Hygroscopic Properties of Internally Mixed Particles of Ammonium Sulfate and SuccinicAcid Studied by Infrared Spectroscopy

Lorena Minambres, Marıa N. Sanchez, Fernando Castano, and Francisco J. Basterretxea*Departamento de Quımica Fısica, Facultad de Ciencia y Tecnologıa, UniVersidad del Paıs Vasco/EuskalHerriko Unibertsitatea, Campus de Leioa, B. Sarriena, s/n, Leioa 48940, Spain

ReceiVed: February 5, 2010; ReVised Manuscript ReceiVed: April 23, 2010

Atmospheric aerosols typically consist of inorganic and organic material. Although the organic fraction canaffect the behavior of mixed organic/inorganic particles, their physical properties are not well-understood. Inthis work, infrared spectra of internally mixed submicrometer particles of ammonium sulfate and succinicacid have been studied at ambient temperature in an aerosol flow cell. The spectra of dried particles showdistinct features relative to the pure components, as a result of ion-molecule interactions between the inorganicand organic species. The hygroscopic behavior of the particles has been followed for several organic/inorganicmass ratios, showing that around equimolar composition, the mixed particles uptake water in a broad rangeof relative humidities (30-80%), substantially lower than the deliquescence relative humidity of the mixedsystem near 80%. Infrared spectra at predeliquescence relative humidities reveal that succinic acid is partiallyor completely in the liquid phase at much higher concentrations that those corresponding to a saturated solutionof succinic acid. This behavior is proposed to arise from the ion-molecule interactions between the organicand inorganic components, which unstabilize the crystal structure relative to the pure solids and cause loss oftranslational order in the crystal, bringing about an increase in the Gibbs energy of the solid particles andallowing the uptake of water molecules prior to the deliquescence point. The obtained results show that waterabsorption prior to full deliquescence in this system has to be taken into account because it extends the rangeof relative humidities at which particles are partially or completely liquid.

Introduction

Atmospheric aerosols have a considerable impact on airpollution and human health.1,2 They influence visibility in thetroposphere and the earth’s radiative energy balance due to theirlight scattering properties and changes in atmospheric chemicalpathways. Atmospheric particles can absorb water and growwith increasing relative humidity (RH), with a possible changeof phase, which, in turn, affects their properties. Aerosol particlestypically consist of inorganic and organic material and elementalcarbon. The most abundant water-soluble inorganic ions aresulfate, nitrate, calcium, chloride, sodium, and ammonium.Organic matter is composed of a wide variety of organiccompounds, which can amount to 30-50% of fine particulatemass, depending on location.3-6 Field studies indicate that20-70% of condensed-phase organic matter is water-soluble7

and includes dicarboxylic acids such as oxalic, malonic, andsuccinic, which have been identified at a variety of sources,urban and remote.8 The organic fraction can affect propertiesof atmospheric aerosol particles such as light scattering,hygroscopicity,phasetransition,solubility,orchemicalreactivity.9-13

Despite its importance, the physical state of mixed organic/inorganic aerosol particles is not well-characterized.14

In laboratory studies, the most effort has been devoted tocarboxylic acids such as formic, acetic, succinic, glutaric, oxalic,pyruvic, and malonic acids.8,9,15-18 There has been some debateabout the effect of organics on the hygroscopicity of inorganicaerosols, some data suggesting that most organics have anegative effect on the growth factor or evaporation rate ofinorganic aerosols and others pointing out that specific

organic-inorganic interactions result in both positive andnegative effects on the hygroscopic properties.15 The generaltrend appears to be that water-soluble organics may enhancewater uptake or may have little impact, whereas insolubleorganics tend to inhibit water uptake by atmospheric aerosol.Succinic acid has a very high deliquescence relative humidity(DRH > 94%) and has been proposed to behave as insolublespecies in atmospheric aerosols once they are crystallized.19

The hygroscopic properties of the succinic acid/ammoniumsulfate (SA/AS) mixed system have been studied by severalinvestigators. Choi and Chan15 reported that the organic acidenhances water absorption of AS with respect to the pureinorganic salt, although the deliquescence behavior and the DRHof (NH4)2SO4 at 80% are not affected. Similar results wereobtained by other authors.20,21 On the other hand, Hameri etal.22 concluded that the deliquescence behavior of mixed succinicacid and ammonium sulfate particles is not completely defined,with particles seeming to exist both as crystalline and solutiondroplets well below the DRH of pure ammonium sulfate.Marcolli and Krieger18 have proposed that water uptake inmixtures of AS and dicarboxylic acids before full deliquescenceshows significant deviations with respect to thermodynamicequilibrium that may be caused by morphological effects.Recently, Ling and Chan23 have reported partial deliquescenceof AS/SA particles at RHs of 80 to >90%.

Previous studies on internally mixed particles of SA and AShave been carried out mostly by electrodynamical balance orDMA techniques. As far as we know, no detailed infraredspectroscopy studies have been presented for this system up tonow. Infrared spectroscopy is a sensitive technique to character-ize aerosol composition, water content, and particle phase, and

* To whom correspondence should be addressed. Phone: +34 94 6012532. Fax: +34 94 601 3500. E-mail: franciscojose.basterretxea@ehu.es.

J. Phys. Chem. A 2010, 114, 6124–61306124

10.1021/jp101149k 2010 American Chemical SocietyPublished on Web 05/05/2010

has been used to study other internally mixed organic/inorganicparticles.24 The spectral features of aerosols can also revealinformation about molecular interactions and the formation ofnew species. In the present work, we study the water uptakeproperties of internally mixed ammonium sulfate/succinic acidparticles for different compositions and show that the presenceof succinic acid enhances the water uptake of ammonium sulfateat RHs lower than the full deliquescence of AS, changing thephases of particles. We rationalize the observed behavior interms of ion-molecule interactions that change the energy ofthe mixed solid with respect to the pure compounds.

Experimental Section

Figure 1 sketches the experimental arrangement used.25 Mixedaqueous solutions of succinic acid (SA) and ammonium sulfate(AS) are formed by dissolving the corresponding solids (SA:Sigma-Aldrich, g99.0%; AS: Alfa Aesar, 99.95%) into deion-ized water at concentrations ranging from 0.02 to 0.05 kg ofsolute per dm3 of water and different solute mixing ratios.Aerosols are formed by adding the solutions to a commercialconstant output atomizer (TSI 3076), in which pure N2 (Praxair,99.990%) is injected at 2.5 bar, yielding submicrometer aerosolparticles.

The formed aerosol flow either enters or bypasses a diffusiondrier (TSI 3062). The relative humidity is controlled by mixingtwo flows of N2 (Praxair 99.990%), one saturated with watervapor from a temperature-controlled water bubbler and the otherdry. Both flows are subsequently added to the aerosol flow. Thesum of both flows is kept constant in order to maintain theaerosol concentration unchanged. The highest RHs are obtainedby heating the water bath to about 35 °C to increase the amountof water vapor, although this did not affect the temperature inthe aerosol cell much, which only increased by 1 °C. Theresultant flow enters a 1 m long, 50 mm diameter infrared pyrexabsorption cell with ZnSe windows at its ends at ambienttemperature. RH values are measured with a digital thermohy-grometer (Hanna HI93640N) in the central section of the flowexiting the aerosol cell, with a measurement error of 3% for 50< RH < 85% and 5% in the rest of the interval.

An infrared light source (ORIEL 6580) gives collimatedinfrared radiation that goes lengthways through the aerosol cellaxis. The outcoming radiation is directed to a FT-IR spectrom-

eter (Nicolet Magna 860) to record an extinction spectrum from700 to 4000 cm-1. In order to reduce ambient water infraredabsorption, the optical path is sealed and flushed by a currentof dry air. Background spectra are recorded before aerosolspectra with the aerosol cell evacuated by a rotary diaphragmpump. Sample interferograms are averaged by collecting typi-cally 200 scans at 4 cm-1 resolution.

A small portion of the aerosol flow is diverted to the entranceof an aerodynamic particle spectrometer (TSI 3321) and acondensation particle counter (TSI 3781) to probe particleaerodynamic diameter distribution26 and total particle concentra-tion, respectively. The particle spectrometer can size particlesin the 0.50-20 µm interval, with a resolution of 32 channelsper decade. The measured aerodynamic diameters of dryparticles fit reasonably well to log-normal distributions, fromwhich the aerodynamic median diameter (Dj a) and the standarddeviation (σ) can be extracted. Geometric median diameters arethen obtained by the formula27 Dj g ) Dj a(F0/F)1/2, where F0 isthe unit density and F is the particle density. A Stokescorrection26 has to be applied to the size distribution values forparticle densities below 0.9 or above 1.1 g cm-3. As an example,values of Dj g ) 0.68 µm and σ ≈ 1.44 are obtained for succinicacid particles formed from a 0.05 kg/L aqueous solution.

Results and Discussion

A. Infrared Extinction Spectra. Figure 2 shows extinctioninfrared spectra of succinic acid solid and aqueous particles.The absorption lines due to gaseous water have been removed,although some small features remain around 1500 and 3600cm-1. The 2300-2400 cm-1 range, in which atmospheric CO2

absorption appears, has been suppressed for clarity.The infrared spectrum of the solid particles matches that

available in the literature for the condensed phase.28 It has abroad absorption band in the 2000-3400 cm-1 region with anumber of prominent peaks due to O-H stretching in theintermolecular hydrogen-bonding structure and intramolecularC-H stretching.29 In crystalline succinic acid, the molecularunit is rather planar and forms linear chains via hydrogenbonds.30 In the 800-1800 cm-1 region, there are several narrow

Figure 1. Experimental setup for the study of hygroscopic propertiesof aerosols. CPC: condensation particle counter; APS: aerosol particlespectrometer; IR source: infrared light source; FT-IR: Fourier transforminfrared spectrometer.

Figure 2. Infrared extinction spectra of succinic acid particles formedby atomizing an aqueous solution. Top: spectrum of undried aqueousparticles. Bottom: spectrum of solid particles after passing through adiffusion drier. Gaseous water and carbon dioxide lines have beenremoved.

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features, of which the most intense is centered at 1700 cm-1

and originates from a CdO stretching vibration, whereas thebands in the 700-1000 cm-1 range have been assigned to crystalchain vibrations.31 However, the broad band centered at 937cm-1 and the low intensity one at 812 cm-1, which also appearin the spectrum of gaseous succinic acid,28 can be assigned tointramolecular vibrations. These two bands are also present inthe spectrum of the dissolved acid, although somewhat displaced(at 958 and 846 cm-1, respectively). The band at 1700 cm-1 isalso found in the spectrum of the aqueous particles, overlappingwith nearby liquid water absorption, but the bands between 800and 1600 cm-1 in the solid disappear or transform in the aqueousphase. These bands will be used to follow the phase transforma-tion of succinic acid from solid to aqueous solution, especiallythe lower wavenumber region, as it is free of interference fromgaseous and liquid water absorption. Finally, the broad andintense liquid water band centered at ∼3400 cm-1 and thesuccinic acid broad band centered at ∼3000 cm-1 are overlapped.

The infrared spectrum of solid ammonium sulfate particlesin the 800-4000 cm-1 region (not shown) is well-known32 andconsists of the ν3(T2) stretching mode of the SO4

2- ion at 1115cm-1 and two modes due to the NH4

+ ion, the broad ν3(T2)stretching between 2800 and 3400 cm-1 and the ν4(T2) bendingat 1430 cm-1. For aqueous ammonium sulfate particles, theobvious changes are the moderate shifts of the ν4(NH4

+) andν3(SO4

2-) bands to higher and lower wavenumbers, respectively.The ν3(NH4

+) band is extensively overlapped with the liquidwater absorption band.

Figure 3 presents infrared absorption spectra of internallymixed SA/AS dry particles in the 800-1200 cm-1 rangeobtained for drying particles formed from a 0.05 kg of soluteper liter at various f(SA) compositions, where f(SA) stands fordry mass fraction of succinic acid (e.g., f(SA) ) 0.20 means20% SA and 80% AS). For comparison, the spectrum of pureAS particles at the same total concentration is also shown.

The solid mixture produces three noticeable spectral changeswith respect to their separate components. The first is the gradual

decrease of the ν3(SO42-) band center wavenumber from the

pure sulfate at 1115 to 1097 cm-1 for f(SA) ) 0.9. The secondis the broadening of the same band, which increases from afwhm of 34 cm-1 for the pure sulfate to 77 cm-1 at f(SA) )0.6, more than twice the first value, and decreases for f(SA) )0.9. The band shape shows an asymmetry (especially for f(SA)) 0.6 and 0.9) indicative of overlapped components. It is to benoted that the region near 1115 cm-1 is free of overlapping withSA bands; therefore, the spectral changes with composition mustarise from the ν3(SO4

2-) band. No similar effect has beenobserved in the ν4(NH4

+) band. Finally, Figure 3 shows theappearance of two new weak bands most visible at f(SA) )0.6 centered at 841 and 975 cm-1, close to succinic acid bands,and absent from the spectra of the separate solid components.Interestingly, these two band positions are approximatelycoincident with bands of aqueous succinic acid.

The previous effects can be attributed to ion-moleculeinteractions between succinic acid molecules and sulfate ions.The small red shift (maximum 17 cm-1) of the intramolecularν3(T2) stretching mode of the SO4

2- ion at 1115 cm-1 whenmixed with succinic acid in a solid environment may arise froma decrease of the internal force constants of the sulfate ion dueto the appearance of intermolecular bonds with succinic acidmolecules.33 The broadening of the sulfate ν3(T2) mode maybe due to various reasons,33 among which are transverse optic/longitudinal optic splitting of infrared-allowed bands, loss oftranslational symmetry due to the more disordered structure inthe mixed solid, and anharmonic coupling of the low-frequencymode due to the formation of hydrogen bonds between theCOOH group and the oxygens of the sulfate ion. Band splittingis visible at f(SA) ) 0.6 and 0.9 (Figure 3), and band asymmetrycan be seen even at f(SA) ) 0.2. We think that the presence ofsurrounding succinic acid molecules lowers the local symmetryaround the sulfate ion, removing the degeneracy of the T2 (triplydegenerate) mode of sulfate. We have reproduced the measuredsulfate band contours at several f(SA) by adding up threeGaussian shaped bands, each of approximately the same fwhmof a pure sulfate band at 1115 cm-1 and a different relativeintensity for each composition. The results are presented inFigure 4.

At f(SA) ) 0.2, the experimental band shape can be fitted toa Gaussian centered at 1115 cm-1 of higher intensity and twoweaker sidebands. Ion-molecule interactions are expected tobe weak at this composition, and the band shape of sulfate isonly slightly perturbed. For f(SA) ) 0.6 and 0.9, the contributionof the sidebands increases, making band splitting more evident,in accordance with a higher number of SA molecules surround-ing sulfate ions and originating stronger interactions. Theappearance of the new weak bands at 841 and 975 cm-1 forf(SA) ) 0.6 can also arise from vibrational coupling phenomenabetween succinic acid molecules and sulfate ions. These bandpositions are almost coincident with two bands of aqueoussuccinic acid, the result being that the environment of sulfateand ammonia ions surrounding succinic acid molecules producesband shift effects similar to those produced by water molecules.These weak bands have not been detected at lower f(SA) values,probably due to low signal. These low frequencies correspondto the intramolecular chain vibration region of the dicarboxylicacid, and it is proposed that the presence of sulfate and ammoniaions slightly changes the vibrational frequencies of the chainto higher wavenumbers due to repulsive forces in the lattice.33

B. Deliquescence and Efflorescence Behavior of Particles.The deliquescence and efflorescence behavior of SA/AS inter-nally mixed particles was investigated by monitoring their

Figure 3. Infrared absorption spectra of internally mixed SA/AS dryparticles at several compositions, where f(SA) stands for dry massfraction of succinic acid. The stars show the position of weak bands.The spectrum of pure ammonium sulfate particles is also shown at thebottom.

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infrared extinction spectra for different f(SA) as a function ofrelative humidity. For deliquescence experiments, atomizedparticles prepared from the 0.020 kg/dm3 starting solutionconcentration were dried by passing them through the diffusiondrier and were mixed with adequate flows of humid nitrogento get the desired RH. The water content of particles as afunction of RH was measured by recording the absorption bandarea of liquid water from 2800 to 3600 cm-1, after subtractingSA and AS spectral features. The results are presented in Figure5.

For pure ammonium sulfate particles (f(SA) ) 0) thedeliquescence curve agrees with the results already published;34,35

no water is taken up by the particles until RH ) 80%, wherethe particles deliquesce abruptly. At the other extreme, whenalmost all solute is succinic acid (f(SA) ) 0.90), we get similarresults as those for pure SA;19 particles remain completely dryuntil RH near saturation at ∼98%, when they begin to take up

water. At f(SA) ) 0.25, the onset of water uptake is locatednear that of pure ammonium sulfate. However, at compositionsapproaching a 1:1 solute mass ratio (f(SA) ) 0.40 and 0.60),the behavior of the mixture changes qualitatively; particles beginto take up water at very low RHs. The starting values of therelative humidity at which we observe liquid water in theextinction spectrum for f(SA) ) 0.40 and 0.60 are near 40%,much lower than those for any of the separate components. Wehave performed additional measurements at other compositionsthat have not been included in Figure 5 for clarity, but theyfollow the same trend. Figure 6 shows the variation of the onsetof water uptake as a function of the succinic acid mole fractionx(SA) for all of the compositions studied.

To plot Figure 6, it has been taken into account that succinicacid can evaporate partially upon atomizing with respect toammonium sulfate,22 thus altering the composition of the originalmixture. To quantify this effect, we have recorded the infraredspectra of an aqueous solution of succinic acid and ammoniumsulfate at a 1:1 solute mass ratio and also that of atomizedparticles from the previous solution, without further drying ordiluting. Calculating the integrated absorbance ratio A(SA)/A(AS) for two given bands of SA and AS in both bulk solutionand atomized particles, the change in relative composition uponatomization was obtained. For a 1:1 mass ratio for the solution,we measured a ratio of 0.8:1 (SA/AS) in the particles, indicatingnear to 20% evaporation of succinic acid relative to ammoniumsulfate. In Figure 6, it can be seen that the points near x(SA) )0.38 do not follow a smooth curve. We think that this resultcomes from measurement errors in the RH of the water uptake

Figure 4. Sulfate ion absorption bands in mixtures of SA and AS insolid particles at a number of solute mass ratios. The experimental bandcontours can be reproduced by the sum of three Gaussian curves (namedg1, g2, and g3) of different relative intensities.

Figure 5. Deliquescence curves showing liquid water integrated areasin the 2800-3600 cm-1 region versus the relative humidity forinternally mixed particles of ammonium sulfate and succinic acid atvarious compositions, where f(SA) stands for dry mass fraction ofsuccinic acid.

Figure 6. Relative humidity at which the onset of water uptake isobserved in deliquescent particles versus the mole fraction of succinicacid for internally mixed particles of ammonium sulfate and succinicacid.

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onset, as it can be seen that the corresponding f(SA) ≈ 0.5 wateruptake curves in Figure 5 have small slopes near the onset,resulting in RH determination error. The efflorescence behaviorof internally mixed particles of succinic acid and ammoniumsulfate atomized from a 0.020 kg/L solution was investigatedfor several compositions, and the results are shown in Figure7.

Aqueous particles were mixed with a flow of dry nitrogen,and the integrated area of liquid water in the particles wasrecorded as a function of the relative humidity. The data inFigure 7 are in accordance with the results already reported;8,15,19

particles crystallize at RH ≈ 50%, and no discernible differencesare observed with the composition of the particles.

Information about the particle phase in the deliquescenceexperiments has been obtained from the spectral features of themixed particles. The band at 1200 cm-1 has been chosen tomonitor solid succinic acid as it does not overlap withammonium sulfate, aqueous succinic acid, or liquid water bands.Furthermore, dissolved succinic acid can be followed byobserving the nearby band at 1179 cm-1. The phase distinctionfor ammonium sulfate is more involved; the sulfate ion band at1115 cm-1 in the pure solid is perturbed by succinic acid, andit is obscured within the features of the aqueous sulfate band at1105 cm-1. Ammonium ion bands centered at 1425 (solid) and1455 cm-1 (solution) are not affected by mixing with succinicacid, but these bands are broad and extensively overlapped.Although spectral features of the former bands have beenprocessed to extract the solid and aqueous separate components,no quantitative results of AS phases have been obtained.

The results for the phase analysis obtained from the deli-quescence spectra can be summarized as follows. For 0 e f(SA)e 0.3, succinic acid and ammonium sulfate are in the solid phasefor the entire range of RH values up to 80% (spectral featuresare not so clear for higher RHs), which is in agreement with nowater uptake observed until high RH. For 0.3 < f(SA) e 0.6,spectral lines of both solid and dissolved succinic acid appearat RHs as low as 13%, the coexistence of both phases continuesup to RH ≈ 50%, and only bands of dissolved SA are presentat higher RHs. These results are in accordance with the onsetof water uptake at low RHs observed in this interval. Figure 8shows an example of the observed phase changes for f(SA) )0.6 particles.

Bands of solid succinic acid and sulfate are present at thelowest RH, but at RH ) 35%, the band of solid succinic acid

at 1200 cm-1 is no longer neat, whereas the band of the liquidat 1179 cm-1 emerges. At RH ) 68%, all succinic acid is inthe liquid phase. The spectral bands above 1200 cm-1 and atRH ) 68% are due to imperfect subtraction of gaseous waterlines. Regarding the AS phase, in Figure 8, the band shape atRH ) 1% is different from those at RH ) 35 and 68%, whichare indistinguishable. This indicates that at least part of AS isin solution. Finally, for 0.7 e f(SA) e 0.9, the results indicatethat solid and dissolved phases of SA coexist for a wide rangeof RHs. At f(SA) ) 0.70, only solid succinic acid appears upto RH ≈ 30%, followed by coexistence of solid and dissolvedphases for RH in the 30-75% interval. At higher humidities,only the bands of the liquid are visible. At f(SA) ) 0.90, solidand liquid coexistence of SA is observed starting at RH ≈ 65%until the highest RH values measured, although the solid ispredominant. No attempt has been made to follow AS phasesin this composition range since ammonium sulfate absorptionis low at this mole fraction.

Particle composition can be calculated from their infraredspectra. The number of molecules of a given species i per unitvolume of aerosol sample Ni is obtained by measuring itsintegrated band absorbance and applying the Beer-Lambertlaw32

where Ai is the integrated band absorbance (cm-1), σj i theintegrated absorption cross section per molecule (m molecule-1),and z the optical path length (m). For liquid water, the integratedabsorption cross section from 2800 to 3600 cm-1 has beencomputed from pure water data36 to be σjH2O ) 1.3 × 10-18 mmolecule-1. To calculate integrated band absorbances fromaerosol extinction spectra, one needs to subtract the scatteringcomponent first, which can be calculated by Mie theory26 if theoptical constants and size distribution of the particles are known.

Figure 7. Efflorescence curves showing liquid water integrated areasin the 2800-3600 cm-1 range versus the relative humidity for internallymixed particles of ammonium sulfate and succinic acid at variouscompositions, where f(SA) stands for dry mass fraction of succinic acid.

Figure 8. Infrared spectra of internally mixed particles of succinicacid and ammonium sulfate at f(SA) ) 0.6 showing solute phasechanges. The absorption band of pure ammonium sulfate is also shownfor comparison.

Ai )σiNiz

2.303 × 102

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As the optical constants of succinic acid are not available, thescattering contibution of the liquid water extinction spectrumhas been removed by simply subtracting the sloping baselinepresent at high wavenumbers to obtain integrated absorbancesAH2O. We have checked that this method yields approximateresults from our previous Mie calculations in aqueous aerosols.25

The absorption cross section for dissolved succinic acid has beencomputed from the integrated absorbance of an aqueous solutionof succinic acid of known composition in an infrared cell forliquid samples in the 1118.6-1305.06 cm-1 interval. Theabsorption cross section of solid succinic acid is then calculatedby measuring the integrated absorbances of solid and aqueousparticles separately in the aerosol cell at the same flow to ensurethe same particle number density and applying the formula σs

) σaqAs/Aaq.The relative composition of particles on average can be

calculated under these conditions from their spectra and thecorresponding absorption cross sections. As an example, at f(SA)) 0.6 and RH ) 68%, in which all succinic is liquid, NH2O/NSA,aq ≈ 9. This value is in contrast with the solubility ofsuccinic acid in water of 7.2 g/100 mL at ambient temperature,37

corresponding to a molecule ratio of NH2O/NSA ≈ 90, which isthe proportion expected at the deliquescence RH, about 10 timesbigger than that in our experiment. For f(SA) ) 0.90, solid anddissolved succinic acid phases coexist, although the solid phaseis predominant; for example, at RH ≈ 98%, the liquid to solidratio of succinic acid is calculated to be NSA,s/NSA,aq ≈ 10.

The perturbations in the spectra of internally mixed solidparticles of SA and AS can be related to their water uptakeproperties. For particles consisting mostly of SA, the magnitudeof the intermolecular interactions is weak; therefore, thehygroscopic behavior of the particles is very similar to that ofpure AS, with no water uptake until full deliquescence. As themixing ratio approaches an equimolar mixture, the crystalstructure order of AS will be lowered, and the NH4

+-SO42-

Coulombic interaction energy will diminish as a result of theinclusion of succinic acid molecules. These interactions causethe system to raise its Gibbs free energy, with the result ofadmitting water molecules in the 35-80% RH range before thecomplete deliquescence of ammonium sulfate in order to lowerits energy. Water uptake will likely start at the outer shell ofthe particles (probably polycrystalline) which are graduallydissolved, although the particles have not taken up enough waterto deliquesce completely. For particles consisting mainly ofsuccinic acid (e.g., f(SA) ) 0.90), solid and aqueous phasescoexist, possibly in the form of a solid core containing most ofthe particle mass, surrounded by an external shield of dissolvedsolute, according to one of the morphological structuresproposed for aerosol particles.38

We have compared the observed deliquescent behavior withthe predictions of the extended AIM aerosol thermodynamicsmodel, which is one of the models widely used to predicthygroscopic properties of aerosol particles and can be usedonline via its Web site (http://www.aim.env.uea.ac.uk/aim/aim.php). The method can be applied to mixed inorganic/organicaqueous solutions.39 For a 1:1 mol ratio of SA and AS, the modelpredicts identical results as those in pure AS; no water uptakeuntil RH ) 80%, when prompt deliquescence takes place andthe solid abruptly incorporates a large amount of water to forma saturated salt solution. However, delayed deliquescence overa range of relative humidities has been observed previously inthis and other systems.18,22 It has been suggested40 that thecoexistence of core and film in micrometer-sized crystals cannotbe due to surface effects and must be attributed to some other

factor such as the physical or chemical state of the initial solubleparticle. Several laboratory studies indicate that, for near-equimolar mixtures of AS and SA particles, there is nosignificant water uptake prior to the AS deliquescence RH.21,23

Prenni et al.21 reported a DRH of 76.3%, practically equal tothat of pure AS. Ling and Chan23 observed partial deliquescenceof AS/SA internally mixed particles at a 1:1 mol ratio and RHsfrom 80 to >90%, with AS being observed to dissolve at 80%RH while SA remained as a solid for RHs as high as 90%.Although our results indicate that the amount of uptaken waterat RH < 80% is rather small, its presence is clearly revealed byinfrared absorption spectroscopy, which is a sensitive techniquefor detecting liquid water. Water uptake at low RHs in AS/SAinternally mixed particles can arise from the influence of particleformation conditions in different experiments. It may be thecase that the internally mixed solid particles formed in ourexperiment after drying may be influenced by kinetic effectsrather than by thermodynamic ones, leading to the formationof metastable solid phases that affect the particle hygroscopicproperties. Raman spectra of AS/SA particles23 have manifestedthe formation of such metastable forms upon crystallization.Infrared spectroscopy data presented in this work help tocomplement the information about the AS/SA system byindicating the presence of ion-molecule interactions in inter-nally mixed solid particles.

Conclusion and Atmospheric Implications

Our results indicate that the internal mixing of ammoniumsulfate and succinic acid in submicrometer particles producesion-molecule interactions (manifested in the infrared spectraof the particles) that reduce cation-anion Coulombic attractiveforces, analogous to the role of a dielectric reducing theattraction between two opposite charges, and increases structuraldisorder in the crystals with respect to pure components. Thesechanges will produce a slight increase in the Gibbs free energyof the mixed system, which has the effect of admitting moreliquid water in the particles in order to lower the energy of thesystem, forming particles in which all SA and at least part ofAS are in the liquid phase at mixing ratios approachingequimolarity, although full deliquescence of AS particles islargely unaffected by the presence of SA.

Water absorption prior to full deliquescence is importantbecause it extends the range of RH values at which liquid-phasereactions can take place.16 The presence of water-solubleorganics internally mixed with ammonium sulfate aerosol canincrease the range of conditions under which the aerosol is asolution, which in turn will influence the gas uptake propertiesand chemical reactivity of the particles. Our results suggest thatthis effect, mainly present in water-soluble organic acids, alsomanifests in some low-solubility acids due to specific physi-cochemical interactions. For succinic acid and ammoniumsulfate, the effect is stronger at 50 wt % fraction, at whichparticles will be liquid (at least partially) for a broad range ofRHs. This behavior, if generally applicable, would have anoticeable impact on the phase of chemically mixed aerosol inthe atmosphere.

Acknowledgment. The authors are grateful to Ministerio deCiencia e Innovacion (Madrid) for grant-in-aids (CGL2008-06041/CLI and Consolider CSD-2007-00013), to GobiernoVasco/Eusko Jaurlaritza (Vitoria-Gasteiz) for general supportthrough a Consolidated Research Group grant, and to Univer-sidad del Paıs Vasco/Euskal Herriko Unibertsitatea (UPV/EHU).L.M. thanks UPV/EHU for a research grant.

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