations at continental background areas – a parametric sensitivity study

Abstract. Aitken mode particles are potentially an important source of cloud droplets in continental background areas. In order to find out which physico-chemical properties of Aitken mode particles are most important regarding their cloud-nucleating ability, we calculated the number of cloud droplets formed on Aitken mode particles, CD2, with an adiabatic air parcel model. The model output was analyzed using a global sensitivity analysis method that quantifies and ranks the relative importance of the considered input parameters to the total variance of CD2. The results show that unless the particle surface tension or the mass accommodation coefficient of water is strongly reduced due to the presence of surface-active organics, the parameters describing the size distribution are generally more important than the particle chemical composition. In the absence of such compounds, the chemical composition may have roughly an equal importance with the size distribution only at low updraft velocities characterized by maximum supersaturations below 0.1%. Furthermore, the largest source of variability is generally the particle number concentration, followed by the particle size. The performed sensitivity analysis revealed that the variability of the particle chemical composition may dominate the total variation of CD2 if: 1) the value of α varies at least one order of magnitude more than what is expected for pure water surfaces (10−2–1), or 2) the particle surface tension varies more than roughly 30% under conditions close to reaching saturation.

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On the contribution of Aitken mode particles to cloud droplet populations at continental background areas – a parametric sensitivity study

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-7-6077-2007 ◽

2007 ◽

Vol 7 (3) ◽

pp. 6077-6112

Author(s):

T. Anttila ◽

V.-M. Kerminen

Keyword(s):

Surface Tension ◽

Particle Size ◽

Chemical Composition ◽

Pure Water ◽

Particle Surface ◽

Chemical Properties ◽

Sensitivity Study ◽

Geometric Standard Deviation ◽

Cloud Droplets ◽

Aitken Mode

Abstract. Aitken mode particles are potentially an important source of cloud droplets in continental background areas. In order to find out which physico-chemical properties of Aitken mode particles are most important regarding their cloud-nucleating ability, we applied a global sensitivity method to an adiabatic air parcel model simulating the number of cloud droplets formed on Aitken mode particles, CD2. The technique propagates uncertainties in the parameters describing the properties of Aitken mode to CD2. The results show that if the Aitken mode particles do not contain molecules that are able to reduce the particle surface tension more than 30% and/or decrease the mass accommodation coefficient of water, α, below 10−2, the chemical composition and modal properties may have roughly an equal importance at low updraft velocities characterized by maximum supersaturations <0.1%. For larger updraft velocities, however, the particle size distribution is clearly more important than the chemical composition. In general, CD2 exhibits largest sensitivity to the particle number concentration, followed by the particle size. Also the shape of the particle mode, characterized by the geometric standard deviation (GSD), can be as important as the mode mean size at low updraft velocities. Finally, the performed sensitivity analysis revealed also that the chemistry may dominate the total sensitivity of CD2 to the considered parameters if: 1) the value of α varies at least one order of magnitude more than what is expected for pure water surfaces (10−2–1), or 2) the particle surface tension varies more than roughly 30% under conditions close to reaching supersaturation.

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Air mass physiochemical characteristics over New Delhi: impacts on aerosol hygroscopicity and cloud condensation nuclei (CCN) formation

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-6953-2020 ◽

2020 ◽

Vol 20 (11) ◽

pp. 6953-6971 ◽

Cited By ~ 2

Author(s):

Zainab Arub ◽

Sahil Bhandari ◽

Shahzad Gani ◽

Joshua S. Apte ◽

Lea Hildebrandt Ruiz ◽

...

Keyword(s):

Chemical Composition ◽

Size Distribution ◽

Cloud Condensation Nuclei ◽

Chemical Properties ◽

Industrial Emissions ◽

Condensation Nuclei ◽

Aerosol Composition ◽

Time Resolved ◽

Air Mass ◽

Air Masses

Abstract. Delhi is a megacity subject to high local anthropogenic emissions and long-range transport of pollutants. This work presents for the first time time-resolved estimates of hygroscopicity parameter (κ) and cloud condensation nuclei (CCN), spanning for more than a year, derived from chemical composition and size distribution data. As a part of the Delhi Aerosol Supersite (DAS) campaign, the characterization of aerosol composition and size distribution was conducted from January 2017 to March 2018. Air masses originating from the Arabian Sea (AS), Bay of Bengal (BB), and southern Asia (SA) exhibited distinct characteristics of time-resolved sub-micron non-refractory PM1 (NRPM1) species, size distributions, and CCN number concentrations. The SA air mass had the highest NRPM1 loading with high chloride and organics, followed by the BB air mass, which was more contaminated than AS, with a higher organic fraction and nitrate. The primary sources were identified as biomass-burning, thermal power plant emissions, industrial emissions, and vehicular emissions. The average hygroscopicity parameter (κ), calculated by the mixing rule, was approximately 0.3 (varying between 0.13 and 0.77) for all the air masses (0.32±0.06 for AS, 0.31±0.06 for BB, and 0.32±0.10 for SA). The diurnal variations in κ were impacted by the chemical properties and thus source activities. The total, Aitken, and accumulation mode number concentrations were higher for SA, followed by BB and AS. The mean values of estimated CCN number concentration (NCCN; 3669–28926 cm−3) and the activated fraction (af; 0.19–0.87), for supersaturations varying from 0.1 % to 0.8 %, also showed the same trend, implying that these were highest in SA, followed by those in BB and then those in AS. The size turned out to be more important than chemical composition directly, and the NCCN was governed by either the Aitken or accumulation modes, depending upon the supersaturation (SS) and critical diameter (Dc). af was governed mainly by the geometric mean diameter (GMD), and such a high af (0.71±0.14 for the most dominant sub-branch of the SA air mass – R1 – at 0.4 % SS) has not been seen anywhere in the world for a continental site. The high af was a consequence of very low Dc (25–130 nm, for SS ranging from 0.1 % to 0.8 %) observed for Delhi. Indirectly, the chemical properties also impacted CCN and af by impacting the diurnal patterns of Aitken and accumulation modes, κ and Dc. The high-hygroscopic nature of aerosols, high NCCN, and high af can severely impact the precipitation patterns of the Indian monsoon in Delhi, impact the radiation budget, and have indirect effects and need to be investigated to quantify this impact.

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Glycine in aerosol water droplets: a critical assessment of Köhler theory by predicting surface tension from molecular dynamics simulations

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-519-2011 ◽

2011 ◽

Vol 11 (2) ◽

pp. 519-527 ◽

Cited By ~ 19

Author(s):

X. Li ◽

T. Hede ◽

Y. Tu ◽

C. Leck ◽

H. Ågren

Keyword(s):

Molecular Dynamics ◽

Surface Tension ◽

Molecular Dynamics Simulations ◽

Global Climate ◽

Pure Water ◽

Model Systems ◽

Water Droplets ◽

Cloud Droplets ◽

Kohler Theory ◽

Dynamics Simulations

Abstract. Aerosol particles in the atmosphere are important participants in the formation of cloud droplets and have significant impact on cloud albedo and global climate. According to the Köhler theory which describes the nucleation and the equilibrium growth of cloud droplets, the surface tension of an aerosol droplet is one of the most important factors that determine the critical supersaturation of droplet activation. In this paper, with specific interest to remote marine aerosol, we predict the surface tension of aerosol droplets by performing molecular dynamics simulations on two model systems, the pure water droplets and glycine in water droplets. The curvature dependence of the surface tension is interpolated by a quadratic polynomial over the nano-sized droplets and the limiting case of a planar interface, so that the so-called Aitken mode particles which are critical for droplet formation could be covered and the Köhler equation could be improved by incorporating surface tension corrections.

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Reduced Rates of Water Loss and Chemical Properties of Skin Secretions of the Frogs Litoria caerulea and Cyclorana australis

Australian Journal of Zoology ◽

10.1071/zo96046 ◽

1997 ◽

Vol 45 (1) ◽

pp. 13 ◽

Cited By ~ 30

Author(s):

Keith Christian ◽

David Parry

Keyword(s):

Chemical Composition ◽

Water Loss ◽

Neutral Lipids ◽

Pure Water ◽

Chemical Properties ◽

Chemical Components ◽

Litoria Caerulea ◽

Skin Secretions ◽

Reduced Rate ◽

Almost All

We measured the rates of water loss in two Australian hylid frogs: the arboreal Litoria caerulea and the terrestrial burrowing frog Cyclorana australis. We measured the latter species with and without cocoons. Both species showed reduced rates of water loss compared with ‘typical’ amphibians that lose water as if from a free surface. Cocooned C. australis had very low rates of water loss. We examined the chemical composition of skin secretions rinsed (using only high-pure water) from both species and the cocoon material from C. australis. The chemical composition of the material from these three sources was generally similar and consisted of 5–10% neutral lipids and 78–85% proteinaceous material. The fact that the terrestrial species has a high resistance to water loss is unusual given that almost all other known species of non-cocooned frogs with reduced rates of water loss are arboreal. The chemical similarity of the skin secretions and cocoons from this species suggest that the reduced rate of water loss in this species is linked to its ability to form a cocoon. Amino acid composition of the material indicated that a sclerotisation process may occur upon oxidation of the secretions. This would result in a physical barrier to water loss in the cocoons and possibly a thin physical proteinaceous barrier on the skin of both species in the absence of cocoons. We suggest that the high proportion of proteins in the skin secretions cannot be ignored, and that it may, in conjunction with the lipids, produce an effective waterproofing barrier in both species. We suggest that chemical components other than lipids also may be important in frogs from other continents, and complete compositional analyses of frog ‘mucus’ are required before we can fully understand the nature of the mechanisms involved in reduced rates of water loss in amphibians with and without cocoons.

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Temporal and spatial variations of aerosol physical and chemical properties over West Africa: AMMA aircraft campaign in summer 2006

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-4463-2010 ◽

2010 ◽

Vol 10 (2) ◽

pp. 4463-4500 ◽

Cited By ~ 3

Author(s):

A. Matsuki ◽

B. Quennehen ◽

A. Schwarzenboeck ◽

S. Crumeyrolle ◽

H. Venzac ◽

...

Keyword(s):

Chemical Composition ◽

Chemical Properties ◽

West African Monsoon ◽

West African ◽

Size Distributions ◽

Physical And Chemical Properties ◽

African Monsoon ◽

Physical And Chemical ◽

Aitken Mode ◽

And Chemical Properties

Abstract. While West Africa is recognized as being one of the global hot-spots of atmospheric aerosols, the presence of West African Monsoon is expected to create significant spatial and temporal variations in the regional aerosol properties through mixing particles from various sources (mineral dust, biomass burning, sulfates, sea salt). To improve our understanding of the complexity of the aerosol-cloud system in that region, the African Monsoon Multidisciplinary Analysis (AMMA) project has been launched, providing valuable data sets of in-situ and remote sensing measurements including satellites for extended modeling. The French ATR-42 research aircraft was deployed in Niamey, Niger (13°30' N, 02°05' E) in summer 2006, during the three special observation periods (SOPs) of AMMA. These three SOPs covered both dry and wet periods before and after the onset of the Western African Monsoon. State of the art physico-chemical aerosol measurements on the ATR-42 showed a notable seasonal transition in averaged number size distributions where (i) the Aitken mode is dominating over the accumulation mode during the dry season preceding the monsoon arrival and (ii) the accumulation mode increasingly gained importance after the onset of the West African monsoon and even dominated the Aitken mode after the monsoon had fully developed. An extended analysis of the vertical dependence of size spectra, comparing the three observation periods, revealed that the decreasing concentration of the Aitken mode particles, as we move from SOP1 (June) to SOP2a1 (July), and SOP2a2 (August), was less pronounced in the monsoon layer as compared to the overlying Saharan dust layer and free troposphere. In order to facilitate to all partners within the AMMA community radiative transfer calculations, validation of satellite remote sensors, and detailed transport modeling, the parameters describing the mean log-normally fitted number size distributions as a function of altitude and special observation periods were summarized and subsequently related to simultaneously performed measurements of major aerosol particle chemical composition. Extended TEM-EDX analysis of the chemical composition of single aerosol particles revealed dominance of mineral dust (aluminosilicate) even in the submicron particle size range during the dry period, gradually replaced by prevailing biomass burning and sulfate particles, after the onset the monsoon period. The spatial and temporal evolution from SOP1 to SOP2a1 and SOP2a2 of the particle physical and chemical properties and associated aerosol hygroscopic properties are remarkably consistent.

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Surface Tension of Surfactant-Containing, Finite Volume Droplets

10.5194/egusphere-egu2020-106 ◽

2020 ◽

Author(s):

Bryan Bzdek ◽

Rachael Miles ◽

Jussi Malila ◽

Hallie Boyer ◽

Jim Walker ◽

...

Keyword(s):

Surface Tension ◽

Climate Models ◽

High Surface ◽

Particle Surface ◽

Volume Ratio ◽

Droplet Surface ◽

Submicron Particles ◽

Cloud Droplets ◽

Droplet Shape ◽

Size Dependent

Surface tension influences the fraction of atmospheric particles that become cloud droplets. Recent field studies have indicated that surfactants, which lower the surface tension of macroscopic solutions, are an important component of aerosol mass. However, 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. For surfactants to be relevant to particle activation into cloud droplets, multiple parameters must be considered. First, the concentration of surfactant in the initial particle must be sufficiently large that surface tension depression is maintained during activation, despite the dilution that occurs as water condenses onto the particle. Second, the high surface to volume ratio of micron and submicron particles necessitates partitioning a larger fraction of the surfactant molecules to the particle surface than in a typical solution, resulting in a depletion of the bulk concentration and an increase in the surface tension relative to a bulk sample. Third, the timescale for establishing equilibrium at the droplet surface must be known. The interplay of these parameters highlights the necessity of direct measurements of picolitre droplet surface tension.This presentation will describe two cutting-edge approaches we have developed to directly measure the surface tension of microscopic droplets. In the first approach, ejection of ~20 &#181;m radius surfactant-containing droplets from a dispenser excites oscillations in droplet shape that can be used to retrieve the droplet surface tension on microsecond timescales. These measurements allow investigation of surfactant partitioning timescales in aerosol and, crucially, test the assumption that droplet surfaces are generally in their equilibrium state. In the second approach, the coalescence of ~8 &#181;m radius droplets is investigated. Coalescence excites droplet shape oscillations which again permit quantification of droplet surface tension. We demonstrate that surfactants can significantly reduce the surface tension of finite sized droplets below the value for water, consistent with recent field measurements. This surface tension reduction is droplet size dependent and does not correspond exactly to the macroscopic solution value. A new monolayer partitioning model confirms the observed 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 and a 30% increase in cloud droplet number concentration, in line with a radiative cooling effect larger than current estimates assuming a water surface tension by 1 W&#183;m-2. The results imply that one single value for surface tension cannot be used to predict the activated aerosol fraction.

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The Kelvin versus the Raoult Term in the Köhler Equation

Journal of the Atmospheric Sciences ◽

10.1175/2008jas2720.1 ◽

2008 ◽

Vol 65 (12) ◽

pp. 4004-4016 ◽

Cited By ~ 40

Author(s):

Heike Wex ◽

Frank Stratmann ◽

David Topping ◽

Gordon McFiggans

Keyword(s):

Surface Tension ◽

Cloud Condensation Nuclei ◽

Cloud Droplet ◽

Sensitivity Study ◽

Hygroscopic Growth ◽

Cloud Droplets ◽

Simple Estimate ◽

Growth Regime ◽

Insoluble Material ◽

Cloud Droplet Number Concentration

Abstract A comprehensive sensitivity study was carried out examining the sensitivity of hygroscopic growth and activation as modeled with the Köhler equation. Different parameters in the Köhler equation were varied within the range of their currently known uncertainties. The parameters examined include not only those describing the nature of the soluble substances in a particle/droplet and the surface tension σ of the droplet solution, but also the recently proposed representation of parameters coupling the Raoult and Kelvin terms (i.e., partitioning of solute between the surface and bulk phases, although the recently proposed adsorption to wettable but insoluble material was not considered). The examined variations cause significant changes in both hygroscopic growth and activation. Whereas the hygroscopic growth regime below 95% RH is insensitive toward the surface tension σ, σ has a large influence on the activation, increasing with decreasing particle size. This implies that a cloud condensation nuclei (CCN) closure, connecting particle hygroscopic growth to activation, has to account for an influence of the examined substance on σ of the particle, especially for smaller particles in the size range from 50 to 100 nm. A simple estimate showed that a lowering of σ by only 10% can cause a change in the activated fraction (i.e., in the cloud droplet number concentration) of at least 10%–20%. Where organic molecules are present in sufficient concentration to reduce σ, surface tension may be an important factor in determining the activation of aerosol particles to cloud droplets.

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Post-flight analysis of the aerosol impact on size distributions of warm clouds’ droplets, as determined in situ by cloud and aerosol spectrometers

10.5194/egusphere-egu2020-13576 ◽

2020 ◽

Author(s):

Denisa Elena Moacă ◽

Sorin Nicolae Vâjâiac ◽

Andreea Calcan ◽

Valeriu Filip

Keyword(s):

Size Distribution ◽

Pure Water ◽

Weather Forecast ◽

Cloud Microphysics ◽

Atmospheric Environment ◽

Size Distributions ◽

Cloud Droplets ◽

Cloud Particle ◽

Microphysical Properties

The influence of aerosol on the various aspects of the atmospheric properties as well as on the energetic balance is widely recognised in the scientific community and this issue is currently subject to worldwide intense investigations. Among the multiple ways aerosol particles are impacting the atmospheric environment, their interference with the phase transformations of the atmospheric water is of particular importance. Cloud microphysics, on the other hand, is one of the key components in weather forecast and, therefore, in pursuing daily domestic activities ranging from agriculture to energy harvesting and aviation. The micro-physical processes taking place in clouds are strongly influenced by the spatiotemporal variation of the size distribution of the cloud droplets. In this context, as in situ investigations of clouds seem appropriate, one of the most useful types of instruments is casted into the generic name of Cloud and Aerosol Spectrometer (CAS) that can be mounted on specialized research aircraft. The CAS working principle relies basically on measuring the forward scattering cross section (FWSCS) of light with a certain wavelength on a cloud particle and comparing it to the FWSCS computed for pure water spheres. The eventual matching of these values leads to assigning a certain value for the measured particle&#8217;s diameter. The light wavelength is usually chosen in a range where pure water has virtually no absorption. However, atmospheric aerosol frequently mixes up with cloud droplets (starting even from the nucleation processes) and alters their optical properties. By increasing absorption and/or refractivity with respect to those of pure water, one can easily show that the FWSCS-diameter diagram changes drastically by becoming smoother and with an overall significant decrease in absolute values. This means that a CAS will systematically count &#8220;contaminated&#8221; cloud droplets in a lower range of diameters, thus distorting their real size distribution. This effect is inherently degrading the objectivity of CAS measurements and should be more pronounced when levels of sub-micrometer sized aerosol increase at the cloud altitude. The present study aims at pointing out such correlation in order to estimate the reliability of size distributions (and of the ensuing cloud microphysical properties) obtained by CAS.

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Chemical composition and droplet size distribution of cloud at the summit of Mount Tai, China

10.5194/acp-2016-1175 ◽

2017 ◽

Author(s):

Jiarong Li ◽

Xinfeng Wang ◽

Jianmin Chen ◽

Chao Zhu ◽

Weijun Li ◽

...

Keyword(s):

Chemical Composition ◽

Size Distribution ◽

Droplet Size ◽

Atmospheric Aerosols ◽

Ph Value ◽

Inorganic Ions ◽

Droplet Size Distribution ◽

Dilution Effect ◽

Water Soluble ◽

Cloud Droplets

Abstract. Chemical composition of 39 cloud samples and droplet size distribution in 24 cloud events were investigated at the summit of Mt. Tai from July to October 2014. Inorganic ions, organic acids, metals, HCHO, H2O2, sulfur(IV), organic carbon, element carbon as well as pH and electrical conductivity were analyzed. The acidity of the cloud water significantly decreased from a reported value of pH 3.86 in 2007–2008 (Guo et al., 2012) to pH 5.87 in the present study. The concentrations of nitrate and ammonium were both increased since 2007–2008, but the overcompensation of ammonium led to the increase of the mean pH value. The microphysical properties showed that cloud droplets were smaller than 26.0 μm and the most were in the range of 6.0–9.0 μm. The maximum droplet number concentration (Nd) was associated with droplet sizes of 7.0 μm. Cloud droplets exhibited a strong interaction with atmospheric aerosols. High PM2.5 level resulted in higher concentrations of water soluble ions and smaller sizes with more numbers of cloud droplets, and further gave rise to relatively high acidity. High degrees of relative humidity facilitated the formation of large cloud droplets and led to high liquid water contents under low PM2.5 level. The cloud droplets to wet deposition acted as an important sink of soluble material in the atmosphere and the dilution effect of the water content should be considered when estimating concentrations of soluble components in the cloud phase.

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In situ chemical measurement of individual cloud residue particles at a mountain site, South China

10.5194/acp-2017-23 ◽

2017 ◽

Author(s):

Qinhao Lin ◽

Guohua Zhang ◽

Long Peng ◽

Xinhui Bi ◽

Xinming Wang ◽

...

Keyword(s):

Chemical Composition ◽

South China ◽

Chemical Properties ◽

In Situ Observation ◽

Mountain Range ◽

Mixing State ◽

Cloud Droplets ◽

Air Masses ◽

Aerosol Mass

Abstract. To estimate how atmospheric aerosol particles respond to chemical properties of cloud droplets, a ground-based counterflow virtual impactor (GCVI) coupled with a real-time single-particle aerosol mass spectrometer (SPAMS) was used to assess the chemical composition and mixing state of individual cloud residue particles in the Nanling Mountain Range (1,690 m a.s.l.), South China, in January 2016. The cloud residues were classified into nine particle types: Aged elemental carbon (EC), Potassium-rich (K-rich), Amine, Dust, Pb, Fe, Organic carbon (OC), Sodium-rich (Na-rich) and Other. The largest fraction of the cloud residues was the aged EC type (49.3 % by number), followed by the K-rich type (33.9 % by number). Abundant aged EC cloud residues that internally mixed with inorganic salts were found in air masses from northerly polluted areas. The number fraction (Nf) of the K-rich cloud residues significantly increased within southwesterly air masses from fire activities in Southeast Asia. In addition, the Amine particles represented 0.2 % to 15.1 % by number to the cloud residues when air masses changed from northerly to southwesterly sources. The Dust, Fe, Pb, Na-rich and OC particles had a low contribution (0.5–4.1 % by number) to the cloud residues. An analysis of the mixing state of cloud residues showed that the Dust and Na-rich cloud residues were highly associated with nitrate. Sulfate intensity increased in the aged EC and OC cloud residues and decreased in the Dust and Na-rich cloud residues relative to both ambient and interstitial particles. A comparison of cloud residues with interstitial particles indicated that a higher Nf for K-rich particles and a lower Nf for the aged EC particles were found in the cloud residues. Relative to the ambient and interstitial particles, the cloud residues exhibited larger size distributions. To our knowledge, this study is the first report on in situ observation of the chemical composition and mixing state of individual cloud residue particles in China. This study increases our understanding of the impacts of aerosols on cloud droplets in a remote area of China.

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