the effect of low solubility organic acids on the ... · the effect of low solubility organic acids...

Atmos. Chem. Phys., 14, 11409–11425, 2014www.atmos-chem-phys.net/14/11409/2014/doi:10.5194/acp-14-11409-2014© Author(s) 2014. CC Attribution 3.0 License.

The effect of low solubility organic acids on the hygroscopicity ofsodium halide aerosolsL. Miñambres, E. Méndez, M. N. Sánchez, F. Castaño, and F. J. Basterretxea

Departamento de Química Física, Facultad de Ciencia y Tecnología, University of the Basque Country, UPV/EHU,Campus de Leioa, B. Sarriena, s/n, Leioa 48940, Spain

Correspondence to:L. Miñambres ([email protected])

Received: 28 November 2013 – Published in Atmos. Chem. Phys. Discuss.: 18 February 2014Revised: 13 August 2014 – Accepted: 31 August 2014 – Published: 29 October 2014

Abstract. In order to accurately assess the influence of fattyacids on the hygroscopic and other physicochemical prop-erties of sea salt aerosols, hexanoic, octanoic or lauric acidtogether with sodium halide salts (NaCl, NaBr and NaI)have been chosen to be investigated in this study. The hy-groscopic properties of sodium halide sub-micrometre par-ticles covered with organic acids have been examined byFourier-transform infrared spectroscopy in an aerosol flowcell. Covered particles were generated by flowing atomizedsodium halide particles (either dry or aqueous) through aheated oven containing the gaseous acid. The obtained re-sults indicate that gaseous organic acids easily nucleate ontodry and aqueous sodium halide particles. On the other hand,scanning electron microscopy (SEM) images indicate thatlauric acid coating on NaCl particles makes them to aggre-gate in small clusters. The hygroscopic behaviour of coveredsodium halide particles in deliquescence mode shows differ-ent features with the exchange of the halide ion, whereas theorganic surfactant has little effect in NaBr particles, NaCl andNaI covered particles experience appreciable shifts in theirdeliquescence relative humidities, with different trends ob-served for each of the acids studied. In efflorescence mode,the overall effect of the organic covering is to retard the lossof water in the particles. It has been observed that the pres-ence of gaseous water in heterogeneously nucleated particlestends to displace the cover of hexanoic acid to energeticallystabilize the system.

1 Introduction

Marine aerosol is one of the most abundant types of nat-ural particulate matter in the Earth’s troposphere. Sea saltparticles play an active role in the Earth’s radiative bal-ance, influence mass transfer of gaseous compounds andcloud-precipitation mechanisms, contribute to the formationof cloud condensation nuclei and have highly reactive sur-faces that take part in heterogeneous and multiphase chemi-cal reactions (Andreae and Rosenfeld, 2008; Carslaw et al.,2010; O’Dowd and De Leeuw, 2007; Finlayson-Pitts, 2003;Lewis and Schwartz, 2004; Quinn and Bates, 2011; Rossi,2003). They also can take up significant amounts of water,exhibiting deliquescence and efflorescence properties underatmospheric conditions (Freney et al., 2009; Martin, 2000;Metzger and Lelieveld, 2007; Mikhailov et al., 2013; Wiseet al., 2012) that can change the particles’ phase and size,together with other interrelated physicochemical properties:for example, water uptake increases particle size, thus favour-ing their sedimentation. In parallel, bigger particles increasethe scattering of solar visible light, thus influencing atmo-spheric radiative transfer and visibility. The presence of wa-ter in atmospheric particles can also change the adsorption oftrace gases and their chemical reactivity (e.g. sulfate chem-istry proceeds by adsorption of gaseous SO2 on aqueous par-ticles, followed by oxidation to sulfate; this pathway is absentin crystalline particles).

Marine aerosol is generated either by the mechanicalaction of the ocean surface (primary sea-salt aerosol), orby gas-to-particle conversion processes (secondary aerosol)mainly in the form of non-sea-salt sulfate and organic species(O’Dowd et al., 1997). Sodium chloride is the principalcomponent of sea salt: typical seawater composition has

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11410 L. Miñambres et al.: The effect of low solubility organic acids

1.05× 104 mg L−1 of Na+ and 1.9× 104 mg L−1 of Cl−

(Lide, 1994). Bromide ions are a minor component of sea-water, and hence of sea salt particles, with a molar ratio ofbromide to chloride of 1 : 650 (Lide, 1994). Despite such asmall contribution to the composition of sea salt particles,bromine plays a comparatively large role in tropospheric seasalt chemistry. The most important is the drop of surface-level ozone concentrations in the Arctic at polar sunrise. Thisis due to the tendency of Br− to segregate to the salt sur-face in the presence of water, substantially increasing Br / Clsurface molar ratios, and to the fact that bromide ions ex-hibit a higher surface reactivity than chloride (Baker, 2005;Ghosal et al., 2008; Zangmeister et al., 2001). Sea salt par-ticles have been shown to be the source of BrO, which isinvolved in catalytic cycles that destroy ozone (Finlayson-Pitts, 2009; Frinak and Abbatt, 2006; Hunt et al., 2004; Readet al., 2008; Von Glasow, 2008). Although the concentrationsof I− present in seawater are much smaller than those ofbromine and chlorine (the molar ratio of I− to Cl− in sea-water is∼ 1 : 106), there is evidence that iodine in the marineboundary layer has an influence on ozone destruction, the ox-idizing capacity of the troposphere, de-noxification, and par-ticle formation (Carpenter, 2003; Saiz-Lopez et al., 2008). Asimilar role to BrO (bromine oxide free radical) is played byIO (iodine oxide free radical), although its source is believedto come from marine algae (Read et al., 2008). Recentlyadvances have been made in quantifying the link betweenseawater chemical processes, and the production, size, andchemical composition of sea-spray aerosol particles by si-multaneous measurements of seawater, particle size distribu-tions, and size-resolved single particle chemical compositionin a laboratory setting reproducing the chemical complex-ity of sea-spray aerosol, including natural seawater, breakingwaves and controlled phytoplankton and heterotrophic bac-teria concentrations (Ault et al., 2013; Prather et al., 2013).It has been shown that the mixing state of sea aerosol is sen-sitive to the presence of heterotrophic bacteria that transformdissolved organic matter.

Organic compounds are present in marine salt aerosol invariable proportions that may represent a large fraction of theaerosol dry mass (Cavalli et al., 2004; Gantt and Meskhidze,2013; Middlebrook et al., 1998). The presence of significantconcentrations of organic matter in marine aerosol was de-tected in earlier studies (Kleefeld et al., 2002; Middlebrooket al., 1998; Putaud et al., 2000). Measures over the NorthAtlantic Ocean have revealed that the organic fraction con-tributes up to 63 % to the sub-micrometre aerosol mass, ofwhich about 45 % is water-insoluble and 18 % water-soluble(O’Dowd et al., 2004). 37 % hydrocarbon and 63 % oxy-genated hydrocarbon speciation was observed for the organicmass indicating that at least 37 % of the organic mass is pro-duced via primary sea-spray (Ovadnevaite et al., 2011a). Itwas found that predominantly organic particles contributebetween 25 and 30 % to general background marine num-ber concentration, 35 % for open ocean nucleation cases,

and 60 % for anthropogenically influenced cases (Bialek etal., 2012). The organic fraction of marine aerosol can behighly enriched due to oceanic biological activity (Ault etal., 2013; Gantt and Meskhidze, 2013; O’Dowd et al., 2002;Ovadnevaite et al., 2011a; Rinaldi et al., 2010). Much of theorganic fraction corresponds to water insoluble fatty acidspresent as surface films on particles (Donaldson and Vaida,2006; Mochida et al., 2002; Tervahattu et al., 2002), but alsoas organic carbon more homogeneously mixed with cationsand anions (Ault et al., 2013).

Moreover, fine-mode marine organic aerosol can have asize distribution independent from that of sea-salt, whilecoarse mode aerosols are more likely to be internally mixedwith sea-salt (Gantt and Meskhidze, 2013). Primary ma-rine aerosols mixed with a surfactant can be generated bywind action on the sea surface, which is covered by a lowsolubility organic layer (Donaldson and Vaida, 2006). Al-ternatively, heterogeneously nucleated particles can formwhen low vapour pressure organic vapours condense on pre-existing aerosol particles, forming a surface coating, whichcan be evenly or unevenly distributed. It has been proposedthat the organic compounds arrange in a hydrophobic organicmonolayer that encapsulates an aqueous particle, forming an“inverted micelle” structure (Ellison et al., 1999). This modelshows agreement with recent molecular dynamic simula-tion results (Chakraborty and Zachariah, 2008). Other mod-els predict that certain fatty acids form pockets of micelleswithin the aerosol, modifying the surface tension of the par-ticle and therefore changing the water uptake properties ofatmospheric aerosols (Tabazadeh, 2005), and that core-shellstructures are not always the most stable (Kwamena et al.,2010), again affecting the particle water uptake propertiesand optical properties.

The presence of an organic film at the surface of a particlemay affect its physical and chemical properties in a numberof ways. The film may act as a barrier to transport across theinterface, inhibiting uptake of atmospheric gases or reactionsbetween gas phase reactants and particle surface, such asthe heterogeneous reaction NaCl(s)+ 2NO2(g) → ClNO(g)+ NaNO3(s) (Donaldson and Vaida, 2006; Finlayson-Pitts,2003). In particular, the surface film can affect the processof cloud condensation nuclei formation and aerosol growthto climatically relevant sizes (Andrews and Larson, 1993;Chuang, 2003). Organic compounds can also change theamount of light scattered by inorganic particles (Dall’Ostoet al., 2010; Fierz-Schmidhauser et al., 2010; Vaishya etal., 2013). Marine primary organic aerosol (POA) can causelarge local increases in the cloud condensation nuclei con-centration by 15 % to more than 100 % (O’Dowd et al., 2004;Ovadnevaite et al., 2011b), and the ambient mass concentra-tion and organic mass fraction of sea-spray aerosol are re-lated to surface ocean biological activity. Despite the consid-erable work that has been carried out in recent years, thereis still much uncertainty about the fundamental propertiesof marine aerosol particles, such as chemical composition,

Atmos. Chem. Phys., 14, 11409–11425, 2014 www.atmos-chem-phys.net/14/11409/2014/

L. Miñambres et al.: The effect of low solubility organic acids 11411

mixing state, hygroscopicity, cloud droplet activation, forma-tion, aging, and removal mechanisms (Gantt and Meskhidze,2013; IPCC, 2013). Several laboratory studies about the ef-fect of organic surfactants, such as palmitic and oleic acids,on NaCl, ammonium sulfate or mineral dust aerosol particlesas a function of relative humidity have been reported em-ploying a variety of experimental techniques, such as electro-dynamic balance, infrared spectroscopy, electrical mobility,optical tweezers, cavity ring-down spectroscopy or nonlin-ear spectroscopy (Cwiertny et al., 2008; Davies et al., 2013;Dennis-Smither et al., 2012; Ebben et al., 2013; Garlandet al., 2005; Hansson et al., 1998; Najera and Horn, 2009;Robinson et al., 2013; Rossi, 2003; Rubasinghege et al.,2013). The general conclusions are that hygroscopic growth,deliquescence relative humidity (DRH) and efflorescence ofthe particles at efflorescence relative humidity (ERH) may beaffected by several factors, such as coating thickness or struc-tural arrangement of the organic film. Special effort has beencarried out to study the morphology and phase partitioning ofaerosol particles consisting of hydrophobic and hydrophilicphases (Ciobanu et al., 2009; Kwamena et al., 2010; Reid etal., 2011; Veghte et al., 2013). On the other hand, moleculardynamics calculations are becoming a commonplace theoret-ical approach in atmospheric aerosol modelling that includessea salt particles mixed with organic molecules (Ma et al.,2011; Sun et al., 2012, 2013; Takahama and Russell, 2011).

As a whole, laboratory studies on inorganic particlescoated with surfactant organics have mainly focused on afew organic molecules, and most of them have been carriedout with ammonium sulfate or sodium chloride. Very fewstudies of hygroscopic behaviour have been carried out onparticles containing bromide or iodide. Furthermore, sodiumchloride, bromide and iodide particles exhibit very differ-ent hygroscopic properties and interact differently with watersoluble dicarboxylic acids such as succinic acid (Minambreset al., 2011). It has been reported that rates of gaseous io-dine emissions during the heterogeneous reaction of O3 withinterfacial iodide are enhanced several-fold by the presenceof alkanoic acids on water, such as octanoic and hexanoicacid (Hayase et al., 2011). In the present work we studythe hygroscopic properties of NaX (X = Cl, Br, I) sodiumhalide salts coated with either one of three different sur-factant carboxylic acid molecules by Fourier-transform in-frared extinction spectroscopy in an aerosol flow tube, aidedby particle sizing methods. The examined acids, all containone carboxylic group at the end of the molecule, are hex-anoic (CH3(CH2)4COOH), octanoic (CH3(CH2)6COOH)and dodecanoic or lauric acid (CH3(CH2)10COOH), here-after shortened as HA, OA and LA, respectively. These acidsbelong to the family of alkanoic acids that make a signifi-cant proportion of the organic compounds emitted from sev-eral sources to the atmosphere, such as seed oil and meatcooking procedures or emission by plants. The substancesemitted in coastal areas can condense onto preexisting ma-rine aerosol and modify their properties. HA, OA and LA

have been observed in the atmosphere of remote marine andcontinental locations (Duce et al., 1983; Gill et al., 1983;Limbeck and Puxbaum, 1999; Samy et al., 2010; Schaueret al., 1999, 2002; Yassaa et al., 2001). OA and LA ex-ist as liquid and solid, respectively, at typical tropospherictemperatures and pressures. HA has higher vapour pressurethan the atmospherically more abundant long chain acidsthat may contribute more substantially to vapour phase pro-cesses. HA, OA and LA have water solubilities of 9.9, 0.68and 0.058 g L−1 at 20◦C, respectively (see Table 1), andhave been selected as they are expected to influence the hy-groscopic behaviour of sea-salt particles differently in viewof their water solubilities: HA has intermediate solubilitybetween highly soluble and highly insoluble organic acids,whereas LA, on the other end, can represent highly insolublefatty acids, OA lying in-between. Due to their overall lowwater solubility, pure fatty acids are not expected to presentsignificant intrinsic hygroscopic properties.

A few studies have been presented describing the effectsof octanoic and lauric acids on the hygroscopicity of NaCl(Hämeri et al., 1992; Hansson et al., 1998; Wagner et al.,1996). The results indicate that formation of organic sur-factant layers tend to slow NaCl deliquescence rate and toslightly lower its DRH. This may affect particle size andphase, changing the amount of scattered solar radiation andalso the adsorption behaviour of trace gases onto particles.Molecular dynamics simulations of water vapour moleculesimpinging on a slab of water coated by octanoic acid filmshowed that the mass accommodation coefficient decreasedwith the degree of surface coverage of the hydrocarbon back-bones (Takahama and Russell, 2011).

2 Materials and methods

Infrared spectroscopy is a well-known sensitive techniqueand has been applied to the study of organic/inorganicaerosol systems (Garland et al., 2005; Najera and Horn,2009). It can yield aerosol composition, water content, andparticle phase. Variations in the wave numbers and widthsof spectral bands (precisely their FWHM: full width athalf maximum) can also reveal information about molecu-lar interactions in mixed systems and the formation of newspecies. Infrared spectra have been combined with electronscanning microscopy (SEM) of particles, a technique that hasbeen demonstrated to be useful to study the chemistry of iso-lated, individual particles of atmospheric relevance (Kruegeret al., 2003; Veghte et al., 2013).

The configuration of the experimental setup used in thiswork is based on a previously described system (Minambreset al., 2010) that has been modified for the present study. Themain elements are depicted in Fig. 1. Sub-micrometric par-ticles are formed by injecting a 0.01 kg L−1 aqueous solu-tion of sodium halide salts (NaCl, NaBr and NaI,≥ 98 %) ina commercial atomizer (TSI 3076). Their relative humidity

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Table 1.Physical properties of the studied carboxylic acids.

Name #C Structural Formula Melting Boiling Water solubilities Vapour pressurePoint (K)a Point (K)a (20◦C, g L−1)b (mbar, 25◦C)c

Hexanoic Acid 6 269.7 477± 4 9.9 0.272

Octanoic Acid 8 289.3± 0.7 510± 4 0.68 5.05× 10−3

Lauric Acid 12 317± 2 571 0.058 2.18× 10−5

a NIST Chemistry WebBook.b The Merck Index: An encyclopedia of chemicals, drugs and biologicals (11 Ed.), Merck, 1989.c Lide, D. R.: CRC handbook of chemistry and physics, 6–64 to 98, CRC press, Boca Raton, FL, 1994.Note: #C Number of Carbon Atoms.

(RH) can be controlled (from 0 to around 95 %) by combin-ing two serially connected diffusion driers and a flow of N2with a controlled amount of water vapour. RH is measuredwith a digital thermo-hygrometer (Vaisala Humicap HMT337) placed at the exit of the aerosol flow cell (see Fig. 1).The measurement error is 1 % RH in the 0–90 % RH range,and 1.7 % outside it. Hygrometer absolute RH values wereperiodically checked against a calibration curve obtained byrecording the integrated infrared absorption of water vapour(measured from 2166 to 1188 cm−1 in the H2O bendingν2fundamental band) flowing through the aerosol cell at se-lected RH values (measured with the hygrometer). Measure-ments are carried out after the RH of the aerosol flow reachesa constant value with fluctuations within the RH measure-ment error. The inorganic particles are coated by passing theaerosol flow (1.8 L min−1) through a heated cell that containsa sample of either hexanoic (99 %), octanoic (≥ 98 %), orlauric acid (≥ 98 %). Table 1 summarizes the most relevantphysical properties of these acids.

The heating cell consists of an horizontally set cylindricalborosilicate glass tube 30 cm long having 3 cm internal di-ameter, that has two smaller glass tubes (30 and 20 cm long,1 cm internal diameter) coaxially attached at its ends. Acidsample (either liquid or solid) is placed uniformly along thecentral tube. The whole cell is thermally isolated by wrap-ping it with alumino-silicate refractory ceramic fiber. To al-low for sufficient vaporization of the acid, the central tubeand exit arm of the cell are heated up to 100◦C by meansof flexible resistors coiled around them. The temperatures atboth cell locations (T1 refers to the central part,T2 to the exitarm, see Fig. 1) are controlled by placing two K-type thermo-couples at the cell outer walls. To form heterogeneously nu-cleated particles,T1 was varied from 75 to 100◦C andT2from 60 to 90◦C. Higher temperature indicates higher con-centration in the gas phase leading to enhanced condensa-tion and larger particles and therefore thicker coating. Thetemperature measurement errors are estimated to be in the

Figure 1. A diagram of the experimental setup. RH is measuredby a thermo-hygrometer with the sensor placed at the exit of theaerosol flow cell. The heating cell where the organic acid sample islocated can be heated by two independent resistors at two differenttemperatures:T1 (75–100◦C) in the central part of the tube andT2(60–90◦C) at the exit arm.

4–7◦C range (the higher value corresponds to the highesttemperature). A prominent baseline shift was observed in allcases, in agreement with particle formation. This shift in-creases withT1 and T2, indicating bigger particles. Purelyhomogeneously nucleated particles were formed by pass-ing a flow of gaseous nitrogen through the heated oven atT1 = 80–100◦C andT2 = 60–90◦C containing the carboxylicacid. This coating method can get a reproducible amount offatty acid on particles in a fast and convenient way, and hasbeen used by other authors in laboratory experiments (Ab-batt et al., 2005; Garland et al., 2008; Rouvière and Am-mann 2010; Stemmler et al., 2008). Although in the tropo-sphere the whole process of heterogeneous nucleation of or-ganic vapours takes place at overall lower temperatures, inour experiment the heated organic vapour gets in contact withthe nitrogen gas flow at ambient temperature that effectively



cools the vapour by rapid heat exchange (the number den-sity of nitrogen molecules in the gas flow is much higherthan the number density of organic vapour molecules in thetube). Eventually, cooled gas-phase organic molecules het-erogeneously nucleate on salt particles in the nitrogen gasflow. In that way, the way to generate heterogeneously nu-cleated particles can be assumed to follow the same physicalprocess than in the atmosphere, although the temperaturescan vary over a broad range.

The final aerosol flux was directed simultaneously toa condensation particle counter (CPC, either TSI 3781 orMSP 1040XP models, inlet flow 0.6 L min−1), an aero-dynamic particle spectrometer (APS,TSI 3321, inlet flow1.0 L min−1) and a Fourier-transform infrared spectrometer(Nicolet Magna 860), to obtain particle number, size distri-bution and their infrared extinction spectra, respectively. In-frared extinction spectra are recorded in the 650–4000 cm−1

range and 4 cm−1 resolution. Infrared radiation from a colli-mated source (ORIEL 6580) travels lengthways a 1 m long,50 mm diameter Pyrex absorption cell at ambient tempera-ture with zinc selenide windows. The outgoing radiation isdirected to the infrared spectrometer (Fig. 1), where the in-frared beam is divided into two. Both beams take slightlydifferent path lengths, and recombine to construct an inter-ferogram. The recombined intensity is recorded as the pathlength difference is changed. By applying the Fourier trans-form technique, the variation of the intensity with the wavenumber is retrieved. The optical path is sealed and flushedby a current of dry air to reduce interference from ambientwater and carbon dioxide. Background spectra are recordedafter pumping out the aerosol cell. Sample spectra are aver-aged by collecting typically 32 scans. To complete analyti-cal on-line methodology, particle shape and size of both pureand mixed particles were determined off-line using a JEOLJSM-7000F scanning electron microscope (SEM), equippedwith a Schottky field emission gun (FEG) and an OxfordInca Pentafet X3 energy dispersive X-ray analyzer (EDX).The EDX microanalysis was performed using an accelerat-ing voltage of 20 kV and a current density of 10−10 A with aworking distance of 10 mm. The aerosol of interest was col-lected at the exit of the extinction flow cell onto a glass slide,and particles were coated with an Au layer (20 nm) depositedby evaporation using a Quorum Q150T Sputter Coater to pro-vide electrical conductivity.

Particle size distribution in the 0.5–20 µm is retrieved byan aerodynamic particle spectrometer (TSI 3321), that give atail in the 0.5–3.5 µm range. Information about the size dis-tribution of pure salt particles in the 0–0.5 µm range was ob-tained by processing the SEM images with the help of the Im-ageJ software [http://rsbweb.nih.gov/ij/docs/intro.html]. Theobtained distribution fitted satisfactorily to a lognormal dis-tribution with a count median diameter of 46 nm andσ = 2.0.Particles appear mostly isolated without a tendency to aggre-gate. Representative examples of number size distributions

of pure NaCl particles and also covered with hexanoic acidare presented in Fig. 2.

The amounts of liquid-water and a given organic acid inthe particles can be calculated on average from measured ab-sorbances in their infrared spectra. The number of moleculesNi of a given speciesi per unit volume of aerosol sample isrelated with the integrated band absorbance of that speciesvia the Beer–Lambert law (Weis and Ewing, 1996):Ai =

σ iNiz/2.303×102, whereAi is the integrated absorbance ofa given band (cm−1), σ i the integrated absorption cross sec-tion per molecule (m molecule−1) of that band, andz is theoptical path length of the aerosol flow cell (m). Theσ i valuecan be taken from the literature or measured independently.The concentration of Na+ and X− ions cannot be quantifiedby this method, as monoatomic ions do not present vibra-tional spectra.

3 Results and discussion

3.1 Infrared spectra of pure carboxylic acids

The infrared absorption spectra of bulk HA, OA and LArecorded at ambient temperature are presented in Fig. 3. Thespectra of HA and OA were recorded in an infrared cell forliquids, whereas for LA one drop of the sample dissolved inethanol was deposited on a BaF2 window until solvent evap-oration, after which the absorption spectrum of the film wasrecorded. The main absorption bands are common to all threeacids, with small differences in band position and intensity.

The sharp carbonyl stretching band can be seen near1710 cm−1, the broad band in the 2500–3500 cm−1 rangehas been assigned to associated COO–H stretchings (broad-ened by intermolecular association by hydrogen bonding),whereas the group of three peaks in the 2800–3000 cm−1

range, exhibiting a different resolvable structure for the dif-ferent acids, has been assigned to –C–H stretchings. A morecomplex band system appears in the 800–1500 cm−1, spe-cific of each acid. On the other hand, the gas phase infraredspectra of the three acids (NIST Chemistry Webbook:http://webbook.nist.gov/chemistry, not shown in Fig. 3) showseveral differences with the bulk phase spectrum: the intenseC = O band locates in the 1780–1790 cm−1, whereas a nar-row band appears near 3580 cm−1 (COO–H free stretch), ab-sent in the condensed phase. Overlapped bands appearing inthe 2800–3000 cm−1 range are coincident with peak posi-tions in bulk phase spectra. Finally, a number of bands arepresent in the 800–1600 cm−1 region, several of which canbe distinguished from condensed phase spectra.

Figure 3 also shows the extinction spectra of pure, homo-geneously nucleated particles. Bands belonging to each acidwere detected in all cases, their absorption intensity growingwith increasingT1 andT2. CPC measurements confirmed thepresence of particles that were assumed to be composed ofpure carboxylic acids. For HA the obtained spectra is mostly


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FIGURE 2 982 983 984

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FIGURE 2 982 983 984

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FIGURE 2 982 983 984

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0.771.99

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Figure 2. Representative particle size distributions of dry NaClaerosol. Panel(a) has been obtained by imaging particles depositedon a glass slide by SEM technique and counting them with theImageJ software [http://rsbweb.nih.gov/ij/docs/intro.html]. Panels(b) and (c) have been obtained by sizing particles by an aerody-namic particle spectrometer; pure NaCl particles(b) and heteroge-neously nucleated particles with hexanoic particles(c) are shown.

coincident with the gas phase spectrum. A weak band locatedat near 1730 cm−1 has been assigned to the C = O stretchoriginating from small particles of liquid HA due to homoge-neous nucleation. This band is 21 cm−1 displaced to higherwave numbers with respect to bulk liquid HA, possibly dueto surface effects in small particles: due to the interactionsbetween surface molecules and the surrounding medium, thesurface region has different structural properties (and thus

33

FIGURE 3 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037

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Figure 3. Infrared extinction spectra of HA(a), OA (b) and LA (c)in different conditions: bottom spectra are from bulk phase acid;medium spectra corresponds to homogeneously nucleated acid par-ticles; upper spectra are from heterogeneously nucleated acids ontoNaCl particles. In all cases, the upper spectra have been increasedfor clarity (increasing factor:× Number).

spectroscopic features) than the core of the particles (Fi-ranescu et al., 2006). This hypothesis is supported by thespectrum of liquid HA adsorbed at the air/water interface byvibrational sum-frequency spectroscopy (Soule et al., 2007),that locates the C = O band in the 1726–1730 cm−1 range,


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depending on the polarization conditions. For OA, the C = Oband was peaked at near 1700 cm−1, but broader than theone corresponding to condensed phase. Also an overlappingband system in the 1540–1650 cm−1 range was observed. Nolines of gaseous OA were detected. For LA, no gas band fea-tures were present, in accordance with its low vapour pres-sure. Further evidence of the presence of particles is given bythe baseline increase to higher wave numbers (Fig. 3), whichis indicative of particle scattering (Hinds, 1998). This effectis more pronounced asT2 is increased. A broad band in the3100–3500 cm−1 range is observed for homogeneously nu-cleated hexanoic and octanoic acid that is absent in the bulkspectrum. This feature may arise from the presence of smallamounts of liquid-water outgassed from the acid that becometrapped into the particles (Safar et al., 1994).

The most notable differences in band wave number andbandwidth for the three acids are observed for the C = Ostretching band, and are summarized in Table 2. These differ-ences can be significant, as they can be related to surface ef-fects that can give information about the particles. Carbonylbandwidth in the bulk acids is in the 20–29 cm−1 range,and increases with molecular mass. These values change inhomogeneously nucleated organic particles, either increas-ing (HA and OA) or decreasing (LA). Relative variations inits magnitude are in the 25–75 % range.

3.2 Infrared extinction spectra of heterogeneouslynucleated NaX particles with carboxylic acids

Representative infrared extinction spectra of heteroge-neously nucleated sodium halide particles are shown inFig. 3. The band intensities of heterogeneous nucleationspectra are much higher than those of homogeneous nucle-ation (e.g. 4 : 1 for NaCl / OA atT1 = 90◦C, T2 = 80◦C). ForNaX / HA particles, bands caused both by gaseous and con-densed phase HA were observed. The latter increase in inten-sity with T1 andT2, whereas the former remain constant. ForNaX covered with OA or LA, practically all infrared bandsoriginate from condensed phase, gas phase OA bands be-ing very weak or absent. The observed carbonyl absorptionband wave number and bandwidth for the various acids arecollected in Table 2. The changes in these magnitudes withrespect to their bulk phase values are indicative of organicmolecule/inorganic ion interactions, and can be used to ad-dress the effect of the ionic salt environment near the organicacid molecules. For all acids the C = O stretch wave numberof the acid coating on NaX varies with the salt and is betweenthe wave number of the corresponding bulk acid and that ofthe homogeneously nucleated acid particles (Table 2). In allcases, bulk wave number of C = O is around 70 cm−1 lowerthan in the gas phase, bulk LA showing the lowest wavenumber (1700 cm−1). On the other hand, the bandwidths ofthe C = O stretch originating from heterogeneously nucle-ated particles depend on the nature of the salt, the organicacid and the degree of covering: for HA-covered particles,

the full width half-maximum (FWHM) can reach 40 cm−1,doubling the bulk HA value, whereas for LA-coated parti-cles it is smaller than the bulk acid bandwidth. The relation-ship of these results with the hygroscopic properties will bediscussed later.

3.3 Morphology of pure and mixed particles

SEM images of pure NaCl and LA particles, and of NaClparticles after covering them with LA, were recorded andare presented in Fig. 4. This technique was not well suitedto study particles that included OA or HA due to their highvapour pressure at room temperature that hinders their ma-nipulation in the SEM vacuum chamber. Images of pureNaCl particles show particles of cubic form as expected butwith their edges somewhat rounded, as a result of a shortexposure of deposited particles to ambient air before beingcoated with the gold layer. As NaCl is very hygroscopic,the particles have taken up a small amount of gaseous wa-ter enough to change their original morphology (see Fig. 4a).

Images of LA particles (Fig. 4b) show a much smallernumber of particles that tend to form big aggregates, typi-cally of 1–2 µm length. This is in accordance with previousstudies (Gadermann et al., 2008). The particles are amor-phous and elongated. Images of NaCl particles depositedjointly with LA (after heterogeneous nucleation, see Fig. 4c)show a small number of particles, much fewer than in thecase of pure NaCl, although the initial amount of NaClaerosol was identical in both cases (this may be due to thelow affinity of the mixture with the supporting material or tohigher tube losses). Most particles present cubic form, andtend to appear as aggregates. Although pure LA particles canbe observed, they are very scarce. A thin layer covering theNaCl particles can be observed, smaller NaCl particles ap-pearing usually immersed in a surfactant drop. Thus it canbe said that a thin layer of lauric acid is deposited on NaClparticles, acting as glue that tends to link individual NaClparticles.

3.4 Deliquescence and efflorescence of heterogeneouslynucleated NaX particles with HA, OA and LA

3.4.1 Infrared spectra of particles at various RHs

To examine the deliquescence behaviour, dry NaX particlescoated with each of the carboxylic acids were mixed with aflow of gaseous water at different RHs. As a representativeexample, Fig. 5 shows three spectra of NaBr particles cov-ered with OA at various RHs. The presence of liquid watercan be detected and quantified by the broad band centered atnear 3400 cm−1. For all cases, no infrared absorption bandsarising from aqueous dissolved acids were detected, so in allthe subsequent discussion all the organic acids are assumedto be undissolved in liquid water.



Table 2.Wave numbers of the C = O stretching band in various conditions for HA, OA and LA. Units in cm−1.

Hexanoic acid (HA) Octanoic acid (OA) Lauric acid (LA)

band FWHM band FWHM band FWHMmaximum maximum maximum

gasa 1780 26 1780 62 1790 30bulk 1710 20 1713 26 1700 29homog. nucl. 1730 26 1700 46 1710 21heter. NaClb 1717 27–40c 1707 19 1710 19heter. NaBrb 1711–1720c 27–37c 1713 23 1708 26heter. NaIb 1720 33 1713 36 1707 13

a NIST Chemistry Webbook:http://webbook.nist.gov/chemistry.b Dry particles.c Depending on the amount of acid deposited onto the salt particles.

Figure 4. SEM images of:(a) pure NaCl particles;(b) pure LAparticles;(c) NaCl particles covered with LA.

In all cases, we paid special attention to the C = O stretch-ing band of the acids near 1700 cm−1, and we analyzed it as afunction of RH, organic acid, and halide anion measured. InOA / NaX and LA / NaX particles the carbonyl band absorp-tion intensity of condensed phase acid keeps constant withRH. For NaX / HA particles, on the contrary, the band inten-sities of liquid HA decrease as RH increases, although theintensities of gaseous HA remain unchanged. As an exam-ple, the spectra in Fig. 6 show the intensity variation of thecarbonyl band near 1700 cm−1 as RH is varied for liquid andgaseous HA in NaBr particles in deliquescence and efflores-cence modes. It can be seen that, while the C = O band inten-sity of gaseous HA keeps roughly constant with RH, the bandintensity for condensed HA lowers at higher RH in deliques-cence mode. The largest decrease was observed in NaBr, andthe smallest in NaI (not shown). In all salts, the particles re-tained liquid HA at RHs higher than their DRH. On the other

35

FIGURE 5 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064

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1000 1500 2000 2500 3000 3500 Wavenumber (cm -1)

RH=79.6%

RH=61.6%

RH=0.2%

Figure 5. Infrared spectra of NaBr particles after passing throughthe heated reservoir containing OA and exposed to different RHs.

hand, the spectrum baseline in Fig. 6 deliquescence spectraalso decreases at high wave number as RH increases (due todecrease of particle scattering), indicating a thinner coatingof the particles.

The efflorescence behaviour of coated aqueous NaX parti-cles was investigated by passing NaX aqueous particles alongthe heated oven containing the carboxylic acid vapour. For allthe systems at RH near saturation, the spectra show bands ofcondensed phase organic acid. The intensity of these bandskeeps roughly constant with RH in OA and LA, but liquidHA band intensities decrease notably as RH is reduced (afactor in the range 3–7 from RH≈ 100 % to 27 %, dependingon the salt). Figure 6 shows the case for NaBr / HA. Also thescattering signal is decreased with RH, indicating that parti-cles get smaller.

3.4.2 Deliquescence and efflorescence curves

Deliquescence and efflorescence curves were recorded bymeasuring the integrated absorbance of liquid water in theparticles in the 3400–3600 cm−1 range and plotting the




36

FIGURE 6 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109

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RH=82.2%

RH=13.3%

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RH=100%

RH=76.8%

RH=48.4%

RH=24.1%

Figure 6. Spectra showing the evolution of the infrared absorp-tion intensity of the C = O band of hexanoic acid near 1700 cm−1

with RH in NaBr particles in deliquescence and efflorescence mode.Also the shaded region indicates the selected area for measuringliquid-water abundance in the particles (see text).

values versus RH. This interval was selected as it is mostlyfree of interference with nearby absorption spectral fea-tures. The scattering component of the liquid-water extinc-tion spectrum was removed by subtracting the sloping base-line present at high wave numbers to obtain integrated ab-sorbances. The results for all the systems are presented inFig. 7. The curves for the pure inorganic salts have also beenmeasured and are included in the figure. Hereafter, we de-scribe the effect of the various acids in each of the inorganicsalts as a function of RH with a special attention towards thedeliquescence and efflorescence behaviour of the mixed par-ticles.

1. NaCl particles. The deliquescence curve of NaCl / HAfollows the same trend to that of pure NaCl particles: nowater uptake is detected until near 73 % RH, where par-ticles abruptly become liquid. Pure NaCl particles deli-quesce at DRH (298 K) = 75.3 % (Tang and Munkel-witz, 1993), a similar value. However, SEM resultsshow that particles uptake small amounts of water be-fore sudden deliquescence (as evidenced by the particlecurved edges in Fig. 4), although they are not enough to

change their size. The quantity of liquid water taken upby NaCl particles is unaffected by the presence of theHA surfactant. However, for NaCl / OA and NaCl / LA,particle deliquescence occurs near to 56 % RH, substan-tially lower than the value corresponding to pure NaCl.These results are in agreement with previous reports inwhich a DRH of 70 % was observed for NaCl particlescovered with OA and LA acids (Hansson et al., 1998).In NaCl / OA, the particles uptake larger amounts of wa-ter vapour than in pure NaCl, whereas the opposite isobserved for NaCl / LA.

The efflorescence curves for all the three acids locatethe ERH close to 40 %. That value is in agreement withprevious works using the same technique (Cziczo andAbbatt, 2000; Weis and Ewing, 1999) that located theERH of pure NaCl particles at 40±5 % RH. The curvesare coincident in the RH = 20–60 % range, but divergetowards higher RHs. For all acids particles retain largeramounts of water than pure NaCl in the RH = 60–95 %,the quantities being in the order LA > OA > HA. Alsothe amount of HA and OA in the particles decreasesas liquid water is removed from them until the ERH isreached, whereas no change in the amount of LA is ob-served with RH.

2. NaBr particles. According to Fig. 7 data, NaBr / HAparticles deliquesce at somewhat higher RH than pureNaBr particles: liquid water in NaBr / HA particles isnot detected until 50 % RH. This value does not changewith the degree of coating, and contrasts with the valueof DRH = 37 % for pure NaBr particles (Minambres etal., 2008). On the other hand, OA and LA as surfactantsdo not have any effect on the deliquescence behaviourof NaBr. The deliquescence curves are practically co-incident with those of pure NaBr. Also, the amount ofwater taken up is similar to pure NaBr, except for OA-covered particles, that uptake larger amounts of waterfor RH > 70 %. Efflorescence curves for all acid surfac-tants are very similar to pure NaBr (ERH = 23 %) in the20–60 % RH (although OA retains slightly more waterat all RHs), but at RH > 60 % acid-covered particles re-tain higher amounts of liquid water than pure NaBr (upto double for NaBr / LA at 90 % RH). Thus the pres-ence of the organic covering causes water loss to happenmore gradually than in the pure salt at high RHs.

3. NaI particles. The deliquescence curves of acid-coveredNaI particles exhibit substantial differences with respectto the pure salt. Whereas pure NaI particles take up wa-ter at all RHs (Minambres et al., 2011), NaI / HA andNaI / LA particles do not uptake water until RH = 16and 21 %, respectively. Liquid water is not detectedin NaI / OA particles until RH = 75 %. It can be con-cluded that organic acid surfactant substantially retardsthe uptake of water in NaI particles, especially OA.



Figure 7. Deliquescence and efflorescence curves of NaX (X = Cl, Br, I) particles covered with HA, OA and LA. The curves for the pureinorganic salts are shown by lines. Left and right panels show deliquescence and efflorescence curves, respectively. Liquid-water mass iscalculated as explained in Sect. 2.

The amount of liquid water in HA and LA-coveredparticles in the RH = 20–80 % is higher than in pureNaI. The efflorescence curve of NaI / LA is practicallycoincident with the pure NaI curve in the RH rangemeasured. However, the HA-covered particles lose wa-ter more gradually, retaining higher amounts of waterthan pure NaI in the RH = 30–80 % range. Finally, OA-covered particles follow closely the pure salt curve forRH > 80 %, but retain more water at lower RHs. Thegeneral tendency is that the presence of acid surfactantretains more water in the particles, except for LA, whichshows little effect.

3.4.3 Discussion of deliquescence and efflorescenceprocesses

The obtained results on the hygroscopicity of sodium halideparticles covered with organic acids having low water solu-bility show an overall complex behaviour. The deliquescence

curves of Fig. 7 indicate that the water uptake process is de-pendent on both the properties of inorganic salts and those ofthe organic acids. Although the effect of the acids on the wa-ter uptake of NaBr particles is small, in case of NaCl particlesthey produce a lowering of the DRH with respect to pure saltparticles. On the contrary, in NaI these acids prevent particlesto uptake liquid water at low RHs, unlike in pure NaI, whichadmits water condensation at RH as low as 6 %. This retard-ing effect is especially attributed to OA. In efflorescence acoherent behaviour is observed for all systems. The presenceof the organic acid makes the particle to retain more water ata given RH, all curves converging at the ERH.

The observed hygroscopic behaviour can be due to severalfactors. The morphology of particles can sometimes influ-ence their hygroscopic behaviour. However, all NaX solidshave an octahedral crystal structure, so all NaX dry particlesare expected to be cubic. This has been verified for NaCl par-ticles by their SEM images in Fig. 4. Thus we do not expect a



dependence of particle morphology in the hygroscopicity ofdifferent NaX salts. Another possibility is that each organicacid interacts differently with each inorganic salt. Organicacids in the surface of inorganic salts will tend to orientatetheir polar groups facing the salt surface so that ion-dipoleinteractions will arise that will diminish the system’s Gibbsfree energy. The polarizability of the halogen atoms increasesin the order Cl (14.7) < Br (21.8) < I (35.1, all in atomic units)(Lide, 1994), so differences in the interaction magnitude areexpected. In addition, the surface of particles will becomemore populated by anions as halide polarizability increases(Jungwirth and Tobias, 2001). Our measurements show that,for the same acid, the DRH increases with the polarizabil-ity of the halide anion. This behaviour also reflects the trendfor pure NaX particles. The acid carbon chain length mayalso influence hygroscopic behaviour. In NaCl and NaBr, alonger chain produces a decrease in the DRH for the HA-OAsequence (17 % in NaCl, 13 % in NaBr). However, further in-crease in the chain length (LA) does not produce any effectin the DRH. On the other hand, in NaI the previous tendencydoes not take place, and there is a noticeable increase in DRHfor OA. Other effects may be responsible for this behaviour.It has been proposed that the structure of the monolayersformed with insoluble surfactants determines their resistancetowards gas uptake (Stemmler et al., 2008). Fatty acids forma highly ordered film in the so-called liquid condensed state,whereas in the liquid expanded state they form a less orderedfilm and do not hinder the uptake. In that way the differencesin retardation in water uptake can arise from the different de-grees of compression of such films (Donaldson and Vaida,2006; Takahama and Russell, 2011). This can be one of thereasons for the observed behaviour in NaI / OA particles, al-though we cannot verify it experimentally. Another possibil-ity is that gaseous water transport occurs through open sec-tions of the surface that can be due to incomplete packing bythe organic film or to random fluctuations (Donaldson andVaida, 2006). SEM images of NaCl / LA do not reveal anyopen section in the surface, although we cannot provide datafor the rest of the acids. The observed salt-specific behaviouris in accordance with the observed deliquescent behaviour ofinternally mixed particles formed of sodium halide and watersoluble organic acids (Minambres et al., 2011).

The hygroscopic behaviour can be correlated with thespectral features of the organic acids shown in Table 2, wherethe wave number and the bandwidth of the C = O band in dif-ferent environments are shown. The band-centre wave num-bers shift in the−6/+10 cm−1 range in the presence of NaXsalts. The bandwidth (FWHM) of the C = O vibration forthe bulk acids is in the 20–29 cm−1 range, but changes sub-stantially for heterogeneously nucleated particles, that spanthe 13–62 cm−1 range (Table 2). For bulk HA, the C = Obandwidth is 20 cm−1, but increases in NaX particles, rang-ing from 27 to 40 cm−1. In OA (FWHM of bulk acid is26 cm−1), the bandwidth can either decrease (13 cm−1 inNaCl / OA) or increase (36 cm−1 in NaI / OA). Finally, the

FWHM of NaX / LA particles lies in the 19–26 cm−1 range,taking smaller values than in bulk LA (29 cm−1). The wave-number shift and broadening of a spectral band may arisefrom a change in the internal force constant of the C = Obond due to the formation of intermolecular bonds with othermolecules, for example hydrogen bonds with water (Kalsi,2007). These results are indicative of the effect of the ionicsalt environment near organic acid molecules.

Although it is not easy to establish clear correlationswith the changes in deliquescence behaviour, some remarkscan be outlined. For systems with NaBr the changes inthe FWHM of the C = O band are the smallest (except forHA / NaBr), which correlate with no change in DRH, exceptfor HA / NaBr particles, that deliquesce at 13 % RH higherthan in pure NaBr. For NaI mixed with an organic acid, thechanges in FWHM are considerable, ranging from−16 cm−1

in LA to +13 cm−1 in HA (Table 2). This indicates strongion–polar head group interaction. However, the explanationfor the water uptake behaviour in this case can be attributedto the hydrophobic effect exerted by the organic acids. Itforms a barrier that prevents the entrance of gaseous watermolecules inside the particles. This effect is very pronouncedfor NaI / OA, and we have not found a satisfactory explana-tion for it. Finally, in NaCl/organic acid systems, the oppositeeffect is observed, in which the presence of acids slightly en-hance condensation of water. This can be attributed to theion–polar group interaction that slightly lowers the Gibbsfree energy of the system, favouring the uptake of gaseouswater molecules. The efflorescence process in NaX aque-ous particles is not substantially affected by the presenceof the acid. As there is not an energy barrier in the efflo-rescence process (in contrast to deliquescence), water losstakes place gradually as RH is lowered. Additionally, thesalting out effect of NaCl will make dissolved organic acidmolecules (specially the most water soluble HA) graduallymove to the surface as the concentration of the salt solutionincreases (Demou and Donaldson, 2002), and will be eventu-ally removed from the surface together with water. However,as the magnitude of the salting out effect for HA is rathersmall (Demou and Donaldson, 2002), this effect is expectedto contribute little in HA.

The results on water uptake and release, together withthe release of HA from particles (exemplified in Fig. 6 forNaBr) give information on how the dynamics of water ex-change in inorganic particles is affected by the presence ofa surface layer having low water solubility. In all deliques-cence curves in Fig. 7, there is a competition for surfacepositions between adsorbed HA and gaseous water, as indi-cated by the evolution of the amounts of liquid HA and wa-ter with RH (or, equivalently, the relative amount of gaseouswater) in Fig. 6. This can be reasoned as follows: in the del-iquescence spectrum of Fig. 6, the amount of liquid HA de-creases as RH (and consequently the amount of gaseous wa-ter) increases, indicating that, if we start from a dry crys-talline NaX particle covered with HA, gaseous water tends



to displace HA molecules away from the particle surface,to try to accommodate themselves on the NaX solid sur-face. This effect can be explained as a consequence of thechange in the Gibbs free energy of the system in the deli-quescence process: as the energetically most stable systemis obtained when gaseous water molecules remain near thesolid NaX surface (producing deliquescence when the num-ber of gaseous water molecules reaches a given value), HAmolecules will tend to be displaced from their surface lo-cations. In efflorescence, however, the observed behaviouris different: removal of liquid water from aqueous particleshaving an organic HA coating due to the decrease of RH isaccompanied by removal of HA from the particle surface.This effect can be explained assuming that liquid HA uni-formly covers the NaX aqueous particle. Water moleculesexiting the particles can interact with HA molecules in theirsurface locations and sweep them away. NaX-gaseous wa-ter interactions must be stronger in NaCl than in NaBr andNaI or, alternatively, NaX-HA interactions will be weaker forNaCl than for NaBr and NaI. For that reason, in NaX / HAparticles, HA is displaced more effectively from the surfaceat the DRH of pure NaCl. In NaBr and NaI, however, thenumber density of gaseous water molecules nearby the parti-cles at the DRH of the pure inorganic particle is not enoughto remove the HA cover, and a higher number of gaseouswater molecules (higher RH) is needed to produce particledeliquescence.

3.5 Estimation of the relative amounts of organic acidand water in the particles

The organic acid / liquid-water mole ratio in the NaX par-ticles can be obtained by applying the Beer–Lambert law(as outlined in Sect. 2) ifAi and σ i are known. Forsamples measured in the same aerosol cell,Norg/NH2O =

AorgσH2O/AH2Oσ org. The absorption cross section of liquidwater has been obtained from pure water data in the 2800–3600 cm−1 range (Downing and Williams, 1975),σH2O =

1.3×10−18 m molecule−1. The absorption cross sections forliquid HA and OA have been obtained by measuring the in-tegrated absorbance of a solution of the acid in methanol ofknown composition in a 150 µm long cell for liquid sam-ples. The C = O band in the 1668–1774 cm−1 range waschosen to compute the integrated absorbance. The obtainedvalues areσHA = 9.87× 10−19 m molecule−1 and σOA =

1.71× 10−19 m molecule−1. From these data, the averageNHA / NH2O andNOA / NH2O ratios have been calculated forthe various salts at several RHs. The results appear in Fig. 8.The obtained results indicate thatNHA / NH2O values arecomprised in the 0.1–0.6 range for RH in the 30–98 % in-terval, the lowest values corresponding to the highest RHs.In contrast, theNOA / NH2O quotient spans from 0.2 to 1.9(for RHs in the 40–96 % range), when again the highest val-ues correspond to the lowest RH conditions. The highestNOA / NH2O ratio (1.9) is obtained for NaI efflorescing par-

Figure 8. NHA /NH2O and NOA/NH2O mole ratios in heteroge-neously coated particles at various relative humidities in deliques-cence and efflorescence conditions. “del” and “effl” stand for deli-quescence and efflorescence. “RH” stands for relative humidity.

ticles. The results in Fig. 8 indicate that, in general, OA-covered particles tend to displace liquid water more effi-ciently than HA-covered particles.

The spectroscopic results indicate that gaseous HA, OAand LA easily nucleate onto dry and aqueous NaX parti-cles. The amount of acid taken up increases with the tem-perature of the oven. Additionally, SEM images of NaClparticles with LA show NaCl particle aggregates, indicat-ing that they are “glued” by lauric acid. However, SEM im-ages provide no visual evidence of the presence of an or-ganic surfactant, so we can conclude that the cover must bemuch thinner than the size of the salt particles. Althoughin principle it is possible that part of the surfactant evapo-rates before particles are covered with gold, LA has a lowvapour pressure (2.2× 10−5 mbar, see Table 1), so we donot expect acid evaporation to alter the sample apprecia-bly. A rough higher limit for the cover thickness can be setat 20 nm, which is the resolution of the SEM images. Thisestimate can be compared with HA and OA cover thick-ness on aqueous particles, calculated assuming that liquidHA or OA arrange in a hydrophobic organic layer in thesurface of an aqueous particle, according to one proposedmodel (Ellison et al., 1999), and taking into account the spec-trally measured relative proportions of water and organicacid. The volume of a spherical aqueous particle of radiusR will be VH2O = (4/3)πR3, whereas the volume of liquidorganic coating of thicknessr, assuming uniform coverage,can be written asVorg = (4/3)π [(R+r)3

−R3]. On the otherhand,Vorg/VH2O = NorgρH2O/(NH2Oρorg), whereρ standsfor density. As theNorg / NH2O ratios have been determinedpreviously, from the previous equations we can obtainr interms of the aqueous particle radiusR:

r /R = [1+ NorgρH2O/(NH2Oρorg)]1/3

− 1.



For typical values ofNHA /NH2O = 0.1,0.3 and 0.6, r /R =

0.035, 0.098 and 0.181, respectively, indicating that thethickness of the organic layer increases roughly linearlywith the number of molecules of liquid HA. For example,if NHA/NH2O = 0.3 andR = 0.3 µm (roughly correspond-ing to NaCl particles appearing in Fig. 4c), the HA coat-ing thickness yields an estimation ofr = 29 nm for a parti-cle coated with HA. This value can be compared with theupper limit of 20 nm estimated for the cover thickness in theLA / NaCl system. The former HA-coating thickness is muchlarger than the estimated thickness of a monolayer, whichis roughly 0.3 nm, assuming a completely extended linearchain and taking into account an average C–C bond lengthof 0.077 nm. However, we cannot affirm whether the parti-cles are uniformly covered. Previous investigations indicatethat the degree of insoluble acid coverage do not substantiallyalter DRH. Complete coverage of inorganic particles by fattyacids had no dramatic effect on NaCl particle DRH (Andrewsand Larson, 1993). On the other hand, DRH of ammoniumsulfate was only slightly lowered for oleic acid thickness upto 109 nm (Najera and Horn, 2009), much bigger than ourestimated thickness of around 30 nm. No variations in DRHwith acid covering thickness were observed in this work.

4 Conclusions and atmospheric implications

This work studied the effect of a surfactant layer of hexanoic,octanoic and lauric acid, which are present in the Earth’stroposphere, on the hygroscopic properties of sodium halidesub-micrometre particles, which are constituents of sea saltaerosol. Infrared extinction spectroscopy together with parti-cle counting and visualizing techniques allowed us to detectthe formation of homogeneously and heterogeneously nucle-ated particles and the variation of their composition in thepresence of variable amounts of gaseous water, leading tothe processes of deliquescence and efflorescence.

It was found that the hygroscopic properties of sodiumhalide particles covered with hexanoic, octanoic or lauricacid change both with the nature of the inorganic salt andthe carboxylic acid. The DRH of NaCl aerosol experiencesa moderate shift with the nature of the organic acid coveringthe particles. The deliquescence of NaCl / OA and NaCl / LAsystems occurs near RH = 56 %, below the DRH of pureNaCl. On the other hand, NaBr covered particles do not sub-stantially alter their water uptake behaviour respect to puresalt particles for OA and LA surfactants, but the DRH ofNaBr / HA particles is about 13 % higher than that of pureNaBr. Finally, organic acid covering on solid NaI particlesacts as a barrier to water uptake; NaI particles deliquescenear 16 and 21 % RH when coated with hexanoic and lau-ric acid, respectively, but the DRH of NaI / OA particles risesto a value higher than 60 %. The general consequence is thatthe water uptake behaviour of sodium halide particles coatedwith organic acids is dependent on the nature of the anion and

the carboxylic acid. This is in accordance with former studiesof sodium halides with succinic acid, which showed a salt-specific behaviour. Consequently, although it is customary toextrapolate the water uptake behaviour of NaCl particles tosea salt aerosol due to the predominance of this species inmarine salt, the detailed picture can be more complex.

Regarding efflorescence process, the obtained results in-dicated that the overall effect of the organic acid cover is toretain higher amounts of water at RH in the 60–90 % rangewith respect to pure NaX particles. In NaCl particles thelonger chain acid (LA) achieved the highest water retention,while the shortest one (HA) provided the lowest. All acids actsimilarly in NaBr aqueous particles, whereas in aqueous NaIparticles OA is the acid that produces higher water retentionat RH in the 60–90 % range. The results showed that this bar-rier effect is dependent on the nature of the organic acid, andcan have important consequences for the troposphere, as theorganic species can determine the amount of liquid water inthe particles and their phase at a given RH. Based on our data,there is no simple correlation between water uptake or crys-tallization processes in coated salt particles and the length ofthe carbonated chain in the carboxylic acid. The complex be-haviour in hygroscopic properties in the different salts cannotbe easily attributed to a single effect, and the results point tomore specific ion-molecule interactions in the NaX/organicacid/H2O systems or to the structure of the organic film onthe particle.

SEM measurements and data deduced from the infraredabsorbance spectra indicated that the covering thickness ofthe obtained salt particles is compatible with an averagevalue of 30 nm. SEM images showed that the effect of lauricacid on NaCl is to agglomerate salt particles, producing big-ger effective particles. It was not possible to observe this ef-fect with the other acids, due to experimental inconveniences.

Several consequences for the atmosphere can be drivenfrom this study: as bromine and iodine ions tend to segre-gate to the surface of marine aerosol particles, and the ef-fect of fatty acids on them can be different as compared tothe more abundant NaCl species, this may influence the sur-face properties of the particles not usually taken into accountin the models. At a given value of ambient relative humid-ity, sea salt particles may have an outer core in which NaBrand NaI are more abundant, and if an organic layer having alow water solubility is present, the interactions of the organiccompound will predominantly take place with bromide andiodide ions. At given conditions of relative humidity in theatmosphere, liquid-water amounts and phase of sea-salt par-ticle outer core may vary regard to the expected behaviourof pure NaCl particles. This has consequences in the hetero-geneous processes taking place between particles and atmo-spheric gases, such as gas uptake and chemical reactivity.



Acknowledgements.The authors are grateful to Ministerio deCiencia e Innovación (Madrid) for grant-in-aids (CGL2011-22441and Consolider CSD-2007-00013), to Gobierno Vasco/Eusko Jau-rlaritza (Vitoria-Gasteiz) for a Consolidated Research Groupgrant (IT520-10), and UPV/EHU for UFI11/23, SEM facili-ties (SGI/IZO-SGIker) and general support. L. Miñambres thanksUPV/EHU for a postdoctoral research grant. We are grateful toCristina Gutiérrez from UPV/EHU for the use of the butanol CPC.

Edited by: T. Petäjä

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